Techniques of Inhalation Anesthesia in Ruminants and Swine

Anesthesia

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Techniques of Inhalation Anesthesia in Ruminants and Swine William]. Tranquilli, D.V.M., M.S.*

Safe inhalation anesthesia requires knowledge of (1) the pharmacology of anesthetics, (2) appropriate induction and maintenance techniques, and (3) anesthetic delivery systems and machines. A variety of inhalation agents are available for use in food animals, halothane being the most popular. In contrast to injectable anesthesia, inhalation anesthesia requires that special attention be given to selection and maintenance of appropriate anesthetic delivery systems and machines. Induction of anesthesia can be achieved with a variety of injectable agents or via mask induction with the inhalation agent itself. A knowledge of each species' laryngeal anatomy and physiologic responses to intubation is essential for safe, efficient airway management. Relatively long periods of inhalation anesthesia can be achieved economically with either a to-and-fro or circle delivery system. A variety of anesthetic machines and vaporizers are commercially available for administration of anesthetic vapors and gases to cattle, sheep, goats, and swine.

GENERAL CONSIDERATIONS Ruminants General anesthesia of large ruminants is necessarily accompanied by periods of recumbency. Clinical observations suggest that dorsal and lateral recumbency are more detrimental to gas exchange than is sternal recumbency. Long periods of abnormal position lead to progressive pulmonary ventilation-perfusion mismatching and eventual right to left shunting of blood through the lungs. A more detailed explanation of this phenomenon is provided elsewhere in this sym-

* Diplomate,

American College of Veterinary Anesthesiologists; Associate Professor, Department of Veterinary Clinical Medicine, University of Illinois College of Veterinary Medicine, Urbana, Illinois

Veterinary Clinics of North America: Food Animal Practice-Vol. 2, No.3, November 1986

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posium. Hypercapnia and a degree of hypoxia may be observed. Diaphragmatic excursion is inhibited by the weight of abdominal organs and their contents, and the muscles of respiration are depressed by the actions of anesthetic agents. Venous return may be reduced by compression of large veins by abdominal organs or a gravid uterus, causing significant reductions in cardiac output. Large quantities of gas are produced in nonfasted ruminants, exacerbating poor diaphragmatic excursion and decreasing functional residual capacity. Excessive salivation or regurgitation of abdominal contents occurs in ruminants and can cause airway obstruction. Active or explosive regurgitation during light anesthesia can result in pulmonary aspiration of ruminal contents. An appreciation of these cardiopulmonary sequelae associated with general anesthesia of ruminants is essential for the planning and completion of a safe anesthetic period. Despite these potential complications, most ruminants can be safely restrained for induction and maintenance of general inhalation anesthesia.

Swine Unlike most ruminants, swine resist minimal restraint when initially handled. In spite of their temperament, most pigs can be trained to accept some forms of manual restraint. Swine respond to reward feeding and enjoy human attention. Pigs have few superficial veins and arteries suitable for cannulation and intravenous drug administration. Considerable variation in accessibility of these vessels exists among swine breeds. In this regard, inhalation anesthesia may be

Figure 1. Mask induction of anesthesia using 4% halothane in oxygen. The mask is held firmly over the snout, making sure not to obstruct the nasal openings.

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preferable to injectable intravenous techniques. The pig's anatomic shape (that is, short neck and legs) makes mechanical or physical restraint very difficult. The long nose can be snared, but this is extremely aversive and should be avoided before induction of anesthesia to minimize stress. A webbed stanchion can be easily constructed, which provides for humane, safe restraint of nearly any size pig (Fig. 1).38 Inhalation anesthesia of swine has not been associated with the severe cardiopulmonary derangements observed in extremely large ruminants. However, anesthesia of large swine is not common, and little comparative data are available from extremely large sows or boars. Halothane and succinylcholine can trigger malignant hyperthermia in susceptible individuals. Malignant hyperthermia has been observed in nearly all swine breeds but is most prevalent in the Spotted swine, Poland China, Pietrain, Large White, and Landrace breeds. In my experience, the Duroc breed appears most resistant.

INHALATION ANESTHESIA The achievement of general anesthesia with volatile and gaseous anesthetic agents depends on the development of a sufficient alveolar anesthetic partial pressure, which is opposed by the uptake of gases from the lung and distribution into body tissues? To induce a sufficient partial pressure of anesthetic gas, proper anesthetic delivery to the patient must occur. Two factors that influence the rise in alveolar partial pressure are inspired concentration and alveolar ventilation. 7 Increased ventilation can rapidly increase anesthetic alveolar partial pressure. Thus, high delivered (inspired) anesthetic concentrations and increased minute ventilation increase alveolar partial pressure of anesthetic gas and shorten the induction period. By removing anesthetic from the alveoli, the uptake of anesthetic gas into body tissues retards the rate of rise of alveolar partial pressure, prolonging induction of anesthesia. Factors that influence the uptake of an inhalation anesthetic are the anesthetic agent's tissue solubility (Table 1), cardiac

Table 1.

Selected Physical Properties of Inhalation Anesthetics SOLUBILITY (PARTITION) COEFFICIENTS

AGENT Halothane Methoxyflurane

Blood/Gas Oil/Gas Rubber/Gas

VAPOR VAPOR PRESSURE CONCENTRATION (% AT 20°C) (MM HG AT 20°C)

2.3

224.0

120.0

243

32

13.0

970.0

630.0

23

3

Enflurane

1.9

98.0

74.0

180

24

Isoflurane

1.4

98.0

62.0

250

33

Nitrous oxide

0.47

1.4

1.2

100

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Table 2. Potency of Five Commonly Used Inhalation Anesthetic Agents Expressed as Alveolar End-Tidal Per Cent (1 MAC*) for Common Domestic Ruminants and Swine AGEKT

CALF

GOAT

SHEEP

PIG

Nitrous oxide

Halothane

6.76 ± 0.03 32

Methoxyflurane

1.09 0.97 0.69 0.73

0.03 29 0.0424 ± __ 11 ± 0.07t24

± ±

0.26 ± 0.0224 0.18 ± 0.Olt 24 1.66 ± 0.0421

Enflurane Isoflurane

0.91 ± 0.0342 1.25 ± 0.0445

1.58 ± 0.1724 1.01 ± 0.06t24

1.55 ± 0.08 8 1.75 ± 0.0141 1.45 ± 0.05 19

* MAC is the minimal alveolar anesthetic concentration (%) that prevents gross purposeful movement to a 60 second application of a painful stimulus in 50 per cent of the animals tested. t Pregnant ewe.

output, and anesthetic alveolar to venous partial pressure difference. 7 If any of these factors is or becomes minimal, uptake is reduced, whereas if any factor is or becomes maximal, uptake is increased. Reduction in anesthetic uptake by the tissues speeds the increase in alveolar partial pressure and thus anesthetic depression, while increased uptake results in the opposite action and prolongs induction. It is reasonable that organs in the vessel-rich group (for example, brain) with high tissue blood flow are the first organs to equilibrate with the alveolar anesthetic partial pressure. Therefore, following equilibration, the minimal alveolar concentration (MAC) or partial pressure (alveolar per cent x 760 mm Hg at sea level) of anesthetic gas can be used as a measure of anesthetic potency? MAC is defined as the minimal alveolar or anesthetic concentration necessary to prevent gross purposeful movement in 50 per cent of the animals given a 60-second application of a noxious stimulus. Inhalation anesthetic agent MAC values that have been determined in ruminants and swine are given in Table 2. The inspired anesthetic concentration necessary for surgical intervention in swine and ruminants can range from 1.5 to 2.5 MAC. Inhalation Agents Nitrous oxide (N 2 0) is a colorless, odorless, tasteless, nonflammable, nonexplosive, inert gas. Its anesthetic potency is minimal compared with that of the volatile anesthetics (Table 2). Nitrous oxide

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induces minimal cardiopulmonary depression and may, in fact, induce a net sympathomimetic effect when used alone or in combination with other inhalation agents. For this reason, N 20 is often combined with volatile inhalation agents to reduce volatile anesthetic agent requirement and cardiopulmonary depression. The use of N 20 is commonplace in human and small animal patients. Although it appears that similar benefits of reduced cardiopulmonary depression occur in ruminants, 11, 20 N 20 has not been routinely advocated. 3, 42 Reasons for this are diverse. Progressive hypoxia is often difficult to avoid in large recumbent anesthetized ruminants. Arterial oxygen tensions may fall below awake values even when the inspired gases are 90 to 95 per cent oxygen. To achieve a significant anesthetic action with N 20, a high delivered (inspired) concentration (50 to 70 per cent) is required, reducing inspired oxygen concentrations. Thus, this likelihood of progressive hypoxemia greatly limits the use of N 20 in large ruminants. Alterations in ventilation and perfusion are not as severe in recumbent sheep, as Pa02 values have been shown to remain relatively high during 120 minutes of halothane anesthesia. 10 Also, rapid entry ofN 20 into air-filled cavities or viscera can cause distention of abdominal organs. 42 Increased abdominal pressure caused by distention of the large fore stomachs reduces diaphragm function and further enhances anesthetic-induced hypoventilation. Because of its concentrating actions on other gases within the alveoli, N 20 is used to induce the "second gas effect'~ and speed induction with more potent inhalation agents.

Volatile Anesthetics Halothane (Fluothane) is the most commonly used inhalation anesthetic in cattle and swine. It is a clear, colorless, volatile liquid that is stored in amber bottles and stabilized with the addition of 0.01 per cent thymol. It possesses physical properties and a potency (MAC) consistent with rapid induction, alteration of anesthetic depth, and recovery from general anesthesia (Tables 1 and 2). Because of its high vapor pressure, halothane is best administered from a precision va~orizer located out of the rebreathing circuit. To minimize cost of tdministration, gases should be delivered through a closed or semiclosed rebreathing circuit. Halothane is a potent cardiopulmonary depressant. 6 This depression is dose-dependent, causing progressive reductions in systemic arterial pressure, cardiac output, stroke volume, and ventricular work as concentration increases. 32 Heart rate is not as readily depressed as other cardiovascular variables. Halothane sensitization of the myocardium to' catecholamines has been observed but may be reversable in both ruminants and swine with alpha-blocking agents such as acepromazine or prazosin. 28 , 39 Ventilation is depressed by halothane. Development of severe hypercapnia is a common occurrence in halothane-anesthetized, spontaneously breathing large ruminants. 13 In spontaneously breathing lambs, halothane (0.75 MAC) depressed min-

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ute ventilation 34 per cent and increased lung and airway resistance by 59 per cent, while lung compliance was unchanged. 29 Halothane is a well-known trigger for the hyperpyrexia syndrome most commonly encountered in swine and humans. Although malignant hyperthermia is probably dependent on a genetically linked defect in muscle, it is a dramatic exaggeration of halothane's ability to hinder calcium movement within muscle. 22 Ruminants do not appear to be genetically predisposed to this condition. 23 Methoxyflurane (Metofane) can be used in ruminants and swine. It has a low vapor pressure and high blood and tissue solubility (Table 1), allowing safe administration from a vaporizer located either within or out of the rebreathing circuit. Methoxyflurane has the lowest MAC value (that is, is the most potent) of all the commonly used inhalation agents (Table 2). Because rapid induction and recovery as well as rapid alteration in anesthetic depth are important for safe anesthesia in ruminants, methoxyflurane is used infrequently.36 In swine, this disadvantage has been minimized by combining N 20 with methoxyflurane to speed induction and recovery.2 Methoxyflurane provides better analgesia and muscle relaxation than does halothane at comparable depths of anesthesia. 13 It induces dose-dependent cardiopulmonary depression similar to halothane. Methoxyflurane's nephrotoxic action reported in man and laboratory animals has not been documented in ruminants and swine. Enflurane (Ethrane) and isoflurane (Aerane) are two new halogenated ether anesthetics possessing good physical and clinical properties for producing general anesthesia in large animals (Tables 1 and 2). These agents have high vapor pressures, requiring the use of precision vaporizers located out of the rebreathing circuit. Although induction and recovery are rapid and anesthetic depth quickly altered, these agents have not been used extensively or evaluated in ruminants and swine. Both agents possess cardiopulmonary depressant actions. They do not sensitize the myocardium to epinephrine as much as halothane and, thus, are less arrhythmogenic. With increasing concentrations of enflurane, muscle twitching and seizure-like activity may occur. This condition has been observed in swine with inspired enflurane concentrations ranging from 4 to 8 per cent. 38 Isoflurane is an isomer of enflurane but does not cause unusual electromyographic responses with increasing concentrations. Although these agents have desirable qualities, economic factors have precluded the routine use of enflurane and isoflurane as general anesthetics in food-producing animals.

ANESTHETIC TECHNIQUES The administration of inhalation anesthesia in cattle, small ruminants, and swine can be achieved in a variety of ways. No single technique is best in all circumstances. Selection of induction technique and anesthetic protocol depends on the size and special needs

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of the patient, available facilities, and abilities of anesthetist and surgeon. Some techniques that have proven successful are reviewed here. The use of preanesthetic agents to induce sedation or tranquilization in ruminants prior to general anesthesia is not routinely necessary. Rapid recovery is a critical factor in the safe completion of general anesthesia of large animals. Preanesthetics that prolong anesthetic action should be given judiciously. Although atropine is effective in reducing salivation for a short period of time, the routine administration of anticholinergic compounds in ruminants is equivocal. In contrast, sedation or tranquilization of pigs is desirable to reduce the stress of physical restraint before induction of anesthesia. Short-acting barbiturates, butyrophenone neuroleptics (droperidol, azaperone), benzodiazepines (diazepam), and phenothiazines (acepromazine) have been successfully employed. Atropine is routinely given to reduce salivation and prevent bradycardia. With intramuscular administration of any preanesthetic agent, a small-gauge needle should be used, with sufficient length to penetrate skin and fat. This ensures that the agent is deposited in muscle, where uptake by blood is more predictable. If possible, preanesthetic agents should be given in a manner that minimizes stress response by the animal. 36 To allow maximal drug effect, the animal should be left undisturbed in quiet surroundings until induction begins. Induction Procedures Induction of anesthesia can be accomplished with rapid bolus of a single injectable agent (for example, thiamylal), often termed "crash induction"; by rapid infusions of mixtures of agents (for example, thiamylal and guaifenesin or xylazine, ketamine, and guaifenesin); or by inhalation of high concentrations of anesthetic vapor (for example, 4 to 5 per cent halothane) via a tight-fitting face mask and rebreathing circuit. Injectable techniques are reviewed elsewhere in this symposium. Small ruminants and swine can be easily restrained during mask induction, while large cattle and swine require expensive mechanical restraint devices. Mask induction is not recommended and is used infrequently in adult cattle. However, this method has been successfully implemented in large sows or boars. Mask Induction. Mask induction of small ruminants and swine can be achieved with a variety of potent inhalation agents. Halothane has been commonly used because of its high volatility, relatively low solubility, and potency (Table 2). Although the newer inhalation agents enflurane and isoflurane may possess more desirable properties than does halothane, their cost of administration at present is prohibitive. Although extremely potent (low MAC value), methoxyflurane possesses a low vapor pressure and high tissue solubility (Table 1). When methoxyflurane is administered alone, these physical properties do not permit rapid mask induction. Anesthesia in small ruminants and swine can be induced with halothane delivered from an adult human or small animal anesthetic

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Figure 2. Stanchion with head catch suitable for induction by inhalation mask oflarge sows and boars.

machine and rebreathing circuit. The animal should be restrained in a standing position or sternal recumbency. Swine weighing up to 120 kg can be restrained in a webbed stanchion (Fig. 1). Large swine can be restrained in a stanchion with a head catch (Fig. 2). A tight-fitting mask placed over the snout minimizes entrainment of room air and anesthetic pollution of the environment (Fig. 3). Following a brief period of pre oxygenation with oxygen flow set at 3 to 5 L per minute, the vaporizer is adjusted to deliver a low concentration of anesthetic (for example, 1 per cent halothane). Following a brief period of acclimation, halothane concentration is increased in a stepwise fashion.

Figure 3. A variety of masks, commercial and homemade, can be used for mask induction of swine and small ruminants.

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One or two minutes is allowed at each incremental increase (1 per cent) until a safe maximal setting is achieved (4 to 5 per cent halothane). Signs of increasing anesthetic depth should be monitored continually. In sheep and goats, anesthetic depression is assessed by noting changes in breathing pattern, palpebral reflex, swallowing movements, eye movements, response to interdigital pressure, and so forth.I8 Regurgitation does not appear to be as common in sheep and goats as in cattle during induction of anesthesia. I7 In pigs, similar signs are monitored. When anesthesia is induced with halothane, special attention should be given to signs of malignant hyperthermia (for example, limb stiffness, increased temperature, erythematous areas of skin). Swine often go through a chewing phase that terminates just before achievement of an anesthetic depth sufficient for endotracheal intubation. Intubation is attempted when jaw tone is relaxed and the tongue can be easily drawn forward, lifting the larynx in a more dorsalcranial position. A second method of mask induction uses N 20 and the second gas effect. When using N 2 0, pre oxygenation for approximately 5 minutes is essential to denitrogenate the patient. During preoxygenation, a low concentration of halothane (1 per cent) or other inhalation agent may be introduced into the rebreathing circuit. At the end of the preoxygenation period, anesthetic is increased to its maximal concentration (for example, 4 to 5 per cent halothane), and N 2 0 is delivered at a rate of approximately 70 per cent of the total gas flow. As the patient becomes depressed, the primary inhalation agent is reduced to a maintenance level (for example, 2 per cent halothane). When chewing motions cease, tracheal intubation is attempted. During the intubation procedure, the mask is necessarily removed, and the patient begins to breath room air. At this time, a rapid decrease in alveolar oxygen or diffusion hypoxia may ensue as N 2 0 quickly diffuses across the alveolar surface from blood, and nitrogen (79 per cent in air) rapidly increases in concentration within the alveoli. Signs of hypoxia and cyanosis may develop, especially if the intubation procedure is prolonged. If intubation is not achieved in a reasonable period (1 to 3 minutes) and the mask has to be reintroduced to deepen anesthesia, 70 per cent N 2 0 should not be used, because this may further exacerbate hypoxia. If cyanosis is noted, the mask should be reapplied and the animal reoxygenated with nearly 100 per cent oxygen until signs of hypoxia are eliminated. Endotracheal Intubation. Tracheal intubation assures a patent airway for delivery of gases and anesthetic vapors while preventing aspiration of saliva and regurgitant. In addition, the application of positive pressure ventilation can be more safely achieved via an endotracheal tube. Controlled ventilation has been performed with a tight-fitting face mask but may force gas into the stomach or rumen. An appropriately sized endotracheal tube should be selected before induction. A slightly smaller and larger tube should also be available. In small ruminants, tracheal diameter can be determined by palpating the trachea. The endotracheal tube's internal diameter is the

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Internal Diameter (mm) of Endotracheal Tubes for Cattle

ANIMAL

INTERNAL DIAMETER (MM)

Large bulls

(750-1000 kg)

30

Average cows

(400 kg)

25

Yearling cows

(300 kg)

20

Calves-6 months (175 kg)

18

Calves-3 months (100 kg)

16

major determinant of resistance to breathing. Thin-walled tubes are preferred for this reason, but care should be taken not to kink or overinflate the cuff, collapsing the tube. Appropriately sized diameters of endotracheal tubes for small and large cattle have been reported and are given in Table 3. 18 Large sheep and goats require an internal diameter of 10 to 16 mm, whereas small sheep will take 6 to 10 mm tubes. Endotracheal tubes designed for use in humans or small animals are satisfactory for small ruminants. In adult cattle, a variety of techniques have been employed to accomplish endotracheal intubation. Cattle are usually restrained with belly bands and halter to a large surgical table where induction is achieved with rapid infusion of a mixture of guaifenesin and ultrashort-acting barbiturate given to effect (Fig. 4). Cattle are placed in lateral recumbency as induction proceeds and the animal relaxes. The use of a mouth or dental wedge is suggested to facilitate the safe passage of the anesthetist's hand and arm into the pharynx. The head is extended and the tongue is drawn out of the mouth. A hand is passed into the pharynx, where the epiglottis and arytenoid cartilages are palpated. The endotracheal tube can then be passed along the operator's arm and guided into the laryngeal opening between the arytenoids and into the trachea. Once the endotracheal tube is connected to the Y-piece of the rebreathing circuit, adhesive tape or gauze is placed firmly around the muzzle in front of the handle of the dental wedge to secure its position (Fig. 5). If a cuffed tube is used, the cuff should be inflated to prevent escape of gases. A pressure of 25 to 30 cm of water pressure is sufficient for cuff inflation. When properly inflated, saliva or foreign material accumulated anterior to the cuff will be removed when the tube is removed with cuff inflated. This may be considered an advantage of using cuffed tubes. However, a noncuffed large Cole tube, having a tapered end that passes into the trachea, can be used safely in adult cattle (Fig. 6). Cole tubes seal adequately within the larynx, allowing for positive pressure ventilation. When correctly placed and anchored, the use of a Cole tube in properly positioned adult cattle (posterior pharynx is elevated with a sandbag) has not been associated with tracheal contamination. 36 The

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Figure 4. Surgical table equipped with head board and belly bands to restrain adult cow during induction of general anesthesia.

mouth wedge should be left in place until recovery from anesthesia is evident and the endotracheal tube removed. A second method of intubation employs the same basic technique, but prior to endotracheal tube passage, a guide tube (for example, horse stomach tube) is passed into the trachea. The endotracheal tube is passed over the guide tube into the trachea, and the guide tube is then removed. This method is often necessary in medium-sized or small cattle. In very small cattle (250 kg or less), intubation may have to be accomplished without the use of a mouth wedge. This can be done when sufficient depth of anesthesia is achieved prior to the insertion of the anesthetist's arm. This technique requires that a person with a small hand and forearm pass the guide tube between the dental arcade into the trachea. A sufficient depth of anesthesia is judged by noting ventral eyeball rotation (Fig. 7) and absence of chewing motion when the tongue is manually extended. It should be emphasized that with this technique, sufficient anesthetic depression is essential if injury to animal and operator is to be avoided. In addition, the occurrence of active or explosive regurgitation is likely when the animal is not adequately anesthetized at the time of intubation. A Rowson laryngoscope (Fig. 8) can be used for placement of an endotracheal tube in adult cattle. The laryngoscope must be used with a guide tube so that the laryngoscope can be removed prior to passage of the endotracheal tube. A lubricant should be applied to the tip of the tube to enhance ease of passage into the trachea. Local anesthetic

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F igure 5. The e ndotracheal tube is secured with adhesive tape placed firml y around th e muzzle in front of the de ntal we d ge handle. The endotracheal tube is taped to the Y-pie ce . The intrave nous infusion set is not di sconnected until intubation is comple te d and a sufficie nt de pth of ane sthe sia is assured .

is not routinely applied to the ruminant's larynx. The possible loss of cough reflex resulting in aspiration at the time of extubation precludes desensitization of the larynx. 13 In calves, sheep, and goats a modified human laryngoscope with blade extension (Fig. 8) is adequate to illuminate and directly visualize the larynx. 36 Unlike that used in adult cattle, the preferred position for intubation is sternal recumbency. As with larger ruminants, the mouth can not be opened widely, making the passage of an endotracheal tube with the laryngoscope in place very difficult. Although an experienced anesthetist can successfully intubate adult sheep or

Figure 6. An assortm e nt of large animal endotracheal tubes. Tube s are eithe r (top two tubes ) or tapered (Cole tubes) to fit into the larynx. Pos itive pre ssure ventilation can be effective ly maintaine d with e ithe r d esign .

c um~ d

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Figure 7. Eyeball position indicative of stage III, plane 2 halothane anesthesia in a steer. Endotracheal intubation and most surgical procedures can be accomplished in stage III, planes 1 and 2.

goats with the laryngoscope in place, a plastic catheter placed in the trachea works well as a guide if direct passage cannot be achieved. As in adult cattle, the laryngoscope is removed, and the endotracheal tube is threaded over the guide into the tracheal opening. A circular rotation movement and water-soluble lubricant placed on the tip of the tube help passage. Once the tube is in place, the cuff is inflated, and the tube is secured with a strip of gauze to the upper or lower jaw. Materials used for anesthetic induction of small ruminants are shown in Figure 9. Although muscle relaxants appear to be effective and safe in ruminants,15 their use to facilitate intubation or enhance muscle relaxation during anesthesia has been rarely necessary. Consequently, these agents are not part of the standard anesthetic regimen used for ruminants. Endotracheal intubation of the pig provides a unique challenge to the anesthetist. As in ruminants, the pig's mouth cannot be opened widely. In addition, the pig has a small larynx that slopes ventrally, creating a sharp angle from the pharynx to tracheal opening (Fig. 10). Laryngospasms are easily elicited, especially in piglets. Postoperative sequelae such as laryngeal spasm or edema are more common in swine

Figure 8. Rowson large animal laryngoscope with two blades. The adult human laryngoscope (bottom) has been modifie d by adding an extension onto the blade.

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Figure 9. A small ruminant anesthetic induction set-up including an assortment of endotracheal tubes, induction agent (thiopental), intravenous catheters, atropine, gauze pledges, tape strips to secure endotracheal tube, laryngoscope with blade extension, and stylets .

Figure 10. A sagittal cross section of the pig larynx. The tip of an endotracheal tube is positioned at the level of the posterior floor of the larynx. Tracheal opening (a) . Dorsal cricoid cartilage (b). Arytenoid cartilage (c) . Ventral cricoid cartilage (d). Thyroid cartilage (e). Entrance to lateral laryngeal ventricle (fl . Posterior floor oflarynx (g). Tip of endotracheal tube (h). Thyroid cartilage (i). Middle laryngeal ventricle (j). Epiglottis (k) .

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than other large animal species. Intubation can be difficult and traumatic, increasing the likelihood of postoperative complications. When postoperative problems occur, rapid reintubation is often impossible and attempts to ventilate with a tight-fitting face mask ineffective. For these reasons, special attention should be paid to proper technique when attempting to intubate swine. 38 In addition, pigs should be observed closely following extubation to ensure adequate breathing. Occurrence of laryngospasm is reduced by achieving a sufficient depth of anesthesia prior to intubation. Spraying the larynx with 2 to 4 per cent lidocaine or equivalent local anesthetic is helpful. Muscle relaxants such as succinylcholine (1 to 2 mg per kg- 1 intravenously) may be given to prevent or treat spasm. 38 When a muscle relaxant is used, mechanical ventilation is required until adequate spontaneous ventilation returns. When a light plane of anesthesia is induced, the pig should be placed in sternal recumbency, and the jaws should be held open with the help of small ropes or gauze strips. The tongue is grasped with a gauze pledge and drawn forward by an assistant. For pigs weighing less than 100 kg, an adult human laryngoscope with extension (Fig. B) is appropriately placed at the base of the tongue, and downward pressure is applied until the tip of the epiglottis is released from above the soft palate. An unobstructed view of the laryngeal opening is thus provided. The use of forceps to grasp the epiglottis is discouraged. A Rowson laryngoscope may be needed in large sows or boars. A guide tube such as a plastic catheter can be passed into the trachea, or direct placement of the endotracheal tube can be attempted. An experienced operator can often pass the endotracheal tube into the larynx with its natural curvature in a ventral position. When the tip of the tube is midway through the larynx, a lBO-degree rotation along the long axis allows the tip to be directed in a dorsal manner so as to pass through the cricoid ring into the tracheal opening. Continuing to advance the tube with the curvature in a ventral direction often causes entrapment of the tip of the tube in the posterior floor of the larynx anterior to the cricoid ring (Fig. 10). The tracheal diameter is surprisingly small in the pig. Comparatively, pigs may have the smallest tracheal diameter of all domestic species. A 50-kg pig will often only require a 7- to 9-mm tube. A 10to 14-mm tube is adequate for adult sows, while small swine (5 to 10 kg) may require only 3- to 4-mm tubes. Palpation of the trachea in small ruminants and swine can provide accurate assessment of tracheal diameter. A selection of tube sizes should be available at induction to ensure that the most appropriately sized tube is used. The use of thin-walled tubes is especially important in pigs because of their small tracheal diameters. Following extubation, endotracheal tubes should be washed and stored in disinfecting solutions. 36 Although inhalation anesthesia can be maintained by delivering gases via small-cuffed tubes placed directly into each nostril, this is not a substitute for tracheal intubation. Anesthesia can be maintained with this method of administration, but airway protection is not provided. Ifcontrolled ventilation is instituted, meteorism may result. 36

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MAINTENANCE OF ANESTHESIA Safe maintenance of inhalation anesthesia requires a knowledge of the clinical signs associated with anesthetic depression, continual monitoring of the patient and anesthetic equipment, and an understanding of anesthetic delivery systems and machines. In addition, when large animals are anesthetized, attention should be given to proper padding of the surgical table or floor. 13, 18, 36 Although cattle appear less susceptible than horses to myositis, the prevention of radial nerve paralYSis or other neuropathy can be enhanced with precautionary measures. Foam rubber padding or air mattresses are helpful when strategically placed under the animal. The down foreleg is passed through an automobile inner tube, which is then placed beneath the shoulder allowing the scapula to fall away from the thorax, relieving pressure on the radial nerve and brachial plexus. Additionally, the down front leg should be drawn forward. Assessment of Anesthetic Depth Similar criteria may be used to assess depth of anesthesia in both ruminants and swine. Classifically, stage III has been considered a sufficient depth of anesthesia to allow surgical intervention. Stage III has been divided into three planes. Plane 1 is characterized by minimal analgesia and a regular pattern of breathing. Nystagmus may be observed but disappears as anesthesia deepens. Laryngeal or esophageal stimulation in ruminants may result in active regurgitation in plane 1. Both ruminants and swine maintain the chewing reflex at this depth. 36 Muscle relaxation is minimal, and palpebral and corneal reflexes are strong. Repeated assessment of corneal reflex is traumatic and is discouraged. The eyeball in cattle may begin a downward rotation, with the cornea being partially obscured by the lower eyelid. 36 This sign is not as predictable in sheep and goats and is harder to assess in these species. In plane 2 of stage III, respiration is stable, laryngeal reflexes are obtunded, the palpebral reflex becomes sluggish, the corneal reflex is still present, and muscle relaxation is adequate for most surgical procedures. In cattle, the eyeball rotates ventrally so as to completely hide the cornea (Fig. 7). This sign will not be as pronounced in the sheep or goat. In swine, similar reflexive signs and cardiopulmonary changes are observed in plane 2, but eyeball rotation is variable and unreliable in assessing depth of anesthesia. In plane 3 of stage III, respiration is further depressed, usually resulting in decreased tidal volume and a rapid respiratory frequency. Muscle relaxation is excellent, even with halothane. In cattle, the eyeball rotates in a dorsal direction, and the cornea becomes centered between the eyelids once again. The pupil is dilated. Plane 3 is considered deep surgical anesthesia, and efforts to reduce anesthetic depression should begin. If anesthetic depth is further increased, stage IV depth may be achieved. Stage IV anesthesia is characterized by a fixed, dilated pupil, absence of the corneal reflex, diaphragmatic

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breathing, and severe cardiovascular depression. Stage IV anesthesia should not be allowed to occur, as death is imminent. 36 Although the clinical signs of anesthetic depression may vary with different inhalation agents (for example, halothane versus isoflurane)/6 work has not been completed characterizing these differences in ruminants and swine. The clinical signs accompanying increasing depths of halothane anesthesia have been recognized and are reported above, but comparisons between halothane and other inhalation agents (for example, isoflurane) at comparable depths (for example, 1 MAC) of anesthesia have not yet been made. No single sign can be used as a reliable indicator of cardiopulmonary function during anesthesia. Monitoring of a variety of signs is essential for accurate assessment of the patient's stability. Routinely monitored signs during inhalation anesthesia should include pulse quality and rate, respiration rate, color of mucous membranes, capillary refill time (CRT), direct or indirect blood pressure, and electrocardiogram. The pulse may be palpated over the median auricular (ruminants and swine), ulnar, metacarpal, or digital arteries. Doppler pulse detection is useful and can be achieved from the palmar distal arteries in cattle. 14 Direct auscultation of the heart can be used as well in smaller ruminants and swine. Normal heart rates in adult cattle range from 45 to 80 beats per minute and may be higher during inhalation anesthesia,45 especially following atropine administration. In small ruminants and swine, the normal heart rate ranges from 60 to 90 beats per minute and may vary greatly during anesthesia. 45 Respiration should be monitored for changes in both rate and tidal volume, which can be assessed from rebreathing bag excursion or with a respirometer. Adult cattle have respiration rates of 10 to 30 breaths per minute, which often double in frequency during anesthesia. 45 Tidal volume decreases to a value of 4 to 6 ml per kg, requiring the use of mechanical ventilation in some cases. The CRT is representative of peripheral perfusion and is a function of both blood pressure and peripheral vascular resistance. The CRT should be less than 2 seconds during anesthesia. In cases in which significant pulmonary shunting is suspected and blood gas analyzing equipment is available, the assessment of arterial pH, PC0 2, and P0 2 provides valuable information of overall cardiopulmonary function. Higher than normal PaC0 2 levels (greater than 50 mm Hg) should be expected in anesthetized spontaneously breathing adult cattle. When exceedingly high PaC0 2 values are encountered (greater than 60 to 70 mm Hg), positive pressure ventilation is effective in correcting hypoventilation. Arterial Pa02 values are often surprisingly low during inhalation anesthesia. However, even when Pa02 values decrease to 70 or 80 mm Hg, oxygen saturation of hemoglobin is greater than 90 per cent. Positive pressure ventilation does not always increase Pa02 values in adult cattle. Pulmonary shunting of blood may increase, and cardiac output is often decreased. Arterial blood gas values determined from an adult anesthetized cow before and after administration of positive pressure ventilation are given in Table 4. When a mixed metabolic and respiratory

610 Table 4.

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Blood Gas Values Obtained from a Holstein Cow Anesthetized with Halothane in Oxygen

METHOD OF VENTILATION*

TIME

pHa

PaC0 2 He)

(MM

Pa0 2 He)

(MM

O2

SATURATION

(%)

90 min

7.229

91.3

91.3

84.0

Positive pressure ventilation

120 min

7.364

62.4

62.4

67.9

Return to spontaneous ventilation

150 min

7.245

89.2

89.2

73.2

Spontaneous ventilation

* Cow was allowed to breath spontaneously for 90 minutes, at which time a blood gas sample was obtained and controlled ventilation initiated. Following 30 minutes of intermittent positive pressure ventilation, another sample was obtained (120 minutes) and the cow was allowed to resume spontaneous breathing. A third sample was taken 30 minutes after resuming spontaneous breathing (150 min). acidosis occurs, large volumes of bicarbonate should not be given injudiciously to correct metabolic acidosis unless the removal of carbon dioxide from the lung is accelerated by instituting intermittent positive pressure ventilation. If this is not done, paradoxical cerebral acidosis may ensue. Monitoring of blood pressure during halothane anesthesia provides the most accurate clinical means of assessing anesthetic depression of cardiovascular function. Indirect methods of measuring blood pressure are not routinely used in ruminants or swine. However, the auricular artery is an excellent site for arterial catheterization when direct measurements are to be made. Following catheterization, direct pressure measurements can be made with an aneroid manometer or pressure transducer and recorder. It is well accepted that halothane produces a dose-dependent depression of blood pressure in mammalian species. However, blood pressure has been reported to increase over the awake value during 1.2 per cent halothane (inspired) anesthesia in adult bulls and steers, while heart rate was unchanged. 43 Cardiac output decreased, while total vascular resistance increased. Arterial pressure also increased when awake cattle were placed in lateral recumbency.43 Maintenance Technique Once endotracheal intubation is accomplished, the tube is connected to the Y-piece of the rebreathing circuit. The posterior pharynx should be elevated with a sandbag or towels to prevent regurgitant from accumulating around the endotracheal tube. Just before the Ypiece of the rebreathing circuit is connected to the endotracheal tube, the anesthesia machine and rebreathing circuit should be filled with the anesthetic-oxygen mixture. This allows the animal to receive the desired anesthetic concentration immediately upon its first breath. 30

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To fill the system, the pop-off valve should be closed, and the Y-piece opening should be capped. The vaporizer is set at the desired concentration (approximately 3.5 to 5.0 per cent in adult cattle), and the oxygen flow rate is set at 8 to 10 L so as to rapidly fill the system until the re breathing bag is filled. The flow meter is then turned off. If the oxygen flush valve is used during induction, 100 per cent oxygen will be delivered into the rebreathing circuit bypassing the vaporizer, diluting anesthetic concentration. When the circuit is connected to the endotracheal tube, the oxygen flow meter is set to deliver 6 to 8 Land the pop-off valve is partially opened to provide a semiclosed system and escape of gases. High flow rates provide for a flushing action of the circuit and speed denitrogenation while continuing to deliver the desired high concentration of anesthetic vapor. Once the animal has stabilized at the desired plane of anesthesia, the flow rate and vaporizer setting can be reduced. High oxygen flow rate (6 to 10 L) and inspired anesthetic concentration (4.5 to 5 per cent) can usually be reduced in 10 to 20 minutes, depending on the assessed depth of anesthesia and surgical requirements. At this time, the depressant actions of the injectable inducing agents should be waning, and anesthesia is maintained by the inhalation anesthetic. Continued high delivered anesthetic concentration (3.5 to 4.0 per cent halothane) is sometimes necessary in adult cattle that have developed significant pulmonary shunting of blood and ventilation-perfusion mismatching. In small ruminants and swine, vaporizers can be set to deliver 1.5 to 2.0 per cent halothane for most surgical procedures. With halothane administration, changes in anesthetic depth can occur rapidly. Within 5 minutes of vaporizer adjustment, significant changes in alveolar end-tidal concentration are observed. This rapid alteration in depth of anesthesia remains a major advantage over many injectable techniques. Constant monitoring and assessment of anesthetic depth are always required, with the goal of providing minimal anesthetic depression commensurate with surgical objectives. Anesthetists should be continually but cautiously attempting to reduce the inspired anesthetic concentration. Many factors such as preanesthetic medication and intraoperative hypothermia can reduce inhalation anesthetic requirement. Delivery of an optimal concentration can be achieved only by constant vigilance in assessing depth coupled with attempts to reduce delivered anesthetic concentrations. In this regard, the anesthetist is as much an artist as a scientist, for whom experience at the task is often the difference between a safe and unsafe anesthetic administration. The use of N 2 0 for maintenance of anesthesia is usually avoided in ruminants. However, in swine, N 2 0 can be administered as part of the maintenance regimen and has the advantage of reducing the requirement of the volatile anesthetic. In order to prevent diffusion hypoxia when the animal begins breathing air, N 2 0 should be discontinued at least 5 minutes before the animal is disconnected from the rebreathing circuit.

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Recovery Period An immediate return to sternal recumbency and recovery to a standing position within 30 minutes after the completion of surgery is desirable. A major advantage of inhalation anesthesia in large ruminants is rapid recovery. Shortly before the end of the surgical procedure (5 minutes), delivered anesthetic vapor concentration is reduced to one half the required maintenance setting. At completion of the procedure, the vaporizer is turned off and the rebreathing system is flushed with O 2 to speed removal of anesthetic gas from the circuit and patient. The endotracheal tube should not be disconnected from the rebreathing circuit until initial signs of recovery appear. Active reflexes, blinking, swallowing, limb movement, chewing, and increased ventilation are readily apparent signs of recovery. The animal should immediately be placed in sternal recumbency to enhance eructation of ruminal gas and alleviate bloat when present. When swallowing or head and leg movements occur, the mouth wedge and tube are removed. The head should be haltered and restrained to prevent self-inflicted trauma. Smaller ruminants and swine can be easily supported in sternal recumbency. The administration of a eNS stimulant (analeptic) such as doxapram is rarely needed in ruminants and swine. If used, stimulation of respiration may lead to increases in the work of breathing and oxygen consumption. Thus, the assurance of a patent airway with an endotracheal tube and oxygen insufflation are important adjunct procedures to the use of analeptics. 31 Tolazoline and yohimbine have been given to bulls or cows premedicated with xylazine. These animals will often have poor minute ventilation manifesting in prolonged recovery periods. In this circumstance, these agents are extremely effective, because they reverse any residual sedative action of xylazine while increasing minute ventilation and thus elimination of volatile anesthetic.

ANESTHETIC EQUIPMENT A variety of anesthetic delivery systems and machines are commercially available (Fig. 11). The anesthetic machine proper consists of vaporizer, flow meters, regulator, and oxygen source. Gases travel from the oxygen source (tank or gas outlet) through the regulator, which reduces the line pressure to a workable level for the flow meter. The flow meter adjusts the oxygen flow to a desired volume of flow per minute into the rebreathing circuit. From the flow meter, a percentage of the total oxygen flow passes through the vaporizer to produce the desired concentration of anesthetic vapor. This mixture of gases is then delivered via the fresh gas line into the rebreathing circuit. Small ruminants and swine weighing up to 140 kg can be anesthetized safely with an anesthetic machine designed for small animals or adult people (Fig. lID). Large-animal machines, although

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Figure 11. A, Narkovet-E large-animal anesthetic machine equipped with a halothane Vapor vaporizer (North American Drager, Telford, Pennsylvania). B, VMS largeanimal anesthesia machine equipped with a halothane Fluotec Mark III vaporizer (Fraser-Harlake, Orchard Park, New York). C, North American Drager large-animal control center with Vapor 19.1 halothane vaporizer and volume-regulated ventilator. D, North American Drager small-animal Narkovet-2 anesthetic machine with Vapor 19.1 halothane vaporizer.

designed for the horse, are ideal for adult cattle. These machines are usually equipped with precision halothane vaporizers placed out of the circuit. With this arrangement, precise anesthetic vapor concentrations can be delivered into the rebreathing circuit. The rebreathing circuit routes expired gases through a soda lime canister for removal of carbon dioxide. Rebreathing circuits can be adjusted to deliver an

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excess of fresh gas (semiclosed system), or oxygen flow can be reduced to meet the oxygen requirement per minute (closed system) of an animal. With the semiclosed system, the pop-off valve must be adjusted to allow excess gases to escape. With the closed system, the exhaust valve may be closed, and the fresh gas flow is set to deliver oxygen at the rate of metabolic consumption. Some advantages of the closed system are reduction in costs, reduced pollution of surgical area with anesthetic vapors, and increased access to information. 27 In recent years, scavenger systems have been employed on semiclosed systems to negate the problem of environmental pollution. The first portable rebreathing system to be used for inhalation anesthesia in adult cattle was the to-and-fro system. 34 Instructions for the construction of a large animal to-and-fro rebreathing canister have been published. 37 The term to-and-fro refers to the back and forth movement of inhaled and exhaled gases through a carbon dioxide canister. To ensure sufficient CO 2 removal, the CO 2 canister should be large enough to provide space between absorbant granules equivalent to or greater than the animal's tidal volume. 37 Because rapid color change of CO 2 absorbent is indicative of rapid CO 2 production, the canister should be transparent. The advantages of the to-and-fro system are efficient CO 2 absorption, simplicity of construction, less resistance to breathing, ease of transport, and adaptability to any anesthetic machine equipped with an out of circuit vaporizer, including small-animal machines (Fig. 12).35 The disadvantages are rebreathing of heated exhaled gases, inhalation of dust from CO 2 canister, propensity to increase mechanical dead space as the CO 2 absorbant granules are exhausted, and necessity of positioning the entire system near the head of the patient. 37 Some of these disadvantages can be minimized by using a semiclosed circuit and higher fresh gas flows. This provides for dilution and a flushing of the exhaled gases, minimizing CO 2 build-up while enhancing heat dissipation within the system. It is not recommended that the to-and-fro system be used with low fresh gas flows. 35 Oxygen flow rates should be set at 6 to 10 L in adult cattle, and vaporizers should be adjusted to deliver 3.5 to 4.5 volumes per cent for induction. For maintenance, the oxygen flow rate is reduced to 3 to 4 L, and the vaporizer setting is decreased to 2 to 2.5 per cent. 35 The circle system is more complex and expensive than the to-andfro system. This sytem routes gases in a circular fashion through the re breathing circuit. The advantages of circle systems are a constant dead space volume and longer rebreathing hoses, which allow for versatility in positioning of the machine and animal. Vaporizers can be mounted in the rebreathing circuit (VIC) or outside the circuit (VOC). It is recommended that highly volatile agents such as halothane be administered with precision vaporizers located out of the rebreathing circuit. Mechanical ventilation with the VIC arrangement or completely closed systems may cause anesthetic overdose. 26 Intermittent positive pressure ventilation can be safely instituted with a back pressure-compensated precision vaporizer placed out of the

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Figure 12. To-and-fro rebreathing delivery system for large animals connected to (A) a small-animal anesthetic machine and (B) an oxygen tank equipped with a flow meter and an isoflurane 19.1 vaporizer.

rebreathing circuit. The ventilator is connected into the circle system at the rebreathing bag port. Mechanical Ventilation The goal of instituting intermittent positive pressure ventilation is to normalize alveolar gas exchange when hypoventilation occurs. During anesthesia, indications for this include anesthetic-induced central depression of respiration, ventilation-perfusion mismatching due to prolonged recumbency, weak ventilatory efforts caused by anesthetic agent or muscle relaxants, and the inability to generate negative pressure within the lung because of thoracic trauma (pneumothorax) or surgical procedure (thoractomy). Positive pressure ventilation can be accomplished with a variety of mechanical ventilators. Two equine commercial units are available in America: the N. A. Drager large-animal control center (Fig. lle) and the J. D. Medical Distributing Company equine ventilator.31 In addition, designs for construction of mechanical ventilators are available. l , 9, 37 Many of these ventilators require the use of Bird respirator units (Fig. 13).9, 37 Some Bird respirators (Mark 7 and 14) must be modified to generate sufficient flow rates for adult cattle (approximately 250 L per minute).25 Ventilators designed for adult humans are adequate for most sheep and goats. Calves and pigs weighing less than 100 kg can also be adequately ventilated with units designed for

616

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Figure 13. Bird respirator used to drive homemade "bag in a barrel" ventilator connected to large-animal anesthetic machine. Air compressor is used to generate air pressure for compression of ventilator bag. The ventilator is connected to the rebreathing circuit at the rebreathing bag port.

the adult human. Ventilators are engineered with double circuits so that rebreathed gas within the bag or bellows does not communicate with the gases (oxygen or air) compressing the bellows. There is no best technique for instituting intermittent positive pressure ventilation. Ventilator setting should be adjusted according to the individual's needs. Ventilation should be improved as efficiently as possible while minimizing undesirable effects. Initial settings for adult cattle are a respiratory frequency of 6 to 9 breaths per minute, tidal volume of 10 to 15 ml per kg (bellows setting of 5 to 8 L in 440 kg cow), and an inspiratory-expiratory ratio of 1 : 2 or greater. These values are similar to those suggested for adult horses. 31 Respiratory frequency of 8 to 12 breaths per minute can be used in small ruminants and swine. Pressures generated within the ventilator and rebreathing circuit should be 30 to 40 mm Hg. Airway pressures of 15 to 20 mm Hg will result. Controlled ventilation is defined as ventilation that is initiated by the anesthetist or the ventilator. Assisted ventilation is positive pressure ventilation that is initiated by the patient's inspiration generating negative pressure within the breathing circuit. If controlled ventilation is desired and muscle relaxants are not used, a respiratory frequency sufficient to induce hypocapnia may be necessary to take control of breathing. Peripheral muscle relaxants are rarely required in ruminants and swine in the clinic setting. In general, during inhalation anesthesia, alveolar hypoventilation and hypercapnia are less likely to occur with controlled ventilation than with assisted ventilation. 31 However, differences in pulmonary function during assisted versus

617

INHALATION ANESTHESIA

controlled ventilation were not observed in young swine anesthetized with ketamine and chloralose, drugs that induce minimal cardiovascular depression. 5 Blood gas derangements progressively worsen in adult cattle during prolonged anesthesia. 34 In the horse, application of either mode of ventilation at the initiation of anesthesia appears to blunt the progressive decline in Pa02 values and hypoxemia that occurs during anesthesia. 33 There is no evidence to indicate that early initiation of intermittent positive pressure ventilation would be less beneficial in blunting progressive hypoxemia in large ruminants. However, in my experience, instituting intermittent positive pressure ventilation following prolonged periods of spontaneous breathing may result in both decreased PaC0 2 and Pa02 values (Table 4). Its effectiveness is ideally assessed by analyzing arterial blood gas tensions or end-tidal alveolar CO 2. Under less ideal conditions, effectiveness of ventilation is usually assessed by chest excursion, anesthetic circuit and airway pressures, frequency and acceptance of forced ventilation, and clinical assessment of cardiovascular function. When controlled ventilation is to be terminated, respiratory frequency is decreased, and PaC0 2 is allowed to increase until spontaneous breathing resumes. It should be remembered that positive pressure ventilation can cause marked depression of cardiac output and cardiovascular function. This is especially evident when thoracic compliance is reduced by positioning and poor diaphragmatic excursion. Cardiovascular function is best maintained by minimizing mean inspiratory airway pressure. Improved ventilation increases alveolar anesthetic concentration. Consequently, the vaporizer setting should be reduced to decrease inspired concentration and prevent anesthetic overdose.

REFERENCES 1. Beerwinkle, K. R., and Witzel, D. A.: A pneumatically driven, electronically controlled respirator for use with large animal inhalation anesthesia systems. Vet. Anesth., 3:110, 1976. 2. Benson, G. J., Hartsfield, S. M., and Thurmon, J. C.: A method for anesthetizing pigs using small animal inhalation anesthetic equipment. Pract. Vet., 49(2):20-

25, 1977. 3. Bowen, J. M., and Seidel, S.: Anesthesia in the cow. Vet. Anesth., 3:100-109,1976. 4. Bowen, J. M., and Yturraspe, D.: Blood gases in anaesthetized adult bovine and their relationship to posture and the duration of halothane anesthesia. Vet. Anesth., 3: 104, 1976. 5. Downs, J. B., Douglas, M. E., Ruiz, B. C., et al.: Comparison of assisted and controlled mechanical ventilation in anesthetized swine. Critical Care Med., 7:5, 1979. 6. Eger, E. 1.: Cardiovascular effects of halothane in man. Anesthesiology, 32:396408, 1970. 7. Eger, E. 1., II: Anesthetic Uptake and Action. Baltimore, Williams and Wilkins Co., 1975. 8. Eisele, P. H., Talken, L., and Eisele, J. H.: Potency of isoflurane and nitrous oxide in conventional swine. Lab. Anim. Sci., 35:76-78, 1985. 9. Fowler, M. E., Parker, E. E., McLaughlin, R. F., Jr., et al.: An inhalation anesthetic apparatus for large animals. J. Am. Vet. Med. Assoc., 143:272, 1963. 10. Fujimoto, J. L., and Lenehan, T. M.: The influence of body position on the blood

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gas and acid base status of halothane-anesthetized sheep. Vet. Surg., 14:169-172, 1985. Garner, H. E., Mather, E. C., Hoover, T. R, et al.: Anesthesia of bulls undergoing surgical manipulation of the vas deferentia. Can. J. Compo Med., 39:250-255, 1975. Gregory, G. A., Wade, J. G., Beihl, D. R., et al.: Fetal anesthetic requirement (MAC) for halothane. Anesth. Analg., 62:9-14, 1983. Hall, L. W., and Clarke, K. W.: Veterinary Anaesthesia. London, Bailliere Tindall, 1983. Heath, R B.: General anesthesia in ruminants. In Jennings, P. B. (ed.): The Practice of Large Animal Surgery. Philadelphia, W. B. Saunders Co., 1984. Hildebrand, S. V., and Howitt, G. A.: Neuromuscular and cardiovascular effects of pancuromium bromide in calves anesthetized with halothane. Am. J. Vet. Res., 45: 1549-1552, 1984. Hodgson, D. S., Steffey, E. P., Woliner, M. J., et al.: Alterations in breathing patterns of horses during halothane and isoflurane anesthetic induction. Proceedings of the Second International Congress of Veterinary Anesthesiologists, 1985, p. 195. Jennings, S.: The use of volatile anesthetic agents in horses and farm animals. Can. Vet. J., 4:86, 1963. Jennings, S.: General anesthesia of ruminants and swine. In Soma, L. R (ed.): Textbook of Veterinary Anesthesia. Baltimore, Williams and Wilkins Co., 1971. Lundeen, G., Manohar, M., and Parks, C.: Systemic distribution of blood flow in swine while awake and during 1.0 and 1.5 MAC isoflurane anesthesia with or without 50% nitrous oxide. Anesth. Analg., 62:499-512, 1983. Lunn, J. K., Liu, W. S., Stanley, T. H., et al.: Peripheral vascular and cardiac effects of nitrous oxide in the bovine. Can. Anaesth. Soc. J., 24:571-585, 1977. Manohar, M., and Parks, C. M.: Porcine brain and myocardial perfusion during enflurane anesthesia without and with nitrous oxide. J. Cardiovasc. Pharmacol., 6:1092-1101,1984. Marshall, B. E., and Wollman, H.: General anesthetics. In Goodman, A. G., Gilman, A. (eds.): The Pharmacologic Basis of Therapeutics. New York, MacMillan Publishing Co., 1980. Newburg, C. A., Lambert, E. H., and Gronert, G. A.: Failure to induce malignant hyperthermia in myotonic goats. Br. J. Anaesth., 55:57-59, 1983. Palahniuk, R J., Shnider, S. M., and Eger, E. 1., II.: Pregnancy decreases the requirements for inhaled anesthetic agents. Anesthesiology, 41 :82-83, 1974. Purchase, 1. F. H.: Some respiratory parameters in horses and cattle. Vet. Rec., 77:859, 1965. Rex, M. A. E.: The current position of closed curcuit anesthesia in small and large animals. Proceedings of the Second International Congress of Veterinary Anesthesiologists, 1985, p. 59. Rex, M. A. E., and Komesaroff, D.: Advantages of closed circuit anesthesia. Proceedings of the Second International Congress of Veterinary Anesthesiologists, 1985, p. 53. Rezakhani, A., Edjtehadi, M., and Szabuniewicz, M.: Prevention of thiopental and thiopental/halothane cardiac sensitization to epinephrine in the sheep. Can. J. Compo Med., 41 :389-395, 1977. Robinson, S. L., Richardson, C. A., Willis, M. M., et al.: Halothane anesthesia reduces pulmonary function in the newborn lamb. Anesthesiology, 62: 578-581, 1985. Rugh, K. S., Zinn, G. M., Paterson, J. A., et al.: Inhalation anesthesia in adult cattle. Lab. Anim. Sci., 35:178-181,1985. Steffey, E. P.: Mechanical ventilation of the anesthetized horse. Vet. Clin. North Am. [Large Animal Pract.] 3(1):97-110, 1981. Steffey, E. P., and Howland, D.: Halothane anesthesia in calves. Am. J. Vet. Res., 40:372-376, 1979. Steffey, E. P., Wheat, J. D., Meagher, D. M., et al.: Body position and mode of ventilation influences arterial pH, oxygen, and carbon dioxide tensions in halothane-anesthetized horses. Am. J. Vet. Res., 38:379, 1977.

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34. Tavernor, W. D.: A simple apparatus for inhalation anaesthesia in adult cattle and horses. Vet. Rec., 73:541, 1961. 35. Thurmon, J. C., and Benson, C. J.: Inhalation anesthetic delivery equipment and its maintenance. Vet. Clin. North Am. [Large Animal Pract.] 3(1):73-96, 1981. 36. Thurmon, J. C., and Benson, C. J.: Anesthesia in ruminants and swine. In Howard, J. L. (ed.): Current Veterinary Therapy: Food Animal Practice. Philadelphia, W. B. Saunders Co., 1986. 37. Thurmon, J. C., Menhusen, M. J., and Hartsfield, S. M.: A multivolume ventilatorbellows and air compressor for use with a Bird Mark IX respirator in large animal inhalation anesthesia. Vet. Anesth., 2:34, 1975. 38. Thurmon, J. C., and Tranquilli, W. J.: Swine anesthesia for cardiovascular research. In Stanton, H. C., and Mersmann, J. H. (eds.): Swine in Cardiovascular Research. Boca Raton, CRC Press, (in press). 39. Tranquilli, W. J., Thurmon, J. C., and Benson, C. J.: Alterations in the arrythmogenic dose of epinephrine induced by xylazine and specific adrenergic receptor antagonists in halothane-anesthetized pigs. Proceedings of the Second International Congress of Veterinary Anesthesiologists, 1985, p. 92. 41. Tranquilli, W. J., Thurmon, J. C., and Benson, C. J.: Anesthetic potency of nitrous oxide in young swine (Sus scrofa). Am. J. Vet. Res., 1:58-60,1985. 42. Tranquilli, W. J., Thurmon, J. C., Benson, C. J., et al.: Halothane potency in pigs (Sus scrofa). Am. J. Vet. Res., 44:1106-1107, 1983. 43. Trim, C. M.: Sedation and general anesthesia in ruminants. Bovine Practitioner, 16:137-143,1981. 44. Trim, C. M., and Semrad, S. D.: Cardiovascular effects of guaifenesin-thiopentalhalothane anesthesia in bulls and steers. Proceedings of the Second International Congress of Veterinary Anesthesiologists, 1985, p. 71. 45. Weiskopf, R. B., and Bogetz, M. S.: Minimum alveolar concentrations (MAC) of halothane and nitrous oxide in swine. Anesth. Analg., 63:529-532, 1984 46. Wirth, D.: Pulse and respiration. In Wirth, D. (ed.): Veterinary Clinical Diagnosis. London, Bailliere, Tindall, and Cox, 1956. Department of Veterinary Clinical Medicine College of Veterinary Medicine University of Illinois Urbana, Illinois 61801