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Preprocedure Considerations
Chapter 44 • Preprocedure Considerations
CHAPTER
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44 Preprocedure Considerations Eric J. Abrahamsen
This chapter covers important topics common to both chemical restraint and anesthesia. Understanding the information contained in this chapter will improve safety and efficacy when employing the techniques presented in subsequent chapters.
Preprocedure Considerations Physical Examination In the field setting a brief physical examination should be preformed, when patient cooperation allows, prior to using a potent chemical restraint technique or anesthetizing a camelid patient. This should include an overall assessment of the patient’s health status, auscultation of the cardiopulmonary systems, and evaluation of locomotor function. This helps determine whether the patient is a suitable candidate and perhaps reduce liability should complications occur. In the hospital setting, packed cell volume (PCV) and total protein (TP) values should be determined in all patients that can be reasonably sampled prior to anesthesia. A complete blood cell count (CBC) and fibrinogen level determination improve the preanesthetic screening process, but for routine procedures in apparently healthy patients, economic and time constraints may limit the laboratory work performed. In all other patients a CBC and fibrinogen level determination should be performed. Additional laboratory work (serum chemistries etc.) should be based on patient status.
Site Selection and Facility Requirements A flat, even surface that provides good footing should be selected. The surface should be soft to reduce risks of injury during induction and recovery. In the field setting, an open grassy area is generally ideal. In the hospital setting, an area with a resilient floor such as hoof matting should be used, if
available. A stall deeply bedded with shavings may be used, although the confined space may interfere with the procedure and increase risk to personnel when larger camelids are involved. The site selected should be free of hazards in all directions for a reasonable distance. Larger camelids are more difficult to control physically and require a somewhat larger “safety zone.” Camelids tend to be patient during recovery from anesthesia. They typically do not attempt to stand until they are awake and functional. Good footing is the primary requirement for achieving a good recovery. Proximity to water, electricity, and vehicle (containing emergency supplies) are also factors to consider when selecting a field site. A calm environment reduces patient anxiety and is always desirable. A small animal surgery table may be used for alpacas and smaller llamas. A foal or small ruminant surgery table or a heavy-duty gurney will be required for larger llamas. A foam pad should be used to cushion the table surface. A shallow trough shaped pad (4-inch base thickness) makes dorsal recumbency easier to maintain and, in many cases, may be used for patients in lateral recumbency. A large animal surgery table is required for larger camels. Llamas and alpacas are typically transferred from the floor to the surgery surface and back manually. It is important to ensure that adequate personnel are available for intended transfers. A 1- to 2-ton overhead hoist may be used to move larger camelid patients into a better working position on the floor (preferably on a foam pad) or onto a surgery table. If the ceiling will not support an overhead mounted hoist without extensive modification, a lateral pull using a wall-mounted winch system may be used to reposition larger patients. Multiple mounting points and a portable winch allow patients to be moved to various areas of the operating theater. Two sets of hobbles connected by rings to the hook of the hoist or winch are used to move the patient. Hobbles are traditionally placed on the pastern but may be positioned above the fetlock, if required, to ensure a secure
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grip on the legs. Hobbles may be fashioned by splicing loops at each end of short segments of 1-inch cotton rope. The body of the rope segment is then fed through each loop to create the sliding component of the hobble. The distance between the loops needs to be sufficient to make placing and removing the hobbles easy, but excessive length reduces the height to which the patient can be lifted using an overhead hoist. Canvas strap hobbles (Shanks Veterinary Equipment, Inc., Milledgeville, IL) are available commercially. The wider, stiffer, canvas strap hobbles require more attention to attain a snug and secure fit. Good facility design makes managing larger patients safer and requires fewer personnel.1
Patient Positioning Camelids continue to salivate when sedated or anesthetized. The degree of protective laryngeal and eye reflexes retained will depend on the chemical restraint or anesthesia technique used and the drug doses selected. Camelids tend to lie down with heavier levels of sedation. Camelid patients that remain standing or are able to maintain sternal recumbency when chemically restrained typically clear saliva effectively. Patients in lateral or dorsal recumbency should be positioned such that saliva runs out of the mouth rather than pooling back near the larynx. In patients in lateral recumbency, this may be accomplished simply by placing a pad under the head–neck junction such that the opening of the mouth is below the level of the larynx (Figure 44-1). A mild downward tilt is generally all that is needed, as use of an overly large pad may impair venous return from the head, resulting in edema formation. Protecting the airway becomes much more challenging when the patient is placed in dorsal recumbency. Camelid patients placed in dorsal recumbency will typically be under the influence of a potent chemical restraint protocol or anesthetized. The body should be elevated to produce a slightly dependent head position. The neck should be gently twisted so that the head rests laterally on the surgery table or floor (Figure 44-2). A shallow trough shaped pad (4-inch base thickness) provides the proper height and makes dorsal recumbency easier to maintain. This positioning places the
mouth opening below the level of the larynx to facilitate saliva egress. The long neck of camelids makes this fairly easy to accomplish. A small pad placed under the head–neck junction may be needed to achieve the proper degree of head tilt. The head must be supported, rather the left hanging, to prevent excessive tension on vital neck structures and reduce the extent of edema formation. Placing the head in a dependent position increases hydrostatic pressures within the blood vessels of the head, which, over time, results in edema formation of the nasal, pharyngeal, and laryngeal tissues, as well as the eyelids. Minimizing the degree of dependence of the head relative to the body to only what is necessary to provide saliva egress may help to reduce the degree of edema formation. The head and neck of a recumbent patient should be extended to minimize impingement of pharyngeal tissue into the airway in a nonintubated patient and prevent kinking of the endotracheal (ET) tube in an intubated patient. Nasal edema is probably the most commonly recognized complication of anesthesia in camelid patients. Camelids with severe nasal edema are able to mouth-breathe but appear stressed when this occurs. Patients that are chemically or medically obtunded may not be able to effectively compensate. It is important to monitor nonintubated patients under the influence of potent chemical restraint or injectable anesthetic protocols and patients recently extubated for signs of respiratory compromise. One must be prepared to respond with ET intubation, if required. In patients exhibiting mild-to-moderate difficulty breathing during recovery, elevating and gently extending the head and neck in a manner that still facilitates saliva egress helps alleviate edema and maintain a patent oral airway. Airway edema generally resolves relatively quickly once the patient is able to keep its head up. Nasopharyngeal intubation, instillation of decongestant products, or both are techniques that have been used to counter the impact of severe nasal edema. Respiratory compromise resulting from laryngeal edema is less common but may be life threatening. Patients exhibiting severe respiratory distress should be endotracheally intubated. Additional information on patient positioning is provided in the “Oxygen Delivery, Muscle and Nerve Protection” section of this chapter.
Endotracheal Intubation The camelid patient should be intubated when the head cannot be positioned to facilitate salvia egress or the
Figure 44-1 Correct head positioning for camelid patients in lateral recumbency.
Figure 44-2 Correct head positioning for camelid patients in dorsal recumbency.
Chapter 44 • Preprocedure Considerations
Figure 44-3 Proper alignment is required to visualize the larynx.
Figure 44-4 One eighth of an inch aluminum stylet, Bivona Aire-Cuf silicone endotracheal tube, Miller 4 laryngoscope blade, Wisconsin 11-ich laryngoscope blade, laryngoscope handle, and gauze tie.
procedure could result in blood or other material entering the airway. Intubation is also required to deliver inhalant anesthetics (saliva production precludes safe mask delivery of oxygen or inhalants). Alpacas and llamas are intubated by direct visualization, as in dogs and cats. Because of the small mouth opening and deep oral cavity in camelids, proper alignment is important to visualize the larynx (Figure 44-3). This is similar to looking through a long, narrow tube. A laryngoscope with an extralong blade aids visualization of the larynx by allowing greater control of the base of the tongue (Wisconsin 11-inch, 14-inch, and 18-inch laryngoscope blades; Anesthesia Medical Specialties, Beaumont, CA) (Figure 44-4). The ET tube must be long enough to place its inflatable cuff below the level of the larynx in the trachea. Oral cavity depth of larger alpacas and llamas typically requires the use of longer 5- to 14-mm silicone “foal” ET tubes (Bivona Aire-Cuf Standard Silicone Endotracheal Tubes with Murphy Eye and Connector, Smiths Medicals). A selection of ET tube diameters should be available to ensure proper fit. Keeping the patient in sternal recumbency with the head elevated reduces the risk of passive regurgitation during intubation. It is important to keep the patient’s mouth pointed downward until the intubation process is imminent to minimize pooling of saliva back around the larynx. An assistant straddles the patient’s back using his or her lower legs to hold the patient in sternal recumbency. The assistant extends the head and neck up toward the individual doing the intubation
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and holds the jaws apart. The assistant’s knees may be used to help control the patient’s head and neck. A reduced level of jaw tone and the absence of the chewing or lingual response to oral manipulation are used to determine when intubation is appropriate. Sliding a large camelid patient down a wall is safer for personnel involved and improves the odds of maintaining sternal recumbency for the intubation process. Additional information on preventing regurgitation is provided in the “Fasting before Anesthesia” section of this chapter. A stylet (one eighth–inch aluminum rod for larger tube sizes and two guttural pouch cannulas glued end-to-end for smaller tube sizes) is used to facilitate intubation of camelids. The thin stylet does not obstruct the view of the larynx. The ends of the stylet should be smooth or rounded to minimize the risk of damaging the mucosal surfaces of the airway. The stylet is guided into the larynx first and the ET tube passed over it into the airway. The stylet must be long enough that it can be grasped above the ET tube as it is advanced down the length of the stylet and into the trachea. With practice, the ET tube can be positioned on the stylet and held in place, with the hand guiding the stylet into the airway, making the process less cumbersome. In large llamas, the size of the oral cavity may be large enough to allow visualization of the larynx while the ET tube is guided into the airway. The stylet is used to stiffen the ET tube. Allowing the stylet to protrude slightly from the end of the ET tube makes placement in the airway easier. Because of the depth of the oral cavity, the presence of glottal folds, and the size of the ET tube required, larger camels are intubated by manually guiding the tube into the airway, as in adult cattle. The anesthetist carries the ET tube into the mouth with one hand and then uses his or her fingers to guide the tube between the arytenoids as the other hand advances the tube. A speculum is required for this technique. The arm or hand size of the individual performing the intubation is the limiting factor in determining the appropriate patient size for this approach. The oral cavity must be large enough to accommodate the operator’s arm and the ET tube. The lower limit for this technique is generally around 300 to 350 kilograms (kg), unless the operator has an exceptionally small arm and hand. A somewhat undersized ET tube may provide additional room for the operator’s arm in marginally sized patients. The operator should wash the arm thoroughly afterward because camelid saliva may irritate the skin of some individuals. A flexible endoscope may be used to guide the ET tube into the larynx. The endoscope serves as both a stylet and a visualization device. This technique has proven useful when traditional methods cannot be used. The endoscope must be small enough to fit inside the ET tube. Camelid patients that are actively spitting prior to anesthetic induction seem to be more prone to regurgitation compared with nonspitters. Feed material is often trapped in the mouth following induction, so extra care must be taken to ensure that it does enter the airway. The small diameter of the tube used in traditional medical suction devices may make removing the coarse feed material difficult at times. Often the fastest and most effective method for clearing the material is to scoop it out using the blade of the laryngoscope. Unfortunately, fasting does not seem to eliminate this rather unpleasant behavior.
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Intravenous Catheters Are “Expensive” and Time Consuming: Are They Really Necessary? A catheter is generally not required when using simple chemical restraint techniques. A catheter should be placed prior to anesthesia in camelid patients. The jugular vein is the most commonly used site for IV catheter placement. The thick fiber coat and neck skin of camelids, combined with poor patient cooperation, may make catheterization of the jugular vein challenging, and carotid sticks do occur. A 14-gauge 5.5-inch catheter is used in most camelid patients. An 18-gauge 2-inch catheter is sufficient for crias. The catheter should be secured to the neck with suture, bandage, or both. The catheter should always be checked before anesthetic induction to ensure that it is still functional. The veins on the external surface of the ear of some camelids are relatively large and accessible. Ear veins can be used to deliver small-volume IV chemical restraint cocktails. Most patients require only modest head restraint when a 25-gauge needle and good technique are used. Ear veins may be used as an alternative site for venous catheter placement. An 18gauge, 20-gauge, or 22-gauge catheter should be used, depending on the vessel size, and secured with a combination of super glue and tape. Because of the increased level of stimulation produced by the catheter stylet, camelid patients are typically less than cooperative to ear catheterization unless chemically or medically obtunded. The cephalic vein may also be used as a site for venous catheter placement, much as it is in small animal patients. The smaller catheters will not provide the flow rate of the 14-gauge catheter but work well otherwise. Camelid owners are frequently concerned about the clipping of fiber, which makes venous catheterization somewhat more challenging. One approach practitioners may find useful is to administer a Ketamine Stun via the ear vein and then place an IV catheter once the patient is recumbent and cooperative. Double Drip or Ruminant Triple Drip may then be delivered to maintain injectable anesthesia or facilitate ET intubation for inhalation maintenance.
Fasting before Anesthesia Fasting has been traditionally recommended for patients undergoing anesthesia. In many equine practices, patients are no longer fasted prior to anesthesia with no adverse consequences and perhaps some real benefits, but some clinicians still believe that it is necessary. Experience with nonfasted emergency cases has shown proper technique to be the most important factor in reducing the risk of regurgitation during induction and intubation in most species. Attempting to intubate a patient with some degree of gag reflex is more likely to result in active regurgitation. Proper induction technique eliminates the gag reflex. Keeping the patient in sternal recumbency with the head elevated reduces the risk of passive regurgitation during intubation. True ruminants tend to bloat in lateral recumbency or when anesthetized, and fasting seems to reduce this problem. Camelids typically do not bloat in lateral recumbency or during anesthesia, but as in true ruminants, passive regurgitation during anesthesia does occur. Camelid patients generally do not need to be fasted for chemical restraint techniques unless a switch to injectable anesthesia is likely to be required to complete the procedure.
Although many practitioners do not fast camelid patients prior to injectable anesthesia, it is currently recommended for patients over 4 months of age. Fasting reduces the gastrointestinal (GI) system volume, which, in turn, reduces pressure on the diaphragm and the potential for passive regurgitation during anesthesia. Proper patient positioning during injectable anesthesia is critical to reduce the risk of aspiration should passive regurgitation occur. Withholding food for 12 to 18 hours (access to water is permitted) has proved effective in minimizing problems during intubation and anesthesia while not producing the adverse effects on GI motility and acid–base status associated with longer periods of fasting. Young animals have minimal energy stores. The risk of hypoglycemia in these patients increases with the duration of anesthesia. “Nursing” patients are typically anesthetized without fasting to reduce the risk of hypoglycemia. Camelid patients younger than 2 months of age should be supplemented with dextrose during anesthesia. Adding 1.25% to 2.5% dextrose to the IV electrolyte solution (10 milligrams per kilogram per hour [mg/kg/hr] for the first hour and 5 milliliters per kilogram per hour [mL/kg/hr] for the balance of anesthesia) is generally sufficient to ensure that blood glucose levels are adequately maintained in these patients. Generally, by 2 months of age, most healthy camelid patients no longer require dextrose supplementation during short anesthetic procedures. Adding 1.25% dextrose to the IV fluids is inexpensive insurance against hypoglycemia in camelid patients 2 to 4 months of age when a longer period of anesthesia is anticipated. Elevated body temperature increases metabolism and the risk of hypoglycemia in younger patients. Patients up to 4 months of age should be supplemented with dextrose when body temperature is significantly elevated. Unless testing is performed during anesthesia, hypoglycemia is typically not recognized until the recovery period. Hypoglycemia produces a stuporous state, and patients typically stall partway through the recovery process. Administration of dextrose has resulted in full recovery with no apparent adverse effects when this has occurred. As with most things medical, prevention is better than treatment. Protracted struggling during physical restraint of unruly young patients may produce substantial elevation of body temperature. Unruly patients should be sedated rather than battled.
Eye Protection When camelid patients are placed in lateral recumbency, care should be taken to ensure that the lids of the down eye are closed. A towel or thin pad may be placed under the down eye to provide further protection. Ophthalmic ointment (bland or antibiotic) should be placed in the eyes to protect them during anesthesia. If the down eye ends up getting bathed in saliva or regurgitation, it should be rinsed out during recovery.
Oxygen Delivery and Muscle and Nerve Protection Information in this section is adapted from the Proceedings of the 2007 British Equine Veterinary Association Congress.2,3 Maintaining adequate delivery of oxygen to tissues is one of the most important goals of the anesthetist. Global oxygen delivery is determined by the combination of cardiac output
Chapter 44 • Preprocedure Considerations and arterial oxygen content. Many sedative and anesthetic agents depress cardiac output. Arterial oxygen content (CaO2) is determined primarily by the level of hemoglobin oxygen saturation (SaO2), which is based on the partial pressure of oxygen in arterial blood (PaO2) and the concentration and affinity of hemoglobin. PaO2 should be approximately five times the inspired level (e.g., PaO2 should be 100 mm Hg on 20% O2 and 500 mm Hg when delivering “100%” O2). Many factors can adversely impact PaO2 during anesthesia. GI mass, bloating, or both exert pressure on the diaphragm, reducing the tidal volume of the patient. Hypoventilation may also be caused by excessive anesthetic depth. Hypoventilation reduces the alveolar oxygen levels (PAO2) and the diffusion pressure it exerts on pulmonary gas exchange. Gravitational effects in large patients redistribute pulmonary circulation to the more dependent regions of the lungs, while ventilation is directed to the more compliant nondependent regions of the lungs mismatch. Diffusion resulting in ventilation perfusion ( V/Q) impairment, typically resulting from pulmonary edema, is a less common cause of decreased PaO2. The partial pressure of carbon dioxide in arterial blood (PaCO2) is used to evaluate the level of respiratory depression. The level of respiratory depression present during anesthesia varies with the technique used and the depth of anesthesia produced. Llamas and alpacas generally exhibit little respiratory depression (PaCO2 40–45 mm Hg) at surgical planes of inhalation anesthesia. Bolus injectable anesthetic techniques typically produce an initial degree of respiratory depression (PaCO2 > 45 mm Hg) that diminishes over time as the plane of anesthesia lessons. Clinically significant respiratory depression (PaCO2 > 60 mm Hg) has been documented during bolus injectable anesthetic techniques in camelid patients. Bolus injection techniques that produce longer periods of anesthesia are more likely to cause significant initial respiratory depression. Even when the PaCO2 remains in the 40- to 45-mm Hg range, clinically significant hypoxemia (SaO2 < 90% and PaO2 < 60 mm Hg) may occur during injectable anesthesia. A pulse oximeter may be used to monitor SaO2 during anesthesia. Hemoglobin (Hb) affinity for oxygen is reportedly higher in alpacas than in most other mammals.4,5 Because of its critical influence on CaO2, SaO2 may be a more valuable parameter to monitor than PaO2 in camelid patients. The fractional inspired oxygen (FIO2) level is 0.2 when patients are breathing room air. Increasing the FIO2 with oxygen insufflation may provide dramatic improvement in PaO2, SaO2, and CaO2 during injectable anesthesia. Perhaps because of their high altitude origins, alpacas and llamas typically exhibit excellent pulmonary gas exchange. PaO2 values are generally 500+ mm Hg during inhalation anesthesia. The ability to correct hypoxemia produced by impaired gas exchange in recumbent or anesthetized large animals receiving “100% O2” (FIO2 approximately 1) is extremely limited. Methods such as intermittent positive pressure ventilation (IPPV) or positive end-expiratory pressure (PEEP) produce occasional improvement in PaO2 but also create adverse mechanical (decreased venous return) and chemical (decreased PaCO2) effects on cardiac output.6,7 Decreases in cardiac output typically exceed any improvements in CaO2 and delivery of oxygen to tissues (DO2) is reduced rather than improved. Because of the influence of the oxygen–hemoglobin dissociation curve, SaO2 and CaO2 are generally maintained at
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levels that are adequate when other aspects of oxygen delivery are properly managed in patients with reduced PaO2 levels. Anemic patients are the exception because they have a reduced oxygen-carrying capacity. Cardiac output (tissue flow) is the most important variable involved in tissue oxygenation. Tissues are able to cope with reductions in arterial oxygen content better than they cope with reductions in blood flow. When CaO2 is reduced, tissue oxygen tension falls, and locally controlled vasodilation occurs. The resulting increase in tissue blood flow improves tissue oxygen delivery. When tissue blood flow is reduced, oxygen extraction from arterial blood is increased, but this mechanism is more limited in its ability to compensate. Arterial blood pressure is monitored as a surrogate of cardiac output. Arterial blood pressure generally provides a reasonable estimation of cardiac output status, but large changes in cardiac output may occur without concomitant changes in arterial blood pressure. In anesthetized horses, cardiac output has been shown to decrease by 30% to 40% when IPPV is instituted.6,7 Increased peripheral resistance maintains arterial blood pressure near preventilation levels, masking the serious decrease in cardiac output. Until clinically useful methods are developed to provide real-time cardiac output values, monitoring arterial blood pressure remains the best defense against poor tissue oxygen delivery during anesthesia. Localized obstruction of blood flow is the typical cause of postanesthetic nerve or muscle complications in large animal anesthesia. Proper positioning of larger patients is important to minimize compressive forces that may result in localized obstruction of blood flow. Placing patients on a thick foam pad during anesthesia distributes the pressure of their weight more evenly, reducing the risk of localized obstruction of blood flow to dependent muscles. For patients in lateral recumbency, the down front leg should be pulled forward to reduce pressure on the radial nerve. The upper front leg should be propped up parallel to the floor or the table surface to minimize the compressive force of its considerable mass. The upper rear leg should be supported in a similar manner. In situations where local tissue blood flow is partially obstructed, reduced arterial oxygen content may contribute to development of postanesthetic neuropathy or myopathy.
Why Is Premedication Important? Veterinarians have been traditionally taught that food animal and camelid patients do not generally require premedication before anesthesia. Actually, most food animal and camelid anesthesia cases involve the use of premedication but not in the manner traditionally used in other veterinary species. Premedication is used to enhance patient control and modify the response to the induction bolus. Premedication may intensify or extend the effects of the induction bolus. Premedication may also minimize the adverse effects of an induction drug. Apprehension and activity alter the distribution of cardiac output, directing a greater portion of blood flow to skeletal muscles. Although many food animal and camelid patients appear calm before anesthetic induction, some degree of apprehension likely exists. Extremely anxious or unruly food animal and camelid patients experience a greater alteration in the distribution of cardiac output. Sedatives such as xylazine or guaifenesin produce a dose-dependent calming effect.
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Reducing the patient’s anxiety and activity directs a greater portion of the cardiac output to the vital organs (centralization of cardiac output). The degree of sedation determines the level of centralization achieved. Centralization of cardiac output does not occur instantaneously. It lags behind peak sedation by a few minutes in calmer patients and longer in extremely anxious or unruly patients. Centralization of cardiac output is desirable because it directs a greater portion of the IV anesthetic induction agent to the target sites in the central nervous system (CNS). When lipid-soluble drugs such as anesthetics induction agents are administered as an IV bolus, it is redistribution from the vital organs to skeletal muscle via continued circulation that ends the clinical effects of the drug. Any increase in the amount of drug sent directly to muscle will decrease the impact of the induction bolus (peak effect and duration). Anesthesia in food animal and camelid patients is typically induced by using a combination of a sedative and ketamine. In small ruminants and camelids, Double Drip (5% guaifenesin with 1 mg/mL of ketamine added) is infused to effect. Anesthesia in small ruminants and camelids may also be induced with a combination of ketamine and diazepam (KetVal). In large ruminants and camels, Double Drip is slowly infused until the first signs of muscle weakness are apparent, and then a bolus of Ket-Val is administered. Administering the sedative (guaifenesin or diazepam) along with the anesthetic induction agent (ketamine) does not provide the benefits achieved with the sedative-first approach used in most other species. Double Drip administration produces obvious signs of sedation before actual anesthetic induction, but the short duration does not allow total centralization to occur. When Ket-Val is used, the onset of diazepam’s sedative effect is quick enough to prevent the neuroexcitatory effects produced by the large dose of ketamine, but little centralization of cardiac output occurs. These induction techniques are generally effective in ruminants and camelids large part because of the calmer demeanor of these patients, but that does not mean that these techniques cannot be improved. Unless contraindicated, anxious or unruly patients should be sedated with xylazine 5 to 10 minutes before the anesthetic induction sequence. The dose of xylazine required will depend on the demeanor of the patient. It could be argued that most healthy ruminant and camelid patients might benefit from a small dose of xylazine 5 to 10 minutes before anesthetic induction because even the calmest of patients will experience some anxiety from the events surrounding anesthetic induction.
Important Information on Selected Drugs Used in Chemical Restraint and Anesthesia α2-Adrenergic Agonists Of the α2-adrenergic agonists available to the veterinary practitioner, xylazine is generally used for sedation of ruminant and camelid patients. Newer, more potent α2-adrenergic agonists such as detomidine, romifidine, and medetomidine are substantially more expensive, and their longer duration of action is often not required. Supplemental doses of xylazine
may be administered to extend the duration of the effect, when required. The initial demeanor of the patient greatly influences the sedation produced by a given dose of an α2-adrenergic agonist. The sedative effect of α2-adrenergic agonists may be overridden by elevated “sympathetic tone.” This results in two characteristic features of α2-adrenergic agonist sedation. The sedative effect produced by lower doses of α2-adrenergic agonists is not as stable. Calm, quiet patients require smaller doses, whereas anxious or unruly patients require larger doses. The ideal dose may be difficult to predict, especially when recumbency is not desired. Experience makes the necessary adjustments easier, but even seasoned practitioners are sometimes surprised. α2-adrenergic agonists may be administered intravenously, intramuscularly, or subcutaneously to produce a dosedependent degree of sedation, muscle relaxation, and analgesia. IV administration of α2-adrenergic agonists provides a faster onset and more intense level of chemical restraint and analgesia. The fairly rapid onset time may be used to advantage, allowing multiple smaller IV doses of an α2-adrenergic agonist to be administered to titrate the effect to the desired level. Intramuscular (IM) administration results in a more gradual onset and provides a longer duration of less intense chemical restraint and analgesia. IM administration is often used when patient cooperation does not allow IV administration or when extended duration is desired. The IM dose is traditionally twice the IV dose one would select for the patient based on the desired level of effect and the patient’s initial demeanor. Subcutaneous (SQ) administration results in the most gradual onset, longest duration, and mildest peak effect. α2-adrenergic agonists possess potent sedative and analgesic properties. These desirable dose-dependent effects are accompanied by a myriad of dose-dependent side effects.8 The cardiorespiratory and GI effects are the most important, although the dose-dependent muscle relaxation produced by α2-adrenergic agonists likely contributes to the incidence of unwanted recumbency. Cardiovascular side effects associated with IV xylazine include decreases in heart rate and cardiac output that may reach 25% when larger doses are administered. IV xylazine produces a biphasic change in blood pressure (initial increase in blood pressure produced by peripheral vasoconstriction, followed by a gradual decrease to below base-line values because of reductions in sympathetic nervous system tone and eventual return to baseline values as the effects of the xylazine administered resolve). Xylazine administered intramuscularly does not produce vasoconstriction, and arterial blood pressure decreases as sedation intensifies and then gradually returns to baseline values as sedation wanes. The cardiovascular effects of the newer, more potent α2-adrenergic agonists such as detomidine are even more potent, producing larger decreases in heart rate and cardiac output. In contrast to xylazine, IV administration of detomidine results in prolonged elevation of blood pressure. Persistent peripheral vasoconstriction in the face of decreased cardiac output does not promote good tissue blood flow. For this reason, I feel that the newer more potent α2-adrenergic agonists should not be routinely used as anesthetic premedications. The cardiovascular changes produced by xylazine are generally well tolerated in the normal healthy patients but may be life threatening in hypovolemic or endotoxic patients.
Chapter 44 • Preprocedure Considerations The sympatholytic effects of α2-adrenergic agonists may exacerbate bradyarrhythmias. α2-adrenergic agonist administration in hyperkalemic patients has resulted in complete heart block. The degree of respiratory depression produced by α2adrenergic agonists is dependent on the drug used and the dose administered. Mild to moderate levels of xylazine sedation produce inconsequential respiratory effects. Respiratory rate typically decreases, whereas tidal volume tends to increase, resulting in only minor alterations in arterial blood gas values. Heavy xylazine sedation may produce clinically significant respiratory depression. Detomidine seems to produce a greater level of respiratory depression, and extremely large doses have resulted in respiratory compromise in horses. The respiratory effects of α2-adrenergic agonists are generally well tolerated in normal, healthy camelid patients. α2-adrenergic agonists produce dose-dependent decreases in GI motility. Xylazine has been reported to increase uterine tone in late gestation.9 α2-adrenergic agonists should be avoided or used with extreme caution in patients with significant cardiorespiratory compromise. Butorphanol, a benzodiazepine, or a slow infusion of guaifenesin may be used to produce sedation and centralization in patients that may not tolerate the adverse cardiorespiratory effects of xylazine. In situations where an α2-adrenergic agonist must be administered to a compromised patient, titrated administration should be used, whenever possible, to reduce risk. Reversal of the α2-adrenergic agonist should be done at the earliest point possible. Titrated reversal once the patient is anesthetized removes the adverse effects of the α2-adrenergic agonist while minimizing the risk of producing an excessively light plane of anesthesia.
α2-Adrenergic Agonist Reversal α2-adrenergic antagonists may be used to reverse the effects of α2-adrenergic agonists at the end of a procedure to facilitate a quicker recovery and minimize the risks of GI complications. IM administration of the reversal agent is preferred in most nonemergency situations because it decreases the risk of CNS excitement or cardiovascular complications. The shorter duration of action of reversal agents when administered intravenously may result in resedation in patients if the α2-adrenergic agonist was administered intramuscularly, especially when larger doses were used. Patient arousal occurs when redistribution reduces blood levels of ketamine or tiletamine to a critical level, although patients are typically not ready to stand at this point. The residual sedative effect of the α2-adrenergic agonist administered is critical in preventing attempts to stand until blood levels of these IV anesthetic agents have decreased sufficiently to ensure a coordinated effort. To reduce the risk of a rough recovery, α2-adrenergic agonist reversal agents should ideally not be administered until the patient is in the sternal position and able to lift its head off the floor. In situations where the patient presents a physical danger to personnel, the reversal agent may be administered when arousal is imminent. The amount of reversal agent used depends on the dose of the α2-adrenergic agonist and duration since administration. The commonly recommended emergency IV doses for yohimbine (0.125 mg/kg) and tolazoline (2 mg/kg) are typically used as the maximum IM dose and reduced to fit the
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circumstances. Atipamezole (20–60 micrograms per kilogram [mcg/kg] IV) and idazoxan (0.05 mg/kg IV) have been used to reverse the effects of α2-adrenergic agonists in ruminants. Doxapram, an analeptic, has produced arousal of patients sedated with α2-adrenergic agonists. When dosed properly, the effects of reversal should start to become evident approximately 8 to 10 minutes following IM administration. Splitting the reversal dose (using both IV and IM routes) may produce quicker recovery while minimizing the risk of resedation. Response to the small IV component will be fairly quick, so the primary IM component should be administered first. Anecdotal reports of deaths associated with tolazoline reversal have created concern regarding the safety of this drug. A case of suspected tolazoline toxicosis in a llama has been reported.10 The patient exhibited generalized weakness, seizures, dyspnea, severe hypotension, tachycardia, and diarrhea after receiving tolazoline (4.3 mg/kg IV followed 45 minutes later by 1 mg/kg IV and 1 mg/kg IM). Large IV doses and rapid IV administration should both be avoided when using tolazoline.
Benzodiazepines Benzodiazepines (diazepam, midazolam) are centrally acting muscle relaxants with sedative effects. Benzodiazepines do not generally produce a beneficial calming effect when used alone in most species, but ruminants and camelids respond favorably. Benzodiazepines produce minimal cardiovascular or GI effects at clinical doses and provide viable options for producing mild sedation in compromised patients. Benzodiazepines may also be combined with other sedatives, and this allows drugs with adverse effects to be used in smaller doses. Diazepam may be administered intravenously or intramuscularly, but IM absorption is unpredictable. Midazolam is water soluble and may be administered intravenously, intramuscularly, or subcutaneously. Midazolam is slightly more potent than diazepam, but the drugs are generally used interchangeably. Now that midazolam is available as a generic formulation, the price differential is negligible.
Butorphanol Butorphanol, an opioid agonist–antagonist, is an analgesic drug with sedative effects. Butorphanol does not generally produce a beneficial calming effect when used alone in most species, but ruminants and camelids respond favorably. Butorphanol produces minimal adverse cardiovascular effects at clinical doses and provides a viable option for producing mild to moderate sedation in compromised patients. Butorphanol may also be combined with other sedatives, which allows drugs with adverse effects to be used in smaller doses. Butorphanol may be administered intravenously, intramuscularly, or subcutaneously.
Guaifenesin Guaifenesin is a centrally acting muscle relaxant with sedative effects. Guaifenesin produces minimal cardiorespiratory effects at clinical doses. Premixed guaifenesin solution (5%) is currently only available from compounding pharmacies. Guaifenesin concentrations of 10% have been shown to cause
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hemolysis in ruminant patients. When mixing guaifenesin from powder stock, solutions of 5% should be prepared. Guaifenesin-based solutions (Double Drip, Ruminant Triple Drip) are used for anesthetic induction and maintenance of injectable anesthesia in food animal and camelid patients. Guaifenesin may also be used to produce mild to moderate preanesthetic sedation in compromised patients that might not tolerate the adverse cardiovascular effects of xylazine. The induction bolus should be drawn up before administration of the solution. Guaifenesin is infused to effect and has an onset time of approximately 1 minute. Observable signs of sedation are used to guide delivery. A slow rate of infusion should be used to allow the clinician to account for the delayed onset when assessing progress and adjusting delivery. As sedation builds the rate of infusion should be decreased to provide time for centralization to occur. Llamas and alpacas generally go down into sternal recumbency when moderately sedated, which allows more latitude when using guaifenesin in this manner. Large ruminants and camels are generally induced while standing. The standing sedation dose of guaifenesin (15–25 mg/kg IV) must be delivered carefully to avoid producing excessive muscle relaxation in these patients. The corrective postural activity that it triggers antagonizes the centralizing effects of the sedation. If excessive muscle relaxation begins to develop, the induction bolus should be administered immediately. Guaifenesin sedation works better in calmer patients and should not be expected to produce significant centralization in extremely anxious or unruly patients. When using Double Drip for anesthetic induction of small ruminant and camelid patients, slowing the rate of infusion once sternal recumbency has been achieved extends the period of sedation to a certain extent, allowing a greater degree of centralization to occur before the point of anesthetic induction is reached.
maintenance anesthesia. Thiopental (1.1 mg/kg IV) is also useful in dulling the protective swallowing reflex present during ketamine-based injectable anesthesia. Tiletamine, a more potent and longer-lasting relative of ketamine, is available only in combination with the benzodiazepine zolazepam under the brand name Telazol. Because of the high cost of Telazol, it is primarily used in large animal practice for “capturing” intractable patients. An apneustic pattern of breathing is often observed in horses following induction with ketamine or Telazol but is not common in ruminant and camelid patients. Both ketamine and tiletamine draw on the sympathetic nervous system reserve to augment cardiac output and blood pressure. This effect helps counter their direct negative inotropic and vasodilatory effects, as well as the negative cardiovascular effects produced by xylazine. Cardiovascular function in normal healthy patients anesthetized with ketamine-based protocols is good to excellent. I cannot emphasize enough the need for caution in administering these seemingly safe drugs to compromised patients in which the sympathetic nervous system reserve may be severely limited. In compromised equine colic patients, I use a reduced dose of ketamine (1.3– 1.75 mg/kg IV, depending on patient status) to produce recumbency. Induction is generally slower when smaller doses of ketamine are used. The cardiovascular status is reevaluated once the patient is recumbent and support measures (dobutamine infusion) instituted, as required, in the induction area. Camelid patients induced in this manner will be at a fairly light, plane of anesthesia and the gag reflex may be present. Additional ketamine administration may be required to minimize the risk of regurgitation during intubation. Using a somewhat undersized ET tube may make the intubation process easier.
Atropine
REFERENCES
Premedication with atropine was commonly recommended in the early days of ruminant anesthesia to counter the profuse salivation in these patients. Atropine administration does not eliminate salivation in ruminant or camelid patients during anesthesia. Atropine tends to reduce the aqueous component of saliva, making it more difficult to clear from the mouth and the oropharynx. Atropine also reduces GI motility. Routine premedication with atropine is unnecessary and increases patient risk. Atropine (0.01–0.02 mg/kg IV) may be used to treat bradycardia or bradyarrhythmias.
1. Abrahamsen EJ: Inhalation anesthesia in ruminants. In Anderson DE, Rings M, editors: Current veterinary therapy food animal practice, ed 5, St. Louis, MO, 2009, Saunders (pp 559-569). 2. Abrahamsen EJ: Dr. Strangeblood Part I—how I stopped worrying and learned to love CO2. In Proceeding of the 2007 British Equine Veterinary Association (BEVA) Congress, Edinburgh, Scotland, 2007. 3. Abrahamsen EJ: Dr. Strangeblood Part II—how I learned to stop worrying about PaO2. In Proceedings of the 2007 British Equine Veterinary Association (BEVA) Congress, Edinburgh, Scotland, 2007. 4. Reynafarje C, Faura J, Villanvicencio D, et al: Oxygen transport of hemoglobin in high-altitude animals (Camelidae), J Appl Physiol 38:806-810, 1975. 5. Sillau AH, Cueva S, Valensuela A, et al: CO2 transport in the alpaca (Lama pacos) at sea level and at 3300 m, Respir Physiol 27:147-155, 1976. 6. Steffey EP, Howland D: Cardiovascular effects of halothane in the horse, J Am Vet Med Assoc 39:611-615, 1978. 7. Hodgson DS, Steffey EP, Grandy JL, et al: Effects of spontaneous, assisted and controlled ventilatory modes in halothane anesthetized geldings, Am J Vet Res 47:992-996, 1986. 8. Campbell KB, Klavano PA, Richardson P, et al: Hemodynamic effects of xylazine in the calf, Am J Vet Res 40:1777-1780, 1979. 9. LeBlanc MM, Hubbell JEA, Smith HC: The effects of xylazine hydrochloride on intrauterine pressure in the cow and mare, Theriogenology 21(5):681-690, 1984. 10. Read MR, Duke T, Toews AR: Suspected tolazoline toxicosis in a llama, J Am Vet Med Assoc 216:227-229, 2000.
Ketamine, Telazol, and Thiopental Ketamine, tiletamine, and the ultra-short barbiturate thiopental are the injectable anesthetic agents used in large animal practice. Ketamine is, by far, the most common injectable anesthetic agent used in large animal practice. Subanesthetic doses of ketamine are also used in chemical restraint techniques. Thiopental is no longer generally used in large animal practice for induction or maintenance of anesthesia but remains the fastest option for restoring an anesthetized state when larger patients get too light during inhalation
CHAPTER
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44 Preprocedure Considerations Eric J. Abrahamsen
This chapter covers important topics common to both chemical restraint and anesthesia. Understanding the information contained in this chapter will improve safety and efficacy when employing the techniques presented in subsequent chapters.
Preprocedure Considerations Physical Examination In the field setting a brief physical examination should be preformed, when patient cooperation allows, prior to using a potent chemical restraint technique or anesthetizing a camelid patient. This should include an overall assessment of the patient’s health status, auscultation of the cardiopulmonary systems, and evaluation of locomotor function. This helps determine whether the patient is a suitable candidate and perhaps reduce liability should complications occur. In the hospital setting, packed cell volume (PCV) and total protein (TP) values should be determined in all patients that can be reasonably sampled prior to anesthesia. A complete blood cell count (CBC) and fibrinogen level determination improve the preanesthetic screening process, but for routine procedures in apparently healthy patients, economic and time constraints may limit the laboratory work performed. In all other patients a CBC and fibrinogen level determination should be performed. Additional laboratory work (serum chemistries etc.) should be based on patient status.
Site Selection and Facility Requirements A flat, even surface that provides good footing should be selected. The surface should be soft to reduce risks of injury during induction and recovery. In the field setting, an open grassy area is generally ideal. In the hospital setting, an area with a resilient floor such as hoof matting should be used, if
available. A stall deeply bedded with shavings may be used, although the confined space may interfere with the procedure and increase risk to personnel when larger camelids are involved. The site selected should be free of hazards in all directions for a reasonable distance. Larger camelids are more difficult to control physically and require a somewhat larger “safety zone.” Camelids tend to be patient during recovery from anesthesia. They typically do not attempt to stand until they are awake and functional. Good footing is the primary requirement for achieving a good recovery. Proximity to water, electricity, and vehicle (containing emergency supplies) are also factors to consider when selecting a field site. A calm environment reduces patient anxiety and is always desirable. A small animal surgery table may be used for alpacas and smaller llamas. A foal or small ruminant surgery table or a heavy-duty gurney will be required for larger llamas. A foam pad should be used to cushion the table surface. A shallow trough shaped pad (4-inch base thickness) makes dorsal recumbency easier to maintain and, in many cases, may be used for patients in lateral recumbency. A large animal surgery table is required for larger camels. Llamas and alpacas are typically transferred from the floor to the surgery surface and back manually. It is important to ensure that adequate personnel are available for intended transfers. A 1- to 2-ton overhead hoist may be used to move larger camelid patients into a better working position on the floor (preferably on a foam pad) or onto a surgery table. If the ceiling will not support an overhead mounted hoist without extensive modification, a lateral pull using a wall-mounted winch system may be used to reposition larger patients. Multiple mounting points and a portable winch allow patients to be moved to various areas of the operating theater. Two sets of hobbles connected by rings to the hook of the hoist or winch are used to move the patient. Hobbles are traditionally placed on the pastern but may be positioned above the fetlock, if required, to ensure a secure
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grip on the legs. Hobbles may be fashioned by splicing loops at each end of short segments of 1-inch cotton rope. The body of the rope segment is then fed through each loop to create the sliding component of the hobble. The distance between the loops needs to be sufficient to make placing and removing the hobbles easy, but excessive length reduces the height to which the patient can be lifted using an overhead hoist. Canvas strap hobbles (Shanks Veterinary Equipment, Inc., Milledgeville, IL) are available commercially. The wider, stiffer, canvas strap hobbles require more attention to attain a snug and secure fit. Good facility design makes managing larger patients safer and requires fewer personnel.1
Patient Positioning Camelids continue to salivate when sedated or anesthetized. The degree of protective laryngeal and eye reflexes retained will depend on the chemical restraint or anesthesia technique used and the drug doses selected. Camelids tend to lie down with heavier levels of sedation. Camelid patients that remain standing or are able to maintain sternal recumbency when chemically restrained typically clear saliva effectively. Patients in lateral or dorsal recumbency should be positioned such that saliva runs out of the mouth rather than pooling back near the larynx. In patients in lateral recumbency, this may be accomplished simply by placing a pad under the head–neck junction such that the opening of the mouth is below the level of the larynx (Figure 44-1). A mild downward tilt is generally all that is needed, as use of an overly large pad may impair venous return from the head, resulting in edema formation. Protecting the airway becomes much more challenging when the patient is placed in dorsal recumbency. Camelid patients placed in dorsal recumbency will typically be under the influence of a potent chemical restraint protocol or anesthetized. The body should be elevated to produce a slightly dependent head position. The neck should be gently twisted so that the head rests laterally on the surgery table or floor (Figure 44-2). A shallow trough shaped pad (4-inch base thickness) provides the proper height and makes dorsal recumbency easier to maintain. This positioning places the
mouth opening below the level of the larynx to facilitate saliva egress. The long neck of camelids makes this fairly easy to accomplish. A small pad placed under the head–neck junction may be needed to achieve the proper degree of head tilt. The head must be supported, rather the left hanging, to prevent excessive tension on vital neck structures and reduce the extent of edema formation. Placing the head in a dependent position increases hydrostatic pressures within the blood vessels of the head, which, over time, results in edema formation of the nasal, pharyngeal, and laryngeal tissues, as well as the eyelids. Minimizing the degree of dependence of the head relative to the body to only what is necessary to provide saliva egress may help to reduce the degree of edema formation. The head and neck of a recumbent patient should be extended to minimize impingement of pharyngeal tissue into the airway in a nonintubated patient and prevent kinking of the endotracheal (ET) tube in an intubated patient. Nasal edema is probably the most commonly recognized complication of anesthesia in camelid patients. Camelids with severe nasal edema are able to mouth-breathe but appear stressed when this occurs. Patients that are chemically or medically obtunded may not be able to effectively compensate. It is important to monitor nonintubated patients under the influence of potent chemical restraint or injectable anesthetic protocols and patients recently extubated for signs of respiratory compromise. One must be prepared to respond with ET intubation, if required. In patients exhibiting mild-to-moderate difficulty breathing during recovery, elevating and gently extending the head and neck in a manner that still facilitates saliva egress helps alleviate edema and maintain a patent oral airway. Airway edema generally resolves relatively quickly once the patient is able to keep its head up. Nasopharyngeal intubation, instillation of decongestant products, or both are techniques that have been used to counter the impact of severe nasal edema. Respiratory compromise resulting from laryngeal edema is less common but may be life threatening. Patients exhibiting severe respiratory distress should be endotracheally intubated. Additional information on patient positioning is provided in the “Oxygen Delivery, Muscle and Nerve Protection” section of this chapter.
Endotracheal Intubation The camelid patient should be intubated when the head cannot be positioned to facilitate salvia egress or the
Figure 44-1 Correct head positioning for camelid patients in lateral recumbency.
Figure 44-2 Correct head positioning for camelid patients in dorsal recumbency.
Chapter 44 • Preprocedure Considerations
Figure 44-3 Proper alignment is required to visualize the larynx.
Figure 44-4 One eighth of an inch aluminum stylet, Bivona Aire-Cuf silicone endotracheal tube, Miller 4 laryngoscope blade, Wisconsin 11-ich laryngoscope blade, laryngoscope handle, and gauze tie.
procedure could result in blood or other material entering the airway. Intubation is also required to deliver inhalant anesthetics (saliva production precludes safe mask delivery of oxygen or inhalants). Alpacas and llamas are intubated by direct visualization, as in dogs and cats. Because of the small mouth opening and deep oral cavity in camelids, proper alignment is important to visualize the larynx (Figure 44-3). This is similar to looking through a long, narrow tube. A laryngoscope with an extralong blade aids visualization of the larynx by allowing greater control of the base of the tongue (Wisconsin 11-inch, 14-inch, and 18-inch laryngoscope blades; Anesthesia Medical Specialties, Beaumont, CA) (Figure 44-4). The ET tube must be long enough to place its inflatable cuff below the level of the larynx in the trachea. Oral cavity depth of larger alpacas and llamas typically requires the use of longer 5- to 14-mm silicone “foal” ET tubes (Bivona Aire-Cuf Standard Silicone Endotracheal Tubes with Murphy Eye and Connector, Smiths Medicals). A selection of ET tube diameters should be available to ensure proper fit. Keeping the patient in sternal recumbency with the head elevated reduces the risk of passive regurgitation during intubation. It is important to keep the patient’s mouth pointed downward until the intubation process is imminent to minimize pooling of saliva back around the larynx. An assistant straddles the patient’s back using his or her lower legs to hold the patient in sternal recumbency. The assistant extends the head and neck up toward the individual doing the intubation
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and holds the jaws apart. The assistant’s knees may be used to help control the patient’s head and neck. A reduced level of jaw tone and the absence of the chewing or lingual response to oral manipulation are used to determine when intubation is appropriate. Sliding a large camelid patient down a wall is safer for personnel involved and improves the odds of maintaining sternal recumbency for the intubation process. Additional information on preventing regurgitation is provided in the “Fasting before Anesthesia” section of this chapter. A stylet (one eighth–inch aluminum rod for larger tube sizes and two guttural pouch cannulas glued end-to-end for smaller tube sizes) is used to facilitate intubation of camelids. The thin stylet does not obstruct the view of the larynx. The ends of the stylet should be smooth or rounded to minimize the risk of damaging the mucosal surfaces of the airway. The stylet is guided into the larynx first and the ET tube passed over it into the airway. The stylet must be long enough that it can be grasped above the ET tube as it is advanced down the length of the stylet and into the trachea. With practice, the ET tube can be positioned on the stylet and held in place, with the hand guiding the stylet into the airway, making the process less cumbersome. In large llamas, the size of the oral cavity may be large enough to allow visualization of the larynx while the ET tube is guided into the airway. The stylet is used to stiffen the ET tube. Allowing the stylet to protrude slightly from the end of the ET tube makes placement in the airway easier. Because of the depth of the oral cavity, the presence of glottal folds, and the size of the ET tube required, larger camels are intubated by manually guiding the tube into the airway, as in adult cattle. The anesthetist carries the ET tube into the mouth with one hand and then uses his or her fingers to guide the tube between the arytenoids as the other hand advances the tube. A speculum is required for this technique. The arm or hand size of the individual performing the intubation is the limiting factor in determining the appropriate patient size for this approach. The oral cavity must be large enough to accommodate the operator’s arm and the ET tube. The lower limit for this technique is generally around 300 to 350 kilograms (kg), unless the operator has an exceptionally small arm and hand. A somewhat undersized ET tube may provide additional room for the operator’s arm in marginally sized patients. The operator should wash the arm thoroughly afterward because camelid saliva may irritate the skin of some individuals. A flexible endoscope may be used to guide the ET tube into the larynx. The endoscope serves as both a stylet and a visualization device. This technique has proven useful when traditional methods cannot be used. The endoscope must be small enough to fit inside the ET tube. Camelid patients that are actively spitting prior to anesthetic induction seem to be more prone to regurgitation compared with nonspitters. Feed material is often trapped in the mouth following induction, so extra care must be taken to ensure that it does enter the airway. The small diameter of the tube used in traditional medical suction devices may make removing the coarse feed material difficult at times. Often the fastest and most effective method for clearing the material is to scoop it out using the blade of the laryngoscope. Unfortunately, fasting does not seem to eliminate this rather unpleasant behavior.
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Intravenous Catheters Are “Expensive” and Time Consuming: Are They Really Necessary? A catheter is generally not required when using simple chemical restraint techniques. A catheter should be placed prior to anesthesia in camelid patients. The jugular vein is the most commonly used site for IV catheter placement. The thick fiber coat and neck skin of camelids, combined with poor patient cooperation, may make catheterization of the jugular vein challenging, and carotid sticks do occur. A 14-gauge 5.5-inch catheter is used in most camelid patients. An 18-gauge 2-inch catheter is sufficient for crias. The catheter should be secured to the neck with suture, bandage, or both. The catheter should always be checked before anesthetic induction to ensure that it is still functional. The veins on the external surface of the ear of some camelids are relatively large and accessible. Ear veins can be used to deliver small-volume IV chemical restraint cocktails. Most patients require only modest head restraint when a 25-gauge needle and good technique are used. Ear veins may be used as an alternative site for venous catheter placement. An 18gauge, 20-gauge, or 22-gauge catheter should be used, depending on the vessel size, and secured with a combination of super glue and tape. Because of the increased level of stimulation produced by the catheter stylet, camelid patients are typically less than cooperative to ear catheterization unless chemically or medically obtunded. The cephalic vein may also be used as a site for venous catheter placement, much as it is in small animal patients. The smaller catheters will not provide the flow rate of the 14-gauge catheter but work well otherwise. Camelid owners are frequently concerned about the clipping of fiber, which makes venous catheterization somewhat more challenging. One approach practitioners may find useful is to administer a Ketamine Stun via the ear vein and then place an IV catheter once the patient is recumbent and cooperative. Double Drip or Ruminant Triple Drip may then be delivered to maintain injectable anesthesia or facilitate ET intubation for inhalation maintenance.
Fasting before Anesthesia Fasting has been traditionally recommended for patients undergoing anesthesia. In many equine practices, patients are no longer fasted prior to anesthesia with no adverse consequences and perhaps some real benefits, but some clinicians still believe that it is necessary. Experience with nonfasted emergency cases has shown proper technique to be the most important factor in reducing the risk of regurgitation during induction and intubation in most species. Attempting to intubate a patient with some degree of gag reflex is more likely to result in active regurgitation. Proper induction technique eliminates the gag reflex. Keeping the patient in sternal recumbency with the head elevated reduces the risk of passive regurgitation during intubation. True ruminants tend to bloat in lateral recumbency or when anesthetized, and fasting seems to reduce this problem. Camelids typically do not bloat in lateral recumbency or during anesthesia, but as in true ruminants, passive regurgitation during anesthesia does occur. Camelid patients generally do not need to be fasted for chemical restraint techniques unless a switch to injectable anesthesia is likely to be required to complete the procedure.
Although many practitioners do not fast camelid patients prior to injectable anesthesia, it is currently recommended for patients over 4 months of age. Fasting reduces the gastrointestinal (GI) system volume, which, in turn, reduces pressure on the diaphragm and the potential for passive regurgitation during anesthesia. Proper patient positioning during injectable anesthesia is critical to reduce the risk of aspiration should passive regurgitation occur. Withholding food for 12 to 18 hours (access to water is permitted) has proved effective in minimizing problems during intubation and anesthesia while not producing the adverse effects on GI motility and acid–base status associated with longer periods of fasting. Young animals have minimal energy stores. The risk of hypoglycemia in these patients increases with the duration of anesthesia. “Nursing” patients are typically anesthetized without fasting to reduce the risk of hypoglycemia. Camelid patients younger than 2 months of age should be supplemented with dextrose during anesthesia. Adding 1.25% to 2.5% dextrose to the IV electrolyte solution (10 milligrams per kilogram per hour [mg/kg/hr] for the first hour and 5 milliliters per kilogram per hour [mL/kg/hr] for the balance of anesthesia) is generally sufficient to ensure that blood glucose levels are adequately maintained in these patients. Generally, by 2 months of age, most healthy camelid patients no longer require dextrose supplementation during short anesthetic procedures. Adding 1.25% dextrose to the IV fluids is inexpensive insurance against hypoglycemia in camelid patients 2 to 4 months of age when a longer period of anesthesia is anticipated. Elevated body temperature increases metabolism and the risk of hypoglycemia in younger patients. Patients up to 4 months of age should be supplemented with dextrose when body temperature is significantly elevated. Unless testing is performed during anesthesia, hypoglycemia is typically not recognized until the recovery period. Hypoglycemia produces a stuporous state, and patients typically stall partway through the recovery process. Administration of dextrose has resulted in full recovery with no apparent adverse effects when this has occurred. As with most things medical, prevention is better than treatment. Protracted struggling during physical restraint of unruly young patients may produce substantial elevation of body temperature. Unruly patients should be sedated rather than battled.
Eye Protection When camelid patients are placed in lateral recumbency, care should be taken to ensure that the lids of the down eye are closed. A towel or thin pad may be placed under the down eye to provide further protection. Ophthalmic ointment (bland or antibiotic) should be placed in the eyes to protect them during anesthesia. If the down eye ends up getting bathed in saliva or regurgitation, it should be rinsed out during recovery.
Oxygen Delivery and Muscle and Nerve Protection Information in this section is adapted from the Proceedings of the 2007 British Equine Veterinary Association Congress.2,3 Maintaining adequate delivery of oxygen to tissues is one of the most important goals of the anesthetist. Global oxygen delivery is determined by the combination of cardiac output
Chapter 44 • Preprocedure Considerations and arterial oxygen content. Many sedative and anesthetic agents depress cardiac output. Arterial oxygen content (CaO2) is determined primarily by the level of hemoglobin oxygen saturation (SaO2), which is based on the partial pressure of oxygen in arterial blood (PaO2) and the concentration and affinity of hemoglobin. PaO2 should be approximately five times the inspired level (e.g., PaO2 should be 100 mm Hg on 20% O2 and 500 mm Hg when delivering “100%” O2). Many factors can adversely impact PaO2 during anesthesia. GI mass, bloating, or both exert pressure on the diaphragm, reducing the tidal volume of the patient. Hypoventilation may also be caused by excessive anesthetic depth. Hypoventilation reduces the alveolar oxygen levels (PAO2) and the diffusion pressure it exerts on pulmonary gas exchange. Gravitational effects in large patients redistribute pulmonary circulation to the more dependent regions of the lungs, while ventilation is directed to the more compliant nondependent regions of the lungs mismatch. Diffusion resulting in ventilation perfusion ( V/Q) impairment, typically resulting from pulmonary edema, is a less common cause of decreased PaO2. The partial pressure of carbon dioxide in arterial blood (PaCO2) is used to evaluate the level of respiratory depression. The level of respiratory depression present during anesthesia varies with the technique used and the depth of anesthesia produced. Llamas and alpacas generally exhibit little respiratory depression (PaCO2 40–45 mm Hg) at surgical planes of inhalation anesthesia. Bolus injectable anesthetic techniques typically produce an initial degree of respiratory depression (PaCO2 > 45 mm Hg) that diminishes over time as the plane of anesthesia lessons. Clinically significant respiratory depression (PaCO2 > 60 mm Hg) has been documented during bolus injectable anesthetic techniques in camelid patients. Bolus injection techniques that produce longer periods of anesthesia are more likely to cause significant initial respiratory depression. Even when the PaCO2 remains in the 40- to 45-mm Hg range, clinically significant hypoxemia (SaO2 < 90% and PaO2 < 60 mm Hg) may occur during injectable anesthesia. A pulse oximeter may be used to monitor SaO2 during anesthesia. Hemoglobin (Hb) affinity for oxygen is reportedly higher in alpacas than in most other mammals.4,5 Because of its critical influence on CaO2, SaO2 may be a more valuable parameter to monitor than PaO2 in camelid patients. The fractional inspired oxygen (FIO2) level is 0.2 when patients are breathing room air. Increasing the FIO2 with oxygen insufflation may provide dramatic improvement in PaO2, SaO2, and CaO2 during injectable anesthesia. Perhaps because of their high altitude origins, alpacas and llamas typically exhibit excellent pulmonary gas exchange. PaO2 values are generally 500+ mm Hg during inhalation anesthesia. The ability to correct hypoxemia produced by impaired gas exchange in recumbent or anesthetized large animals receiving “100% O2” (FIO2 approximately 1) is extremely limited. Methods such as intermittent positive pressure ventilation (IPPV) or positive end-expiratory pressure (PEEP) produce occasional improvement in PaO2 but also create adverse mechanical (decreased venous return) and chemical (decreased PaCO2) effects on cardiac output.6,7 Decreases in cardiac output typically exceed any improvements in CaO2 and delivery of oxygen to tissues (DO2) is reduced rather than improved. Because of the influence of the oxygen–hemoglobin dissociation curve, SaO2 and CaO2 are generally maintained at
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levels that are adequate when other aspects of oxygen delivery are properly managed in patients with reduced PaO2 levels. Anemic patients are the exception because they have a reduced oxygen-carrying capacity. Cardiac output (tissue flow) is the most important variable involved in tissue oxygenation. Tissues are able to cope with reductions in arterial oxygen content better than they cope with reductions in blood flow. When CaO2 is reduced, tissue oxygen tension falls, and locally controlled vasodilation occurs. The resulting increase in tissue blood flow improves tissue oxygen delivery. When tissue blood flow is reduced, oxygen extraction from arterial blood is increased, but this mechanism is more limited in its ability to compensate. Arterial blood pressure is monitored as a surrogate of cardiac output. Arterial blood pressure generally provides a reasonable estimation of cardiac output status, but large changes in cardiac output may occur without concomitant changes in arterial blood pressure. In anesthetized horses, cardiac output has been shown to decrease by 30% to 40% when IPPV is instituted.6,7 Increased peripheral resistance maintains arterial blood pressure near preventilation levels, masking the serious decrease in cardiac output. Until clinically useful methods are developed to provide real-time cardiac output values, monitoring arterial blood pressure remains the best defense against poor tissue oxygen delivery during anesthesia. Localized obstruction of blood flow is the typical cause of postanesthetic nerve or muscle complications in large animal anesthesia. Proper positioning of larger patients is important to minimize compressive forces that may result in localized obstruction of blood flow. Placing patients on a thick foam pad during anesthesia distributes the pressure of their weight more evenly, reducing the risk of localized obstruction of blood flow to dependent muscles. For patients in lateral recumbency, the down front leg should be pulled forward to reduce pressure on the radial nerve. The upper front leg should be propped up parallel to the floor or the table surface to minimize the compressive force of its considerable mass. The upper rear leg should be supported in a similar manner. In situations where local tissue blood flow is partially obstructed, reduced arterial oxygen content may contribute to development of postanesthetic neuropathy or myopathy.
Why Is Premedication Important? Veterinarians have been traditionally taught that food animal and camelid patients do not generally require premedication before anesthesia. Actually, most food animal and camelid anesthesia cases involve the use of premedication but not in the manner traditionally used in other veterinary species. Premedication is used to enhance patient control and modify the response to the induction bolus. Premedication may intensify or extend the effects of the induction bolus. Premedication may also minimize the adverse effects of an induction drug. Apprehension and activity alter the distribution of cardiac output, directing a greater portion of blood flow to skeletal muscles. Although many food animal and camelid patients appear calm before anesthetic induction, some degree of apprehension likely exists. Extremely anxious or unruly food animal and camelid patients experience a greater alteration in the distribution of cardiac output. Sedatives such as xylazine or guaifenesin produce a dose-dependent calming effect.
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Reducing the patient’s anxiety and activity directs a greater portion of the cardiac output to the vital organs (centralization of cardiac output). The degree of sedation determines the level of centralization achieved. Centralization of cardiac output does not occur instantaneously. It lags behind peak sedation by a few minutes in calmer patients and longer in extremely anxious or unruly patients. Centralization of cardiac output is desirable because it directs a greater portion of the IV anesthetic induction agent to the target sites in the central nervous system (CNS). When lipid-soluble drugs such as anesthetics induction agents are administered as an IV bolus, it is redistribution from the vital organs to skeletal muscle via continued circulation that ends the clinical effects of the drug. Any increase in the amount of drug sent directly to muscle will decrease the impact of the induction bolus (peak effect and duration). Anesthesia in food animal and camelid patients is typically induced by using a combination of a sedative and ketamine. In small ruminants and camelids, Double Drip (5% guaifenesin with 1 mg/mL of ketamine added) is infused to effect. Anesthesia in small ruminants and camelids may also be induced with a combination of ketamine and diazepam (KetVal). In large ruminants and camels, Double Drip is slowly infused until the first signs of muscle weakness are apparent, and then a bolus of Ket-Val is administered. Administering the sedative (guaifenesin or diazepam) along with the anesthetic induction agent (ketamine) does not provide the benefits achieved with the sedative-first approach used in most other species. Double Drip administration produces obvious signs of sedation before actual anesthetic induction, but the short duration does not allow total centralization to occur. When Ket-Val is used, the onset of diazepam’s sedative effect is quick enough to prevent the neuroexcitatory effects produced by the large dose of ketamine, but little centralization of cardiac output occurs. These induction techniques are generally effective in ruminants and camelids large part because of the calmer demeanor of these patients, but that does not mean that these techniques cannot be improved. Unless contraindicated, anxious or unruly patients should be sedated with xylazine 5 to 10 minutes before the anesthetic induction sequence. The dose of xylazine required will depend on the demeanor of the patient. It could be argued that most healthy ruminant and camelid patients might benefit from a small dose of xylazine 5 to 10 minutes before anesthetic induction because even the calmest of patients will experience some anxiety from the events surrounding anesthetic induction.
Important Information on Selected Drugs Used in Chemical Restraint and Anesthesia α2-Adrenergic Agonists Of the α2-adrenergic agonists available to the veterinary practitioner, xylazine is generally used for sedation of ruminant and camelid patients. Newer, more potent α2-adrenergic agonists such as detomidine, romifidine, and medetomidine are substantially more expensive, and their longer duration of action is often not required. Supplemental doses of xylazine
may be administered to extend the duration of the effect, when required. The initial demeanor of the patient greatly influences the sedation produced by a given dose of an α2-adrenergic agonist. The sedative effect of α2-adrenergic agonists may be overridden by elevated “sympathetic tone.” This results in two characteristic features of α2-adrenergic agonist sedation. The sedative effect produced by lower doses of α2-adrenergic agonists is not as stable. Calm, quiet patients require smaller doses, whereas anxious or unruly patients require larger doses. The ideal dose may be difficult to predict, especially when recumbency is not desired. Experience makes the necessary adjustments easier, but even seasoned practitioners are sometimes surprised. α2-adrenergic agonists may be administered intravenously, intramuscularly, or subcutaneously to produce a dosedependent degree of sedation, muscle relaxation, and analgesia. IV administration of α2-adrenergic agonists provides a faster onset and more intense level of chemical restraint and analgesia. The fairly rapid onset time may be used to advantage, allowing multiple smaller IV doses of an α2-adrenergic agonist to be administered to titrate the effect to the desired level. Intramuscular (IM) administration results in a more gradual onset and provides a longer duration of less intense chemical restraint and analgesia. IM administration is often used when patient cooperation does not allow IV administration or when extended duration is desired. The IM dose is traditionally twice the IV dose one would select for the patient based on the desired level of effect and the patient’s initial demeanor. Subcutaneous (SQ) administration results in the most gradual onset, longest duration, and mildest peak effect. α2-adrenergic agonists possess potent sedative and analgesic properties. These desirable dose-dependent effects are accompanied by a myriad of dose-dependent side effects.8 The cardiorespiratory and GI effects are the most important, although the dose-dependent muscle relaxation produced by α2-adrenergic agonists likely contributes to the incidence of unwanted recumbency. Cardiovascular side effects associated with IV xylazine include decreases in heart rate and cardiac output that may reach 25% when larger doses are administered. IV xylazine produces a biphasic change in blood pressure (initial increase in blood pressure produced by peripheral vasoconstriction, followed by a gradual decrease to below base-line values because of reductions in sympathetic nervous system tone and eventual return to baseline values as the effects of the xylazine administered resolve). Xylazine administered intramuscularly does not produce vasoconstriction, and arterial blood pressure decreases as sedation intensifies and then gradually returns to baseline values as sedation wanes. The cardiovascular effects of the newer, more potent α2-adrenergic agonists such as detomidine are even more potent, producing larger decreases in heart rate and cardiac output. In contrast to xylazine, IV administration of detomidine results in prolonged elevation of blood pressure. Persistent peripheral vasoconstriction in the face of decreased cardiac output does not promote good tissue blood flow. For this reason, I feel that the newer more potent α2-adrenergic agonists should not be routinely used as anesthetic premedications. The cardiovascular changes produced by xylazine are generally well tolerated in the normal healthy patients but may be life threatening in hypovolemic or endotoxic patients.
Chapter 44 • Preprocedure Considerations The sympatholytic effects of α2-adrenergic agonists may exacerbate bradyarrhythmias. α2-adrenergic agonist administration in hyperkalemic patients has resulted in complete heart block. The degree of respiratory depression produced by α2adrenergic agonists is dependent on the drug used and the dose administered. Mild to moderate levels of xylazine sedation produce inconsequential respiratory effects. Respiratory rate typically decreases, whereas tidal volume tends to increase, resulting in only minor alterations in arterial blood gas values. Heavy xylazine sedation may produce clinically significant respiratory depression. Detomidine seems to produce a greater level of respiratory depression, and extremely large doses have resulted in respiratory compromise in horses. The respiratory effects of α2-adrenergic agonists are generally well tolerated in normal, healthy camelid patients. α2-adrenergic agonists produce dose-dependent decreases in GI motility. Xylazine has been reported to increase uterine tone in late gestation.9 α2-adrenergic agonists should be avoided or used with extreme caution in patients with significant cardiorespiratory compromise. Butorphanol, a benzodiazepine, or a slow infusion of guaifenesin may be used to produce sedation and centralization in patients that may not tolerate the adverse cardiorespiratory effects of xylazine. In situations where an α2-adrenergic agonist must be administered to a compromised patient, titrated administration should be used, whenever possible, to reduce risk. Reversal of the α2-adrenergic agonist should be done at the earliest point possible. Titrated reversal once the patient is anesthetized removes the adverse effects of the α2-adrenergic agonist while minimizing the risk of producing an excessively light plane of anesthesia.
α2-Adrenergic Agonist Reversal α2-adrenergic antagonists may be used to reverse the effects of α2-adrenergic agonists at the end of a procedure to facilitate a quicker recovery and minimize the risks of GI complications. IM administration of the reversal agent is preferred in most nonemergency situations because it decreases the risk of CNS excitement or cardiovascular complications. The shorter duration of action of reversal agents when administered intravenously may result in resedation in patients if the α2-adrenergic agonist was administered intramuscularly, especially when larger doses were used. Patient arousal occurs when redistribution reduces blood levels of ketamine or tiletamine to a critical level, although patients are typically not ready to stand at this point. The residual sedative effect of the α2-adrenergic agonist administered is critical in preventing attempts to stand until blood levels of these IV anesthetic agents have decreased sufficiently to ensure a coordinated effort. To reduce the risk of a rough recovery, α2-adrenergic agonist reversal agents should ideally not be administered until the patient is in the sternal position and able to lift its head off the floor. In situations where the patient presents a physical danger to personnel, the reversal agent may be administered when arousal is imminent. The amount of reversal agent used depends on the dose of the α2-adrenergic agonist and duration since administration. The commonly recommended emergency IV doses for yohimbine (0.125 mg/kg) and tolazoline (2 mg/kg) are typically used as the maximum IM dose and reduced to fit the
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circumstances. Atipamezole (20–60 micrograms per kilogram [mcg/kg] IV) and idazoxan (0.05 mg/kg IV) have been used to reverse the effects of α2-adrenergic agonists in ruminants. Doxapram, an analeptic, has produced arousal of patients sedated with α2-adrenergic agonists. When dosed properly, the effects of reversal should start to become evident approximately 8 to 10 minutes following IM administration. Splitting the reversal dose (using both IV and IM routes) may produce quicker recovery while minimizing the risk of resedation. Response to the small IV component will be fairly quick, so the primary IM component should be administered first. Anecdotal reports of deaths associated with tolazoline reversal have created concern regarding the safety of this drug. A case of suspected tolazoline toxicosis in a llama has been reported.10 The patient exhibited generalized weakness, seizures, dyspnea, severe hypotension, tachycardia, and diarrhea after receiving tolazoline (4.3 mg/kg IV followed 45 minutes later by 1 mg/kg IV and 1 mg/kg IM). Large IV doses and rapid IV administration should both be avoided when using tolazoline.
Benzodiazepines Benzodiazepines (diazepam, midazolam) are centrally acting muscle relaxants with sedative effects. Benzodiazepines do not generally produce a beneficial calming effect when used alone in most species, but ruminants and camelids respond favorably. Benzodiazepines produce minimal cardiovascular or GI effects at clinical doses and provide viable options for producing mild sedation in compromised patients. Benzodiazepines may also be combined with other sedatives, and this allows drugs with adverse effects to be used in smaller doses. Diazepam may be administered intravenously or intramuscularly, but IM absorption is unpredictable. Midazolam is water soluble and may be administered intravenously, intramuscularly, or subcutaneously. Midazolam is slightly more potent than diazepam, but the drugs are generally used interchangeably. Now that midazolam is available as a generic formulation, the price differential is negligible.
Butorphanol Butorphanol, an opioid agonist–antagonist, is an analgesic drug with sedative effects. Butorphanol does not generally produce a beneficial calming effect when used alone in most species, but ruminants and camelids respond favorably. Butorphanol produces minimal adverse cardiovascular effects at clinical doses and provides a viable option for producing mild to moderate sedation in compromised patients. Butorphanol may also be combined with other sedatives, which allows drugs with adverse effects to be used in smaller doses. Butorphanol may be administered intravenously, intramuscularly, or subcutaneously.
Guaifenesin Guaifenesin is a centrally acting muscle relaxant with sedative effects. Guaifenesin produces minimal cardiorespiratory effects at clinical doses. Premixed guaifenesin solution (5%) is currently only available from compounding pharmacies. Guaifenesin concentrations of 10% have been shown to cause
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hemolysis in ruminant patients. When mixing guaifenesin from powder stock, solutions of 5% should be prepared. Guaifenesin-based solutions (Double Drip, Ruminant Triple Drip) are used for anesthetic induction and maintenance of injectable anesthesia in food animal and camelid patients. Guaifenesin may also be used to produce mild to moderate preanesthetic sedation in compromised patients that might not tolerate the adverse cardiovascular effects of xylazine. The induction bolus should be drawn up before administration of the solution. Guaifenesin is infused to effect and has an onset time of approximately 1 minute. Observable signs of sedation are used to guide delivery. A slow rate of infusion should be used to allow the clinician to account for the delayed onset when assessing progress and adjusting delivery. As sedation builds the rate of infusion should be decreased to provide time for centralization to occur. Llamas and alpacas generally go down into sternal recumbency when moderately sedated, which allows more latitude when using guaifenesin in this manner. Large ruminants and camels are generally induced while standing. The standing sedation dose of guaifenesin (15–25 mg/kg IV) must be delivered carefully to avoid producing excessive muscle relaxation in these patients. The corrective postural activity that it triggers antagonizes the centralizing effects of the sedation. If excessive muscle relaxation begins to develop, the induction bolus should be administered immediately. Guaifenesin sedation works better in calmer patients and should not be expected to produce significant centralization in extremely anxious or unruly patients. When using Double Drip for anesthetic induction of small ruminant and camelid patients, slowing the rate of infusion once sternal recumbency has been achieved extends the period of sedation to a certain extent, allowing a greater degree of centralization to occur before the point of anesthetic induction is reached.
maintenance anesthesia. Thiopental (1.1 mg/kg IV) is also useful in dulling the protective swallowing reflex present during ketamine-based injectable anesthesia. Tiletamine, a more potent and longer-lasting relative of ketamine, is available only in combination with the benzodiazepine zolazepam under the brand name Telazol. Because of the high cost of Telazol, it is primarily used in large animal practice for “capturing” intractable patients. An apneustic pattern of breathing is often observed in horses following induction with ketamine or Telazol but is not common in ruminant and camelid patients. Both ketamine and tiletamine draw on the sympathetic nervous system reserve to augment cardiac output and blood pressure. This effect helps counter their direct negative inotropic and vasodilatory effects, as well as the negative cardiovascular effects produced by xylazine. Cardiovascular function in normal healthy patients anesthetized with ketamine-based protocols is good to excellent. I cannot emphasize enough the need for caution in administering these seemingly safe drugs to compromised patients in which the sympathetic nervous system reserve may be severely limited. In compromised equine colic patients, I use a reduced dose of ketamine (1.3– 1.75 mg/kg IV, depending on patient status) to produce recumbency. Induction is generally slower when smaller doses of ketamine are used. The cardiovascular status is reevaluated once the patient is recumbent and support measures (dobutamine infusion) instituted, as required, in the induction area. Camelid patients induced in this manner will be at a fairly light, plane of anesthesia and the gag reflex may be present. Additional ketamine administration may be required to minimize the risk of regurgitation during intubation. Using a somewhat undersized ET tube may make the intubation process easier.
Atropine
REFERENCES
Premedication with atropine was commonly recommended in the early days of ruminant anesthesia to counter the profuse salivation in these patients. Atropine administration does not eliminate salivation in ruminant or camelid patients during anesthesia. Atropine tends to reduce the aqueous component of saliva, making it more difficult to clear from the mouth and the oropharynx. Atropine also reduces GI motility. Routine premedication with atropine is unnecessary and increases patient risk. Atropine (0.01–0.02 mg/kg IV) may be used to treat bradycardia or bradyarrhythmias.
1. Abrahamsen EJ: Inhalation anesthesia in ruminants. In Anderson DE, Rings M, editors: Current veterinary therapy food animal practice, ed 5, St. Louis, MO, 2009, Saunders (pp 559-569). 2. Abrahamsen EJ: Dr. Strangeblood Part I—how I stopped worrying and learned to love CO2. In Proceeding of the 2007 British Equine Veterinary Association (BEVA) Congress, Edinburgh, Scotland, 2007. 3. Abrahamsen EJ: Dr. Strangeblood Part II—how I learned to stop worrying about PaO2. In Proceedings of the 2007 British Equine Veterinary Association (BEVA) Congress, Edinburgh, Scotland, 2007. 4. Reynafarje C, Faura J, Villanvicencio D, et al: Oxygen transport of hemoglobin in high-altitude animals (Camelidae), J Appl Physiol 38:806-810, 1975. 5. Sillau AH, Cueva S, Valensuela A, et al: CO2 transport in the alpaca (Lama pacos) at sea level and at 3300 m, Respir Physiol 27:147-155, 1976. 6. Steffey EP, Howland D: Cardiovascular effects of halothane in the horse, J Am Vet Med Assoc 39:611-615, 1978. 7. Hodgson DS, Steffey EP, Grandy JL, et al: Effects of spontaneous, assisted and controlled ventilatory modes in halothane anesthetized geldings, Am J Vet Res 47:992-996, 1986. 8. Campbell KB, Klavano PA, Richardson P, et al: Hemodynamic effects of xylazine in the calf, Am J Vet Res 40:1777-1780, 1979. 9. LeBlanc MM, Hubbell JEA, Smith HC: The effects of xylazine hydrochloride on intrauterine pressure in the cow and mare, Theriogenology 21(5):681-690, 1984. 10. Read MR, Duke T, Toews AR: Suspected tolazoline toxicosis in a llama, J Am Vet Med Assoc 216:227-229, 2000.
Ketamine, Telazol, and Thiopental Ketamine, tiletamine, and the ultra-short barbiturate thiopental are the injectable anesthetic agents used in large animal practice. Ketamine is, by far, the most common injectable anesthetic agent used in large animal practice. Subanesthetic doses of ketamine are also used in chemical restraint techniques. Thiopental is no longer generally used in large animal practice for induction or maintenance of anesthesia but remains the fastest option for restoring an anesthetized state when larger patients get too light during inhalation