Management of Intraoperative and Postoperative Anesthetic Complications in Ruminants and Swine

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Anesthesia

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Management of Intraoperative and Postoperative Anesthetic Complications in Ruminants and Swine Thomas W. Riebold, D.V.M.*

Fortunately, major complications do not occur frequently during or following well-planned food animal anesthesia. However, one must be vigilant so that unexpected complications can be recognized early and treated effectively. As in other species, complications are most effectively treated by prevention and, therefore, emphasis should be placed on formulation and implementation of a rational anesthetic regimen.

INTRAOPERATIVE COMPLICATIONS Airway Obstruction

Airway obstruction, if left untreated, results in dyspnea, hypoxia, hypercarbia, apnea, and finally cardiac arrest. Animals with airway obstruction have a characteristic respiratory pattern. The thorax expands partially and then remains static during inspiration, collapsing quickly during expiration. Because of the obstruction, limited expansion of the thoracic cavity occurs during inspiration. Negative pressure of inhalation causes rapid recoil of the thorax during exhalation. Lack of air movement from the nares or endotracheal tube confirms airway obstruction. Often adult cattle are intubated for maintenance of general anesthesia. During intubation, the operator's hand is placed in the oropharynx and guides the endotracheal tube into the larynx. Depending on the size of the animal and the individual's arm, airway obstruction

* Diplomate,

American College of Veterinary Anesthesiologists; Veterinary Teaching Hospital, Oregon State University College of Veterinary Medicine, Corvallis, Oregon

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can occur. It is important that intubation be accomplished promptly. If the technique requires over 1 minute, the hand and arm should be withdrawn from the oral cavity to allow the animal to breathe. Swine are more difficult to intubate than ruminants, and rough or traumatic laryngeal manipulation can provoke laryngeal edema and laryngospasm. Often these complications occur during recovery, requiring reintubation or tracheotomy, which is difficult to perform in semiconscious swine. 19 In anesthetized swine, laryngospasm can be successfully treated with succinylcholine, which relaxes the larynx and allows intubation. Topical application of lidocaine to the larynx with an adjustable pattern plant sprayer prevents laryngospasms and permits intubation. 9 Atraumatic technique during intubation prevents trauma and irritation to the larynx. Airway obstruction can occur following regurgitation in ruminants. In lightly anesthetized cattle, active regurgitation can occur during intubation,18 and passive regurgitation can occur at any time during anesthesia due to relaxation of the cardia. If the cause is active regurgitation, the depth of anesthesia should be rapidly increased and the airway established to prevent aspiration. Because rumen contents contain more solid material than the gastric contents of monogastric animals, there is a greater potential for ingesta to obstruct the larynx while the more fluid portion will drain from the mouth. Animals that are not intubated are at greatest risk. Because intubated animals that have regurgitated during anesthesia are at risk following extubation, ingesta should be removed from the buccal cavity prior to extubation. Partial or complete airway obstruction can occur when the endotracheal tube becomes kinked or collapses or when it is accidentally placed in the esophagus. Improper positioning of the head, particularly acute flexion of the neck, can result in kinking of the endotracheal tube, as can malposition of the anesthetic equipment. Overinflation of the cuff can cause collapse of the endotracheal tube. Diffusion of nitrous oxide into the cuff during the anesthetic procedure may also cause collapse of the tube. 4 In each of these instances, absence of movement or diminished movement of the rebreathing bag of the anesthetic machine is noted, along with increased resistance to gas flow during manual compression of the rebreathing bag. Treatment is based on identification and correction of the underlying cause. Following esophageal placement of the endotracheal tube, no movement of the rebreathing bag occurs. In addition, esophageal intubation may provoke regurgitation. 18 Improper tube placement is confirmed by palpation of the tube in the esophagus, lack of gas flow through the tube, and increased resistance to tube passage. Dorsal displacement of the soft palate occurs with some degree of frequency in anesthetized horses. The soft palate moves dorsal to the epiglottis, hindering airflow into the larynx. This condition is usually not encountered in ruminants but can be a potential problem in swine because of their long soft palate that can be displaced dorsally and engage the larynx. This can occur during anesthesia when the trachea is not intubated and following extubation. Airway obstruction

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can be corrected by inducing the animal to swallow, causing the soft palate to return to its normal position. If the anesthetic plane is sufficient to abolish the swallowing reflex, the problem is much more severe because the animal cannot correct itself, resulting in impaired breathing and the possibility of apnea and cardiac arrest. A small stomach tube passed into the oropharynx stimulates swallowing in an animal that is not too deeply anesthetized. If the animal cannot swallow, it should be reintubated until the anesthetic plane decreases to the point at which the animal can swallow. Apnea Hypoxia often occurs in anesthetized animals breathing room air or can result during ventilation-perfusion inequality or right-to-Ieft shunts. 14 , 18 In addition, ruminal tympany and the volume of ingesta and viscera in ruminants hinder diaphragmatic function, increasing the degree of hypoventilation and decreasing functional residual capacity.14 Overdosing with xylazine, the barbiturates, or halothane can cause hypoventilation or apnea. Hypoventilation or apnea can result from neuromuscular blockade induced by peripheral muscular relaxants (for example, succinylcholine) or interaction of anesthetics and antibiotics (for example, aminoglycosides).1 Because ruminants have very low levels of pseudocholinesterase, metabolism of succinylcholine is slow, causing prolonged effects of the drug. Succinylcholine is contraindicated in ruminants except in special circumstances, and it should be used only when equipment for manual or mechanical ventilation is available. Another cause of apnea is anterior migration of local anesthetic in the epidural space, blocking nervous innervation to the diaphragm and intercostal muscles. Whatever the etiology of hypoventilation or apnea, ventilation must be controlled until the cause is identified and corrected. The most effective treatment is tracheal intubation and controlled ventilation, preferably with oxygen, but room air or compressed air can be used. During inhalation anesthesia, alternate compression and release of the rebreathing bag provides ventilatory support. Mechanical ventilators can also be used to provide support in this instance. A demand valve can be used to provide controlled ventilation by intermittent compression and release of a button (Fig. 1).15 This piece of equipment is small, portable, and capable of generating sufficient gas flow to effectively ventilate adult cattle. An E cylinder of oxygen lasts about 15 minutes, becoming progressively less efficient as it empties. Because it is capable of generating high gas flow, use of the demand valve requires caution with small patients to decrease the risk of pulmonary barotrauma, which can lead to tension pneumothorax. Another method of intermittent positive pressure ventilation (IPPV) involves the use of an oxygen cylinder and nasogastric tube. s After connection of the tube to the cylinder, the cylinder valve is opened until gas flow is felt about 18 inches from the tube tip and the tube is passed into the endotracheal tube. By alternate occlusion and release of the endotracheal tube around the smaller tube, IPPV can

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Figure 1. A, The demand valve is connected to an oxygen source with fitting A. B, The patient is attached to the demand valve (B) and ventilated by alternate compression and release ofa trigger (C).

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be delivered, but this method is not as efficient as others. Small food animals «75 kg) can be effectively ventilated with an Ambu bag connected to the endotracheal tube. 18 Periodic compression and release of the bag ventilates the animal. With all techniques, the inspiratory phase should be ended following observation of adequate thoracic expansion. During controlled ventilation, attempts should be made to identify the cause of the apnea. When a specific antagonist (for example, naloxone, 4-aminopyridine, or yohimbine) is available and indicated, it should be used. 22 Symptomatic therapy can include the use of doxapram, an analeptic. If ruminal tympany has caused hypoventilation or apnea, the rumen should be decompressed, preferably by passage of a stomach tube. If that is not possible, the rumen should be trocarized with a 12-gauge needle. When specific therapy is unavailable, controlled ventilation and other supportive therapy (for example, intravenous fluids, heating blankets) should be continued until the animal recovers . If the animal reveals signs of arousal without spontaneous breathing, respiratory support may have resulted in

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hyperventilation, and the respiratory rate should be decreased to allow time for sufficient carbon dioxide accumulation and initiation of spontaneous breathing.

Cardiac Dysrhythmia Cardiac dysrhythmias in anesthetized food animals do not appear as commonly as in other species. The most common dysrhythmia is premature ventricular contraction (PVC). This can occur following induction of anesthesia with thiobarbiturates and is often coupled with a normal beat, giving rise to the term bigeminal rhythm. 10 Usually it is transitory and resolves without treatment. Other dysrhythmias that may occur following thiobarbiturate induction are sinus tachycardia, ventricular tachycardia, multifocal ventricular tachycardia, and ventricular fibrillation. 10 Following the induction period, PVCs are most commonly caused by hypoxia or hypercarbia and pre-existing traumatic myocarditis in cattle. An occasional PVC is usually of little concern, but runs of three or more consecutive PVCs are serious because cardiac output drops dramatically. As a result, coronary perfusion decreases and myocardial hypoxia ensues, increasing the irritability of the ectopic focus. Presumptive diagnosis of PVCs can be made by palpation of a peripheral artery and noting the change in the sequence or rhythm of pulsations or by auscultating and noting the change in rhythm of the heart sounds. The diagnosis can be confirmed by electrocardiography. Therapy is directed at controlling the irritable focus and identification and correction of the underlying cause. The most common causes are hypercapnia and hypoxemia resulting from hypoventilation, exhausted soda lime, and an inadequate fraction of inspired oxygen (F 102)' When proper ventilation does not eliminate the PVCs, lidocaine is administered at 1 to 2 mg per kg intravenously as a bolus. 12 If the PVCs continue or recur after one or two boluses of lidocaine, a slow infusion of lidocaine (40 to 60 /-Lg per kg per minute) should be instituted. Atrial fibrillation can occur in cattle as a sequela to metabolic derangement. Diagnosis can be confirmed with electrocardiography. Most often, it occurs secondary to gastrointestinal obstruction. Usually atrial fibrillation resolves after correction of the primary problem. Because cattle are amenable to physical restraint and local anesthesia, corrective surgery can be performed without general anesthesia. The oculocardiac reflex is well recognized. 10, 17 It is mediated by the trigeminal and vagus nerves and initiated by traction or pressure on the eyeball during surgery. Bradycardia is the usual response, but cardiac arrest can occur. Whenever major ophthalmic surgery (for example, enucleation) is planned, the possibility of dysrhythmias or bradycardia exists. Discontinuation of surgical manipulations, allowing normal heart rate to return, followed by a less traumatic approach is usually sufficient to correct the problem. If the dysrhythmias or bradycardia persists or returns despite gentle tissue handling, atropine

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should be administered at 0.12 mg per kg intravenously to ruminants and 0.04 mg per kg intravenously to swine to correct the problem. Cardiac arrest is best treated by prevention. Often, signs of impending arrest are observed, allowing therapy to be instituted. These signs include hypotension, diminished pulse pressure and capillary refill time, dark blood, decreased bleeding at the incision site, dysrhythmias, and decreased respiratory rate and tidal volume. Cyanosis mayor may not be present. Apnea may occur prior to cardiac arrest or simultaneously. Three types of cardiac arrest can occur. They are asystole, electromechanical dissociation, and ventricular fibrillation. The type of cardiac arrest can be determined with an electrocardiogram. Asystole is characterized by a lack of electrical activity. Electromechanical dissociation is characterized by cardiac electrical activity without myocardial contraction as determined by absence of peripheral pulse or heart sounds. Ventricular fibrillation is characterized by irregular undulating waveforms with no recognizable PQRS complexes. Coarse fibrillation shows waveforms with greater amplitude than fine fibrillation. Without electrocardiography, one can only speculate as to the type of arrest. Treatment of cardiac arrest is based upon the letters ABCD. Establishment of an airway (A) is the first step. If the animal is not intubated, intubation should be performed immediately. Use of a face mask in this instance is unsatisfactory because it increases the chance of gastric or ruminal insufflation and regurgitation. Controlled ventilation (breathing, B) should be instituted with one of the previously discussed techniques. External cardiac massage (C) should be instituted in an attempt to perfuse important organ systems. Because of the rigidity of the thoracic wall, this technique is of limited value, if any, in adult cattle and swine in maintaining circulation. 12 However, it may aid in movement of drugs from a peripheral vein to the central circulation. The technique may be of benefit in goats, sheep, calves, and small swine. The method of cardiac compression varies with the size of the animal. Cardiac massage of smaller food animals can be accomplished by placing the hands directly over the heart. Cardiac massage of large cattle may be accomplished by placing one's knee directly over the heart. The force required for compression varies with the size of the animal and thoracic rigidity. A minimum of60 compressions per minute is desired in small food animals and 40 compressions per minute in large food animals. 12 A pause should occur every five to ten compressions to allow for delivery of one to two ventilatory cycles. Periodic slow compression of the abdomen may promote venous return from large abdominal veins. An electrocardiogram is essential for rational drug therapy (D). If electromechanical dissociation is present, an inotropic agent is indicated to improve myocardial contractility. Inotropic agents include calcium borogluconate (1 ml per kg per hour),16 calcium chloride (1 ml per 10 kg),12 dobutamine (1 to 5 J.1g per kg per minute), 12 dopamine

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(1 to 5 J..Lg per kg per minute)/2 and epinephrine (1 to 3 J..Lg per kg).12 Recently, methoxamine (0.2 to 0.5 mg per kg) and fluids were shown to be effective in electromechanical disassociation. 13 If asystole is present, isoproterenol (0.05 to 0.1 mg per kg per minute)/2 epinephrine, or methoxamine and fluids can be used to initiate contraction. 13 Epinephrine can be repeated at 5-minute intervals. If ventricular fibrillation is present and an electrical defibrillator is not available, chemical defibrillation can be attempted. 13 Paradoxically, epinephrine may be effective in treating ventricular fibrillation. 7 Methoxamine and fluid administration have also been shown to be effective. 13 Lidocaine usually does not correct ventricular fibrillation but does aid in prevention of dysrhythmias afterwards. 13 During arrest, intravenous fluids should be administered rapidly (10 to 20 mg per kg per hour) to expand vascular volume. In animals anesthetized with halothane, epinephrine administration can cause ventricular fibrillation 7 and, therefore, should probably be the last drug used when there are alternatives. Acidosis occurs and can be treated with sodium bicarbonate at 0.5 mEq per kg per 10 minutes as indicated by blood gas analysis. 12 Following resuscitation, use of glucocorticoids should be considered to aid prevention of hypoxia-induced cerebral and pulmonary edema. 13 Treatment of cardiac arrest is often unrewarding, particularly if the underlying cause is not easily and quickly correctable. The use of intrathoracic cardiac massage, although heroic, is probably of limited value toward the animal's long-term survival. If cardiac arrest is a sign of systemic illness (for example pneumonia, shock, or white muscle disease), the chances of survival are very small. If the animal is successfully resuscitated, intensive therapy is often required during convalescence. Economic considerations may limit therapy in food animals. Other Complications Circulatory collapse and eventually cardiac arrest may be caused by dehydration, hemorrhage, inappropriate use of vasodilators, overdose of anesthetic agents and tranquilizers, hypoxemia, and endotoxic shock. The problem is recognized by deterioration of the variables used to assess cardiovascular performance. Decrease in pulse pressure, hypotension (mean arterial blood pressure [AP] < 60 mm Hg), increased capillary refill time, pale mucous membranes, and either bradycardia or tachycardia are noted. Supportive therapy consisting of rapid administration of intravenous fluids (for example, Ringer's lactate), reduction or cessation of anesthetic administration, and the use of positive inotropic agents to improve cardiac output can be instituted. Examples of positive inotropic agents are calcium borogluconate, calcium chloride, dobutamine, and dopamine. If severe bradycardia is present, atropine is indicated at 0.12 mg per kg for ruminants and 0.04 mg per kg for swine. If possible, specific therapy should be implemented to correct the cause (for example, whole blood transfusion, increased ventilation, or increased F 102)'

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Hypothermia can occur during anesthesia. Anesthetic agents interfere with heat regulation. 7 In large food animals, this is usually of little consequence. Because of their large body mass to surface area ratio, body temperature seldom decreases by more than 0.5°C during anesthesia unless a major body cavity is opened. Even if a body cavity is opened, temperature usually decreases less than 2°C. However, in smaller food animals, hypothermia can become a significant clinical problem during prolonged surgery, causing a decrease in anesthetic requirement, and should be suspected whenever recovery is delayed. Warm-water recirculating blankets placed under the animal help maintain normal body temperature. At the least, towels should be placed between the animal and surgery table. Malignant hyperthermia can occur in swine, being more common in some breeds and blood lines than others. Landrace, Hampshire, Pietrain, Yorkshire, and Poland China breeds are most commonly affected. l l It has not been reported in ruminants. Malignant hyperthermia is triggered by the halogenated anesthetic agents or succinylcholine administration. It is a genetic disorder, which once triggered, rapidly progresses to death. Signs include tachycardia, tachypnea, blotchy cutaneous cyanosis, muscle rigidity, respiratory and metabolic acidosis, and increase in body temperature (up to 114°F). Treatment is based on early recognition of signs and discontinuation of anesthetic administration. Dantrolene (a muscle relaxant) is effective both therapeutically and prophylactically at 4.0 mg per kg intravenously.4 However, its expense may limit its use to selected situations. In addition to dantrolene, therapy includes sodium bicarbonate, cold intravenous fluids, ice-water enemas, and bathing. 19 Because mortality is high, emphasis should be placed on prevention. Herd history should be obtained to establish whether stress-related deaths have occurred in related individuals. The barbiturates, steroidal anesthetics, and nitrous oxide are indicated when anesthesia is contemplated in susceptible individuals. 19 Ruminal tympany often occurs during anesthesia owing to fermentation of ingesta and the animal's inability to eructate. As tympany develops, more pressure is placed on the diaphragm, decreasing functional residual capacity and impeding ventilation. 14 In addition, tympany increases the risk of regurgitation. Therapy involves passage of a stomach tube to decompress the rumen. On occasion, one is unable to pass the stomach tube into the rumen. In these difficult cases, placing the animal in sternal recumbency aids the procedure. When that is not possible, the rumen can be decompressed with a 12-gauge needle inserted through the abdominal wall. Fortunately, ruminal tympany is usually of the nonfrothy type and decompression is easily accomplished. External pressure placed on the rumen helps expel gas from the stomach tube. Fasting the animal and depriving it of water before anesthesia quite effectively prevent ruminal tympany. This technique decreases the amount of fermentable ingesta in the rumen. Ruminal tympany can also occur during the use of nitrous oxide because it tends to ac-

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cumulate in gas-filled visci. 4 Discontinuation of nitrous oxide administration along with decompression of the rumen are recommended. Connective tissue is not as fibrous in ruminants' lungs and, therefore, excessive airway pressure can cause emphysema and tension pneumothorax more easily than in the horse. 8 Signs include increased resistance to inspiration because of tension pneumothorax and dyspnea. It is treated by placement of a chest tube and aspiration of the air. It is much easier to prevent than to treat. Do not use excessive airway pressure «30 em H 2 0) when "sighing" the animal or using IPPV. When local or regional anesthesia is used in small ruminants, overdose of local anesthetic can result in toxicity. The maximum calculated safe dose for lidocaine is 13.0 mg per kg. 7 In a 7 -kg pygmy goat presented for dehorning, the maximum dose is 91 mg or 4.5 ml of 2 per cent lidocaine. Signs of toxicity are nystagmus, muscle fasciculations and CNS stimulation progressing to opisthotonus and convulsions, and finally CNS depression and coma. 4 Treatment is directed at controlling the signs. In some cases, tranquilization is sufficient, and in others, general anesthesia is required to control convulsions. When using local anesthesia in small food animals, concentration of the local anesthetic should be diluted and particular attention given to total dose. Intracarotid injection seldom occurs in cattle because of the depth of the carotid artery in the neck. However, in small ruminants, an inadvertent intracarotid injection of a tranquilizer or anesthetic agent can occur. Severe sequelae can result from the irritating effects of the drugs on the CNS. Convulsions can occur and death is quite possible. If convulsions occur, they must be controlled with general anesthesia. Because the sequelae can be so severe, every effort must be made to avoid intracarotid injection. In swine, attempted venipuncture of the anterior vena cava can result in entry of the brachial artery.8 This technique is not recommended for injection of anesthetic drugs. In humans, loss of the hand has occurred following inadvertent injection of thiobarbiturate in the median artery.3 Paralysis of the rear limbs may occur following anterior migration of local anesthetic in the epidural space. In contrast to the horse, food animals do not become frenzied from loss of the use of their rear limbs. After the effects of the local anesthetic agent dissipate, normal nerve function returns. Provision of nonslip footing helps to prevent injuries to the animal if it tries to stand before it is able. The animal should not be encouraged to stand until action of the local anesthetic has dissipated. Assistance should be provided when it does attempt to stand. Greater anterior migration of local anesthetic can occur when the epidural space is smaller than anticipated. This may occur in overly fat or pregnant individuals. 4 ,7

POSTOPERATIVE COMPLICATIONS Thrombophlebitis can occur following perivascular injection of irritating compounds, although it does not appear to be as frequent or

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severe as in horses. For performing intravenous injections, a needle or catheter of sufficient size and length should be placed in the jugular vein and secured. Its accuracy should be verified before and after each injection. If perivascular injection has occurred, early recognition and treatment are essential to avoid massive tissue damage. Following recognition, the area should be infiltrated with normal saline to dilute the injected drug, 2 per cent lidocaine to control venospasm and promote absorption, and, if desired, a corticosteroid to help control inflammation in the area. Once the inflammatory process has started, symptomatic therapy is indicated. Hydrotherapy can be used to minimize swelling, or if desired, a cold hydrocolloidal pack can be incorporated in a bandage around the animal's neck. After the area beings to drain, hydrotherapy should continue. Depending on the severity of the lesion, surgical debridement and ligation of the jugular vein may be required to prevent exsanguination should the vessel wall erode. Injections in the mammary veins of cows should be performed with caution because abscesses can frequently occur. Corneal ulcers can occur following anesthesia. Often the animal's eyes remain open during anesthesia, allowing the cornea to dry. A bland opthalmic ointment should be instilled in the eyes to prevent drying. When the animal is in lateral recumbency, its dependent eye should be closed to prevent contact between the table surface and cornea. Because ruminants salivate copiously during anesthesia and are prone to regurgitation, the head should be positioned so that saliva and rumen contents do not contact the eye. If an ulcer does occur as evidenced by photophobia, lacrimation, and fluorescein staining, routine treatment should be instituted. 21 Food animals tend to recover from general anesthesia without the emergence delirium that can occur in horses. Consequently, long bone fractures or cervical fractures seldom occur. Aspiration pneumonia can occur following regurgitation of rumen or gastric contents. If active regurgitation occurs, the animal may inhale the material deeply into the pulmonary tree, initiating bronchospasm and physical obstruction of the airways. Signs include dyspnea and, depending on severity, cyanosis. Aminophylline can be used for bronchodilatation at 11 mg per kg intravenously over 20 minutes along with oxygen. 2 If the animal survives the initial insult, pneumonia is certain. Broad-spectrum antibiotics are indicated. Silent or passive regurgitation can occur with the same results except that there usually is not as much particulate material in the regurgitant. Similar treatment is instituted. Because of the potential severity of this complication, emphasis should be placed on prevention. Tracheal intubation is recommended and, if this is not possible, the occiput should be elevated to encourage fluids to drain from the mouth and not into the trachea. Postoperative myopathy or neuropathy can occur in cattle but does not appear to occur as often as in horses. It does not seem to be a problem in calves, sheep, swine, or goats. In horses, its occurrence has been linked to hypotension and poor patient padding and posi-

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Correct position of an animal on a surgical table.

tioning. 2o Cattle should be placed on a flat padded surface with the nondependent limbs parallel to the table and perpendicular to the body. The dependent foreleg should be drawn forward after an automobile tire inner tube is placed under the shoulder and elbow to protect the radial nerve (Fig. 2). The inner tube elevates the humerus to diminish contact between it and the table surface, and drawing the limb anterior places the weight of the thorax on the triceps muscle instead of the humerus. When in dorsal recumbency, the animal should be placed squarely on its back on a flat padded surface with the forelegs in a flexed relaxed position and the rear legs flexed and "frogged. " Postoperative myopathy or neuropathy is recognized by muscle weakness and inability to stand. Therapy is symptomatic and includes corticosteroids; phenylbutazone; fluids to maintain hydration, acidbase status, and electrolytes; and vitamin E-selenium compounds. The use of dantrolene (1.0 mg per kg) orally may be considered. Placing the animal in a sling may be helpful but can increase muscle damage. The problem may take several days to resolve and can threaten life. Again, it is better to prevent the problem by positioning the anesthetized animal properly and avoiding excessive depth of anesthesia. REFERENCES 1. Adams, H . R., Teshe, R. H., and Mercer, H. D.: Anesthetic-antibiotic relationships. J. Am. Vet. Med. Assoc., 169:409-412, 1976. 2. Ayres, J. W., Pearson, E. C., Riebold, T. W., et al.: Theophylline and dyphylline concentrations in the horse. Am. J. Vet. Res ., 46:2500-2506, 1985. 3. Brown, S. S., Lyons, S. M. , and Dundee, J. W.: Intra-arterial barbiturates: A study of some factors leading to intravascular thrombosis . Br. J. Anesth., 40:13-19, 1968.

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4. Dripps, RD., Eckenhoff, J. E., and Vandam, L. D.: Introduction to Anesthesia: The Principles of Safe Practice. Edition 6. Philadelphia, W. B. Saunders Co., 1982. 5. Gabel, A. A., Heath, R B., Ross, J. N., et al.: Hypoxia-its prevention in inhalation anesthesia in horses. Proc. Am. Assoc. Equine. Pract.: 179-196, 1966. 6. Gronert, C. A., Theze, R. A . , Milde, J. H., et al.: Catecholamine stimulation of myocardial oxygen in porcine malignant hyperthermia. Anesthesiology, 49:330, 1978. 7. Hall, L. W., and Clarke, K. W.: Veterinary Anesthesia. Edition 8. London, Bailliere Tindall, 1983. 8. Heath, R. B.: General anesthesia in ruminants. In Jennings, P. B. (eds.): The Practice of Large Animal Surgery. Philadelphia, W. B. Saunders Co., 1984, pp. 202-204. 9. Kinyon, G. E.: A new device for topical anesthesia. Anesthesiology, 56: 154, 1982. 10. Lumb, W. V., and Jones, E. W.: Veterinary Anesthesia. Edition 2. Philadelphia, Lea & Febiger, 1984. 11. McGrath, W., Rempel, W. E., Addis, P. B., et al.: Acepromazine and droperidol inhibition of halothane-induced malignant hyperthemia (porcine stress syndrome). Am. J. Vet. Res., 42:195-198, 1981. 12. Muir, W. W., and Bednarski, R M.: Equine cardiopulmonary resuscitation-Part II. Compo Cont. Educ. Pract. Vet. 5:S287-S295, 1983. 13. Muir, W. W., and Bonagura, J.: Cardiovascular emergencies in medical emergencies. In Sherding, R. G. (ed.): Medical Emergencies. New York, Churchill Livingstone, 1985, pp. 37-94. 14. Musewe, V. 0.: Respiratory mechanics, breathing patterns, ventilation and diaphragmatic electromyogram (EMG) in normal, unsedated, adult, domestic cattle (Bos taurus) breathing spontaneously in the standing and the sternal-recumbent body position, and during insufflation of the rumen with air. Ph.D. Thesis. Davis, University of California, 1978. 15. Riebold, T. W., Evans, A. T., and Robinson, N. E.: Evaluation of the demand valve for resuscitation of horses. J. Am. Vet. Med. Assoc., 176:623-626, 1980. 16. Riebold, T. W., Goble, D.O., and Geiser, D. R: Large Animal Anesthesia Principles and Techniques. Ames, Iowa State University Press, 1982. 17. Short, C. E., and Rebhum, W. C.: Complications caused by the oculocardiac reflex during anesthesia in the foal. J. Am. Vet. Med. Assoc., 176:630, 1980. 18. Thurmon, J. C., and Benson, G. J.: Anesthesia in ruminants and swine. In Howard, J. C. (ed.): Current Veterinary Therapy: Food Animal Practice. Philadelphia, W.B. Saunders Co., 1981, pp. 58-81. 19. Thurmon, J. C., and Tranquilli, W. J.: Anesthesia for cardiovascular research. In Stanton, H. C., and Merman, J. H., (eds.): Swine in Cardiovascular Research. Boca Raton, CRC Press, 1985. 20. White, N.A.: Postanesthetic recumbency myopathy in horses. Compo Cont. Ed. Pract. Vet., 4:S44-S52, 1982. 21. Whitely, R. D., and Moore, C. P.: Ocular diagnostic and therapeutic techniques in food animals. Vet. Clin. North. Am. [Large Anim. Pract.], 6:553-575, 1984. 22. Zahner, J. M., Hatch, R C., Wilson, R C., et al.: Antagonism and xylazine sedation in steers by doxapram and 4-aminopyridine. Am. J. Vet. Res., 45:2546-2551,1984. Veterinary Teaching Hospital College of Veterinary Medicine Oregon State University Corvallis, Oregon 97331