Sedation and General Anesthesia in Ruminants Cynthia M. Trim, BVSc, MRCVS, DVA, DACVA Department of Veterinary Medicine and Surgery College of Veterinary Medicine University o f Missouri-Columbia Columbia, Missouri Introduction During rumination in the conscious animal, vagal activity produces reticular contraction forcing ingesta against the There is nothing magical about anesthesia for animals cardia while reflux into the pharynx is dependent upon an classed as Food Producing species. Generally, they will intact parasympathetic nervous system to the respond to anesthetic drugs in the same way as the dog or pharyngoesophageal sphincter and esophagus. Atropine cat, and satisfactory anesthesia can be accomplished with may reduce the incidence of regurgitation by reducing the same drugs that are used in small animal practice, given reticular contractions. However, the risk of regurgitation is the appropriate preparation, equipment, and time. more influenced by the relaxing effect of anesthetic drugs on However, confusion remains as to the "best" anesthetic the pharyngo-esophageal and gastro-esophageal sphincters. technique, e.g., for a bull for penile surgery or a goat for Experimental data indicate that the pharyngo-esophageal cystotomy. In a practice situation, the major drawback to sphincter is the most resistant barrier,1 but clinical the administration of an anesthetic is cost. Often the experience has demonstrated that some sedatives, especially economic value of the animal is insufficient to warrant xylazine,3 and all general anesthetic drugs depress utilization of expensive drugs or to promote acquisition of pharyngeal tone (i.e. the swallowing reflex) in varying necessary equipment; anesthetic machines, surgery tables, degrees. The most important factor, therefore, becomes the and monitoring devices. Consequently, anesthesia becomes degree of pressure inside the rumen. The most effective a question of which is the simplest, most economical, and method of maintaining low rumen pressure and decreasing safest technique that will allow surgery to be performed. It is the incidence of regurgitation is to reduce the volume of difficult to recommend specific anesthetic techniques. While rumenal contents. This can be accomplished by withholding pitfalls can be pinpointed, precautions suggested, and food for 36 hours and water for 6-8 hours prior to the pharmacological actions of drug combinations described, procedure. The adverse effects of withholding food for this the practitioner must make these choices depending on the period of time are apparently minimal in the healthy animal. value of the animal and the availability of equipment. Mitchell and Williams have reported that only a mild This article is concerned with the most frequently metabolic alkalosis occurred in sheep following 48 hours of encountered problems associated with sedation and/or starvation.2 anesthesia in ruminants and gives recommendations to Forceful regurgitation sometimes occurs on induction of reduce or eliminate the risk of these problems during general anesthesia. Typically, in a lightly anesthetized subsequent surgery or surgical procedures. Further, types patient, this occurs during intubation of the trachea. It is and effects of sedatives and general anesthetics which are probable that stimulation of the larynx causes the glottis to used routinely are also considered. close and inspiration during this time results in a strong negative intra-thoracic pressure. This pressure difference Potential Problems causes rumenal contents to enter the esophagus and initiates peristaltic waves which move the material towards the A. Regurgitation and pulmonary aspiration pharynx. Esophageal contractions are easily observed and B. Bloat, influencing adequacy of ventilation warn of impending regugitation. Alternatively, coughing, C. Inadequate oxygenation in the adult bovine stimulated by laryngeal manipulation during intubation, D. Injury in the perioperative period also may initiate regurgitation through an acute rise in A. Regurgitation. Regurgitation of rumenal contents rumenal pressure. Clinical impression suggest that a lower and subsequent pulmonary aspiration is always a potential incidence of regurgitation occurs if surgical anesthesia is hazard of heavy sedation or general anesthesia in the induced in the ruminant before endotracheal intubation is ruminant. performed. Regurgitation while an endotracheal tube is in This paper is published by permission of the Editor, California Veterinarian and the author. d Xylazine, Bayer NOVEMBER, 1981 137 scours... 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Squibb & Sons, Inc. 870-501 Issued: Nov. 1980 place is of little concern provided that the pharynx is cleared mismatch, 1) withhold food for 36 hours to reduce the and drained and the nasal cavity flushed prior to extubation. weight of abdominal contents on the diaphragm, 2) maintain Recommendations: To prevent or reduce regurgitation a good cardiac output and arterial blood pressure by and pulmonary aspiration, management of the ruminant avoiding large doses of sedative or tranquilizers and by under heavy sedation or general anesthesia must include 1) maintaining a light plane of general anesthesia. prior withholding of food for 36 hours and water for 6-8 Treatment: In the absence of an anesthetic machine, hours, 2) endotracheal intubation even though anesthesia is oxygen can be administered by a small stomach tube passed maintained with injectable agents, 3) positioning the head through the ventral nasal meatus or into an endotrachael with the nostrils lower than the larynx to allow free drainage tube. Adequate oxygenation can usually be maintained in of saliva and regurgitated fluid, and 4) avoid moving the the face of moderate ventilation-perfusion mismatch by animal once it is anesthetized. Regurgitation and aspiration delivering 15-25 L O 2/ minute into the pharynx or 10-15 L occurring in the anesthetized adult bovine as a result of O 2/minute into the trachea. rolling it over leaves the veterinarian liable for negligence.3 D. Injury. Injury which might occur during induction B. Bloat and Ventilation. The decrease in intestinal of anesthesia as a result of an awkward fall from standing motility which results from atropine administration, position to recumbency can be avoided by first casting the sedation, or general anesthesia permits gas from bacterial animal with ropes while muscle tone is still present. Tilting fermentation to accumulate. If food has not been withheld, surgical tables and rotating chutes also offer ideal means of distension of the rumen will be evident within one hour. The restraint. increased pressure of the rumen on the diaphragm severely The most frequent type of injury is forelimb lameness interferes with ventilation. The magnitude of this problem is which is manifested upon recovery from sedation or general illustrated in Figure 1 which depicts a three-month-old 100 anesthesia. The lameness is due to radial nerve damage kg Holstein calf which had been anesthetized for one hour and/or myositis caused by ischemia during the period of with only halothane in 100% oxygen. The animal had not recumbency. The clinical appearance of lameness due to been starved. Analysis of an arterial blood sample at this radial nerve damage is illustrated in Figure 3. A 310 kg time showed a pH 7.14, pCO 2 75 mmHg and pO 2 86 Holstein bull was anesthetized for one and one-half hours in mmHg. Normal values are pH 7.40, pCO 2 40 mmHg and right lateral recumbency for radiography with thiopental pO 2 500-600 mmHg. Thus the animal had a severe and halothane. Upon recovery, the bull was unable to extend respiratory acidosis and the oxygen level, although the forelimb and had lost conscious proprioception of the adequate, was dramatically below the value normally foot. The limb was unable to bear weight during the expected on 100% oxygen. This degree of ventilatory following 36 hours. Damage to the radial nerve usually embarrassment may have resulted in death of the animal due occurs where the nerve passes around the shoulder joint or to hypoxia if it had been breathing air (21% oxygen). lies in the musculo-spiral groove of the humerus. Myositis Recommendations: To prevent or reduce the incidence invariably involves the triceps brachii muscles but may also of bloat and impaired ventilation, food should be withheld include the deltoideus, pectoral, or brachchiocephalicus for 36 hours. muscles. Clinical experience indicates that a high incidence Treatment: It is important to prevent bloat occurring of radial nerve damage and/or myositis exists when an since passage of a stomach tube rarely achieves successful