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Some Characteristics of Ruminants and Swine that Complicate Management of General Anesthesia
Anesthesia
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Some Characteristics of Ruminants and Swine that Complicate Management of General Anesthesia E. P. Steffey, V.M.D., Ph.D.*
Numerous characteristics of ruminants and swine complicate anesthetic management. Some of these are so intimidating that general anesthesia is avoided whenever possible in these species despite the fact that it may be convenient, improve surgical conditions, and provide an environment less dangerous to the animal, veterinarian, and support personnel. My aim is to provide an overview of the special and sometimes unique characteristics of ruminants and swine that require careful consideration in formulating an anesthetic plan. For organizational purposes, I highlight species' similarities and differences according to the body-systems approach. My inclusion of specific information is arbitrary at times because there is significant systems pathophysiology overlap.
GENERAL CONSIDERATIONS The size and temperament of animals under consideration are quite varied, and these characteristics markedly influence selection of anesthetic method.
Size Within this focused area of clinical interest, there is tremendous diversity. Swine range in weight from less than 1 kg for newborn piglets to more than 300 kg for adults. The weights of small domestic ruminants fall into a similar lower, although less extreme, upper range. Cattle may reach 1000 kg. * Diplomate, American College of Veterinary Anesthesiologists; Professor and Chairman, Department of Surgery, and Chief, Anesthesia/Critical Patient Care Service, Veterinary Medical Teaching Hospital, University of California School of Veterinary Medicine, Davis, California.
Veterinary Clinics of North America: Food Animal Practice--:Vol. 2, No.3, November 1986
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Animal size determines anesthetic equipment used (for example, endotracheal tubes, breathing circuit). Depending on the extent of anesthetic management required and the type of clinical practice, the diversity of individual size in this animal group requires a large equipment inventory. Animal size also influences animal handling ease and the function or dysfunction of certain body systems.
Temperament Domestic sheep and goats are usually docile and, along with piglets and calves, are manageable without a great deal of excitement. Adult pigs and cattle, however, may be particularly unruly and even combative. Generally, as animal temperament becomes worse, the need for specialized personnel and facilities (for example, physical restraint) or more pronounced preanesthetic sedation (that is, chemical restraint) increases. Unfortunately, even necessary extremes of physical and chemical restraint are often associated with important complications, including exertional hyperthermia or rhabdomyolysis, physical injury to animal, staff, and facilities, and profound chemically induced vital organ (for example, cardiopulmonary) depression.
DIGESTIVE SYSTEM Most common hazards associated with general anesthesia of ruminants relate to the interaction of the digestive and respiratory systems. For example, mechanical blockage of respiratory gas flow may occur because saliva or stomach contents are aspirated into the respiratory passages. Further, the large and unique stomach of the ruminant may in certain circumstances force the diaphragm cranial and drastically reduce lung volume.
Saliva Ruminants secrete a large volume of saliva that is rich in bicarbonate, phosphate, and other ions. Saliva is a major factor in the maintenance of fluid volume and steady-state conditions of pH and ionic composition in the rumen. Secretion in ruminants is continuous (even during general anesthesia), although the rate is altered depending on conditions. Total volume in awake sheep may be 6 to 16 L per 24 hours 11 and about 50 L in adult cattle. 17 Compared with water, bovine saliva has a low surface tension. This property promotes formation of foam. 17 Therefore, saliva both quantitatively and qualitatively serves as an important source of airway obstruction material in anesthetized ruminants whose airway is not protected by endotracheal intubation. Although answers are not readily apparent, other issues relating to the loss of copious quantities of saliva during general anesthesia deserve query. For example, of what importance to the maintenance of body fluid balance is the salivary loss accompanying general anes-
GENERAL ANESTHESIA IN RUMINANTS AND SWINE
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thesia? Furthermore, saliva is rich in bicarbonate and acts as a ruminal buffer. The neutralizing reaction of the large volume of saliva helps to maintain favorable conditions in the rumen for growth of bacteria and protozoa. Of what consequence to ruminal flora stability and postanesthetic morbidity is this perianesthetic salivary loss? Ruminant Stomach The ruminant stomach occupies nearly three fourths of the abdominal cavity and is related closely to the diaphragm. In adult cattle, it has an average capacity of 115 to 150 L. 8 The stomach is composed of the rumen, reticulum, omasum, and abomasum, which in the adult represent about 80, 5, 7, and 8 per cent, respectively, of the total stomach capacity. In sheep and goats, the capacity of the stomach is 15 to 18 L. The acute and long-range vitality of the ruminant are intimately related to a functional stomach. Events related to anesthetic management may depress function and, by influencing factors that impact on the weight and distribution of the ruminant stomach, also compromise cardiopulmonary function. 14 , 15, 26 Rumen Function. The rumen may be regarded as a large fermentation chamber that provides a suitable environment for the continuous culture of a mixed population of bacteria and protozoa. The ingested ration is the substrate on which the microflora grow and produce fermentation end products, which are absorbed from the rumen as a primary energy source. Indeed, the rumen wall in effect serves as a semipermeable membrane separating rumen content and the extracellular fluid volume of the animal. The economy of the whole animal is ultimately linked to events in the rumen. Changes on one side of the rumen influence the other side. Consequently, when the animal's ration is abruptly changed or halted (for example, due to preanesthetic fast) or rumen motility is reduced or halted (for example, by certain preanesthetic and anesthetic drugs), the normal fermentation patterns change. With this change, rumen-content pH is influenced and may additionally depress rumen mobility and destroy large quantities of the normal ruminal microflora. Clinical experience suggests that under usual conditions, relatively healthy ruminants presented for short-term anesthesia cope successfully with these insults. However, when these systems are already compromised by diseases, circumstances related to anesthetic management may overwhelm usual compensatory mechanisms and add to postanesthetic morbidity. Body Position. In order to determine the nature and magnitude of effect of recumbency (sternal) on abdominal events and, in turn, respiratory system mechanics, Musewe studied healthy, awake 14 COWS. He found that when cattle position themselves in sternal recumbency, there is (1) a rise in intraruminal and intraperitoneal pressure; (2) evidence that the diaphragm is forced further into the chest cavity by a higher pressure on the peritoneal side than on the pleural
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side; (3) an increase in respiratory resistance, suggesting a reduction in the caliber of small or intermediate airways associated with a decrease in end-expired lung volume; and (4) a breathing pattern readjustment whereby inspiratory frequency is slowed and duration is prolonged, probably by an alteration in the contractile activity of inspiratory muscles to minimize the energy cost of breathing. Clinical experience and the recent work of Adetunji and colleagues at the Ontario Veterinary College suggest that dorsal (supine) posture produces even greater changes and respiratory embarrassment. 1 Stomach Distention. Food, water, or gas distention of the rumen may magnify the influence of body posture and further depress respiratory function. 14, 15, 23, 26 Sedative and anesthetic drugs also increase the severity of compromise by depressing homeostatic mechanisms that attempt to correct for insults and limit respiratory depression. For example, loss of eructation accompanies heavy sedation or general anesthesia. In the absence of eructation, ruminal gas accumulation relates directly to the rate of gas production. Ruminal bloat can be minimized by animal fasting or at least omission of green grass and other highly fermentable foodstuffs before anesthesia. Postprandial gas production of about 30 L per hour has been reported in cattle. 4 In the absence of an adequate fast, the magnitude of insult produced by gastrointestinal gas production can be decreased by shortening anesthesia and recumbency times. When included in the anesthetic management, nitrous oxide may further augment the expansion of the gastrointestinal tract. 18 Volume expansion occurs because the greater solubility of nitrous oxide in biologic solvents compared with that of gastrointestinal gases such as methane results in greater net transfer of nitrous oxide molecules into the gut. 7 , 21 Regurgitation and Aspiration. The danger of regurgitation and inhalation of stomach contents associated with general anesthesia is present with swine and ruminants. The incidence of pulmonary aspiration is minimized, especially with swine, by withholding water for 6 to 12 hours and food for 12 to 24 hours before induction of general anesthesia and recumbency. Despite a preanesthetic fast of up to 48 hours, regurgitation is especially common in ruminants and may occur during both light (active regurgitation, vomiting) and deep (passive or silent regurgitation) stages of general anesthesia. 22 , 23 Presumably, the active process requires a complicated and coordinated series of unsuppressed reflex mechanisms intent on rejecting unwanted material from the pharynx and other upper digestive tract structures. Passive regurgitation, on the other hand, is presumably the result of relaxed esophageal musculature and transruminal pressure gradients. Normally, nature prevents respiratory system aspiration of foreign material by a series of mechanisms that take advantage of an intact reflex arc and mechanical structures of the upper airway. Whenever sensors are desensitized, nervous pathways are blocked or weakened, or effector structures are weakened (as with sedative or
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anesthetic drugs), the risk of aspiration of foreign material into the airways increases. Experimental evidence, largely from monogastric animals, and clinical experience indicate that the consequences of asplfation of stomach contents depend on the amount and distrubution of inhaled material, its pH, and the presence or absence of food, particulate matter, and bacteria. 22 , 30 In nonruminant animals, perhaps including swine, gastric hydrochloric acid appears to be the major factor responsible for morbidity and mortality following lung aspiration. The toxic effect of acid in the lungs is frequently equated with chemical burn. After acid aspiration, there is immediate reflex airway closure and destruction of both type II alveolar cells, which produce surfactant, and the pulmonary capillary lining cells. The loss of alveolar and capillary integrity results in pulmonary edema and hemorrhage, which in turn cause hypoxemia and arterial hypotension. Under normal circumstances, rumen pH remains within the range of 5.5 to 7.0. During fasting, the ruminal pH of normal animals may rise to B.O. Therefore, presumably of greater importance in ruminants is the pulmonary damage caused by aspiration of solid food particles, which cause reflex airway constriction and mechanically plug airways, and bacteriologically active material, which contaminates lung regions with lethal results.
CARDIOPULMONARY SYSTEMS
Peripheral Circulation. Adult swine tend to be stocky in body build, with thick and often crusty skin. Accordingly, superficial sites for intravenous injection are difficult to locate even in healthy animals. Hypovolemia may further decrease accessibility of peripheral veins. Auricular veins on the external surface of the ear flap are usually most easily located. Respiratory System Lack of familiarity with species' differences in respiratory system anatomy and function has always complicated anesthetic management of animals. Upper Airway. Endotracheal intubation of swine and ruminants can be very difficult. Swine and ruminants have jaws that do not open very widely, and they have long narrow oral cavities and deeply set laryngeal openings. Even with available laryngoscopes, this anatomic arrangement makes direct visualization of the larynx difficult in swine and small ruminants and nearly impossible in adult cattle. Lung. Anatomy. There are major differences in lung anatomy among species. 13 These have recently been reviewed in depth with regard to their influence on differences in lung function and reaction to injury.16, 25 Because of problems associated with endotracheal in-
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tubation, one anatomic difference that deserves added emphasis is that of bronchial supply to lungs. The lungs of domestic animals are divided into lobes. In most animals, inspired air is supplied to the lobes via the trachea, which bifurcates into right and left mainstem bronchi. These principal bronchi then enter the right and left lung, respectively, and continue to branch. Ruminants and swine differ from other species in that the right cranial lobe bronchus arises directly from the trachea at about the level of the third rib rather than from the mainstem bronchus. 8 Function. General anesthesia and recumbency usually result in respiratory depression and impairment of arterial oxygenation. Ruminants and probably large swine, like horses, seem to be at even greater risk than smaller species for developing complications of respiratory function. Marked elevations in arterial carbon dioxide tension (PaC0 2) with an accompanying reduction in arterial pH are regularly observed in deeply sedated, recumbent or anesthetized, spontaneously breathing ruminants. Laboratory20 and clinical experience has been that the magnitude of PaC0 2 increase is generally much greater in ruminants than that found in similarly lightly anesthetized, spontaneously breathing small animals such as dogs 19 and at least young swine. 24 For example, surgically unstimulated healthy dogs lightly anesthetized with halothane in oxygen usually maintain a PaC0 2 below 50 mm Hg (normal awake value is 38 to 44 mm Hg). However, PaC0 2 values from similarly studied cattle are usually greatly in excess of 55 to 60 mm Hg. These findings suggest that in ruminants, anesthetics cause greater depression to the central nervous system medullary respiratory center's drive to breathe or to the diaphragm and intercostal muscles, thereby making the respiratory pump less effective at moving sufficient quantities of gas into and out of the lung. Thus, either anesthetics depress the recognition of a need to breathe, or the events related to general anesthesia reduce the ability to breathe, or both. Studies that further define specific anesthetic effects on respiration in food animals are presently lacking. A second major complication associated with general anesthesia and recumbency of food animals, especially ruminants, is an impairment in arterial oxygenation. Arterial oxygen tension (Pa02) is usually well below values predicted on the basis of inspired oxygen tension (P102) and is manifested as a large difference between alveolar P0 2 and Pa02 (that is, PA02 - Pa02). Indeed, some animals breathing pure oxygen may be hypoxemic (Pa02 < 80 mm Hg). The classic causes of deficiencies in oxygenation are insufficient movement of a volume of air per unit time into and out of the lung (that is, hypoventilation); reduced PI02 (for example, with altitude); diffusion limitation; arteriovenous blood shunt; and mismatching of ventilation and lung blood flow. Because I believe considerations related to hypoventilation and P102 are well understood, and diffusion limitation is not normally considered a problem in healthy animals, I
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will focus brief attention on the imbalance of ventilation (V) and perfusion (0). The primary function of the lung is gas exchaI1ge. The efficiency of gas exchang~ is related to how well fresh gas (V) is matched. with ~enous bloQd.(Q). An ideal situation is a near-perfect match of V and Q (that is, Y/Q = 0.8.-~.0). Regions of lung that are underventilated relative to Q have a V/Q ratio less than 0.8. A shunt is a circumstance in which some blood reaches the arterial system without passing through a ventilated lung unit. The blood does not participate in gas exchange. It is usually thought of as a direct anatomic venous:to.-arterial connection that bypasses functional alveoli; therefore, V/Q = O. Functionally, the same thing occurs when perfusion continues to collap~e9 alveoli (atelectflS!s). Obviously, as the quantity of lung with low V/Q « 0.8) or zero V/Q increases, aortic Pa02 decreases. More in-depth reviews are readily available. 28 , 29 The mechanisms that ut:lderlie respiratory dysfunction in ruminants relate heavily to V and Q inter.reJationships. For example, gravity causes a vertical stratification of V/Q in lungs of standing animals. Prolonged abnormal body positioning usually accompanies. geneql.l anesthesia. Thi~ places an unusual force of gravity on both ~ apd Q, but especially Q, and causes a substantial m~lc~istribution ofV/Q, frequently with resultant overall reduction in V/Q. Weights of tissues overlying the dependent lung (for example, chest wall, diaphragm, gastrointestinal tract and contents) are significant impediments to normal pulmonary function. Not uncommonly, food animals presented for anesthesia are pregnant. Further, changes that mechanically depress lung function are expected to increase with duration of pregnancy and increasing fetal size. These demands for increasing effort coupled with drug-induced depres~ion o( the respiratory pump further augment the maldistribution of V and Q. A common sequela in the more dependent lung areas is atelectasis. Inhaled anesthetics may additionally compromise gas exchange by suppressing the vital regulation of pulmonary circulation via pulmonary hypoxic vasoconstriction, and thereby prevent compensato~y ~hanges that act to minimize the deterioration of this matching of VI Q. 12
NEUROMUSCULAR SYSTEMS Ischemic Myopathy and Neuropathy It is a general clinical impression that improper body position (especially of limbs) and prolonged immobile recumbency may result in postanesthetic myopathy or neuropathy. It is also generally appreciated that the incidence and severity of pathology increase directly with the size of the patient. Therefore, adult cattle and large swine, like horses, are particularly at risk. 5 The exact cause of this syndrome, which manifests itself in the
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immediate postanesthetic period, is unknown and may include multiple etiologies. It is likely that regional ischemia plays an important role. Prolonged tissue compression secondary to immobilization, recumbency, and overlying skeletal and muscle mass result in diminished local blood flow and cellular necrosis. Diminished local blood flow may also result from external compression of major blood vessels or drug-induced reduction in cardiac output or alteration in regional blood flow and perfusion pressure.
Malignant Hyperthermia Malignant hyperthermia (MH) is a stress- or chemically induced, genetic myopathy characterized by a rapid and marked rise in body temperature in humans and animals, usually without sepsis. 2 , 3 The term malignant refers to the rapid progression of events leading to irreversibility and death. In those who are genetically susceptible, MH is an established complication of exposure to some anesthetic or anesthetic adjuvant drugs or to high circulating levels of catecholamines produced by severe stress (for example, porcine stress syndrome). Malignant hyperthermia was first described in human patients by Denborough and colleagues at the Australian National University in 1960 6 and later in swine by Hall and colleagues 9 and Jones and colleagues. Io Although the syndrome has been described in a number of species since then, only in swine does it occur with any regularity. Certain breeds of swine are particularly prone to the condition, including Poland China, Landrace, and Pietrain. The condition is less common in the Large White, Yorkshire, and Hampshire breeds. In addition to anesthetic considerations for the individual animal, MH (or a qualitatively similar syndrome) is of significant concern and economic impact to some commercial swine herds. The syndrome (under these circumstances, more commonly known as porcine stress syndrome) is important to the swine industry because stress initiation in awake susceptible swine causes accelerated metabolism, hyperthermia, and deterioration of muscle resulting in a pale, soft, exudative meat that is unfit for sale for human consumption. Susceptible swine usually show no prior evidence of myopathy but can be identified by exposure to halothane. 27
SUMMARY Successful anesthetic management of food animals depends on knowledge of basic principles and techniques of anesthesia common to most species. When specifically considering food animals, additional emphasis is directed toward animal size, temperament, and anatomy. Respiratory failure induced by a variety of mechanisms is a major complication of special importance in ruminants. Problems relate especially to difficulty of endotracheal intubation, inhalation of saliva and rumen contents, and reduced lung gas volume caused by
GENERAL ANESTHESIA IN RUMINANTS AND SWINE
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abdominal organ (especially rumen)-induced cranial diaphragmatic displacement. When evaluating swine for anesthesia, specific additional management considerations include accessibility of peripheral blood vessels, ease of endotracheal intubation, and porcine malignant hyperthermia.
REFERENCES 1. Adetunji, A., McDonell, W. N., and Pascoe, P. J.: Cardiopulmonary effects of xylazine, acetylpromazine and chloral hydrate in supine cows. Presented at the 2nd International Congress of Veterinary Anesthesia, Sacramento, Oct. 7-10, 1985. 2. Aldrete, J. A., and Britt, B. A. (eds.): Second International Symposium on Malignant Hyperthermia. New York, Grune and Stratton, 1978. 3. Britt, B. A. (ed.): Malignant hyperthermia. Int. Anesthesiol. Clin., 17:1979. 4. Church, D. C.: Digestive Physiology and Nutrition of Ruminants. Edition 2. Corvallis, Oregon, D.C. Church, 1975. 5. Cox, V. S., McGrath, C. J., and Jorgenson, S. E.: The role of pressure damage in pathogenesis of the downer cow syndrome. Am. J. Vet. Res., 43:26-31, 1982. 6. Denborough, M. A., and Lovell, R R H.: Anaesthetic deaths in a family. Lancet, 2:45, 1960. 7. Eger, E. I., II: Pharmacokinetics. In Eger, E. I., II (ed.): Nitrous Oxide/N 2 0. New York, Elsevier, 1985, pp. 81-107. 8. Getty, R: Sisson and Grossman's The Anatomy of the Domestic Animals. Edition 5. Philadelphia, W.B. Saunders Co., 1975. 9. Hall, L. W., Woolf, N., Bradley, J. W. P., et al.: Unusual reaction to suxamethonium chloride. Br. Med. J., 2:1305, 1966. 10. Jones, E. W., Nelson, T. E., Anderson, I. L., et al.: Malignant hyperthermia of swine. Anesthesiology, 36:42-51, 1972. 11. Kay, R N. R: The rate of flow and composition of various salivary secretions in sheep and calves. J. Physiol., 150:515-537, 1960. 12. Marshall, B. E., and Marshall, C.: Anesthesia and pulmonary circulation. In Covino, B. G., Fuzzard, H. A., Rehder, K., et al. (eds.): Effects of Anesthesia. Bethesda, American Physiology Society, 1958, pp. 121-136. 13. McLaughlin, R F., Tyler, W. S., and Canada, R. 0.: A study of the subgross pulmonary anatomy in various mammals. Am. J. Anat., 108:149-165, 1961. 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. Musewe, V. 0., Gillespie, J. R, and Berry, J. D.: Influence of ruminal insufflation on pulmonary function and diaphragmatic electromyography in cattle. Am. J. Vet. Res., 40:26-31, 1979. 16. Robinson, N. E.: Some functional consequences of species differences in lung anatomy. Adv. Vet. Sci. Compo Med., 26: 1-33, 1982. 17. Somers, M.: Saliva secretion and its functions in ruminants. Aust. Vet. J., 33:297301, 1957. 18. Steffey, E. P., and Eger, E. I., II: Nitrous oxide in veterinary practice and animal research. In Eger, E. I., II (ed.): Nitrous Oxide/N 2 0. New York, Elsevier, 1985, pp. 305-312. 19. Steffey, E. P., Gillespie, J. R, Berry, J. D., et al.: Circulatory effects of halothane and halothane-nitrous oxide anesthesia in the dog: Spontaneous ventilation. Am. J. Vet. Res., 36:197-200, 1975. 20. Steffey, E. P., and Howland, D., Jr.: Halothane anesthesia in calves. Am. J. Vet. Res., 40:372-376, 1979. 21. Steffey, E. P., Johnson, B. H., Eger, E. I., II, et al.: Nitrous oxide: Effect on accumulation rate and uptake of bowel gases. Anesth. Analg., 58:405-408, 1979.
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22. Stewardson, R. H., and Nyhus, L. M.: Pulmonary aspiration: An update. Arch. Surg., 112:1192-1197, 1977. 23. 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., 1981, pp. 58-81. 24. 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. 25. Tyler, W. S.: Comparative subgross anatomy of lungs: Pleuras, interlobular septa and distal airways. Am. Rev. Respir. Dis., 128:S32-S36, 1983. 26. Ungerer, T., Orr, J. A., Bisgard, C. E., et al.: Cardiopulmonary effects of mechanical distension of the rumen in nonanesthetized sheep. Am. J. Vet. Res., 37:807-810, 1976. 27. Webb, A. J.: The halothane test: A practical method of eliminating porcine stress syndrome. Vet. Rec., 106:410-412, 1980. 28. West, J. B.: Ventilation/Blood Flow and Cas Exchange. Edition 2. Philadelphia, F. A. Davis Co., 1970. . 29. West, J. B.: Pulmonary Pathophysiology-The Essentials. Edition 2. Baltimore, Williams and Wilkins, 1982. 30. Wynne, J. W., and Modell, J. H.: Respiratory aspiration of stomach contents. Ann. Intern. Med. 87:466-474, 1977. Department of Surgery School of Veterinary Medicine University of California Davis, California 95616
0749-0720/86
$00.00 + $.20
Some Characteristics of Ruminants and Swine that Complicate Management of General Anesthesia E. P. Steffey, V.M.D., Ph.D.*
Numerous characteristics of ruminants and swine complicate anesthetic management. Some of these are so intimidating that general anesthesia is avoided whenever possible in these species despite the fact that it may be convenient, improve surgical conditions, and provide an environment less dangerous to the animal, veterinarian, and support personnel. My aim is to provide an overview of the special and sometimes unique characteristics of ruminants and swine that require careful consideration in formulating an anesthetic plan. For organizational purposes, I highlight species' similarities and differences according to the body-systems approach. My inclusion of specific information is arbitrary at times because there is significant systems pathophysiology overlap.
GENERAL CONSIDERATIONS The size and temperament of animals under consideration are quite varied, and these characteristics markedly influence selection of anesthetic method.
Size Within this focused area of clinical interest, there is tremendous diversity. Swine range in weight from less than 1 kg for newborn piglets to more than 300 kg for adults. The weights of small domestic ruminants fall into a similar lower, although less extreme, upper range. Cattle may reach 1000 kg. * Diplomate, American College of Veterinary Anesthesiologists; Professor and Chairman, Department of Surgery, and Chief, Anesthesia/Critical Patient Care Service, Veterinary Medical Teaching Hospital, University of California School of Veterinary Medicine, Davis, California.
Veterinary Clinics of North America: Food Animal Practice--:Vol. 2, No.3, November 1986
507
508
E. P.
STEFFEY
Animal size determines anesthetic equipment used (for example, endotracheal tubes, breathing circuit). Depending on the extent of anesthetic management required and the type of clinical practice, the diversity of individual size in this animal group requires a large equipment inventory. Animal size also influences animal handling ease and the function or dysfunction of certain body systems.
Temperament Domestic sheep and goats are usually docile and, along with piglets and calves, are manageable without a great deal of excitement. Adult pigs and cattle, however, may be particularly unruly and even combative. Generally, as animal temperament becomes worse, the need for specialized personnel and facilities (for example, physical restraint) or more pronounced preanesthetic sedation (that is, chemical restraint) increases. Unfortunately, even necessary extremes of physical and chemical restraint are often associated with important complications, including exertional hyperthermia or rhabdomyolysis, physical injury to animal, staff, and facilities, and profound chemically induced vital organ (for example, cardiopulmonary) depression.
DIGESTIVE SYSTEM Most common hazards associated with general anesthesia of ruminants relate to the interaction of the digestive and respiratory systems. For example, mechanical blockage of respiratory gas flow may occur because saliva or stomach contents are aspirated into the respiratory passages. Further, the large and unique stomach of the ruminant may in certain circumstances force the diaphragm cranial and drastically reduce lung volume.
Saliva Ruminants secrete a large volume of saliva that is rich in bicarbonate, phosphate, and other ions. Saliva is a major factor in the maintenance of fluid volume and steady-state conditions of pH and ionic composition in the rumen. Secretion in ruminants is continuous (even during general anesthesia), although the rate is altered depending on conditions. Total volume in awake sheep may be 6 to 16 L per 24 hours 11 and about 50 L in adult cattle. 17 Compared with water, bovine saliva has a low surface tension. This property promotes formation of foam. 17 Therefore, saliva both quantitatively and qualitatively serves as an important source of airway obstruction material in anesthetized ruminants whose airway is not protected by endotracheal intubation. Although answers are not readily apparent, other issues relating to the loss of copious quantities of saliva during general anesthesia deserve query. For example, of what importance to the maintenance of body fluid balance is the salivary loss accompanying general anes-
GENERAL ANESTHESIA IN RUMINANTS AND SWINE
509
thesia? Furthermore, saliva is rich in bicarbonate and acts as a ruminal buffer. The neutralizing reaction of the large volume of saliva helps to maintain favorable conditions in the rumen for growth of bacteria and protozoa. Of what consequence to ruminal flora stability and postanesthetic morbidity is this perianesthetic salivary loss? Ruminant Stomach The ruminant stomach occupies nearly three fourths of the abdominal cavity and is related closely to the diaphragm. In adult cattle, it has an average capacity of 115 to 150 L. 8 The stomach is composed of the rumen, reticulum, omasum, and abomasum, which in the adult represent about 80, 5, 7, and 8 per cent, respectively, of the total stomach capacity. In sheep and goats, the capacity of the stomach is 15 to 18 L. The acute and long-range vitality of the ruminant are intimately related to a functional stomach. Events related to anesthetic management may depress function and, by influencing factors that impact on the weight and distribution of the ruminant stomach, also compromise cardiopulmonary function. 14 , 15, 26 Rumen Function. The rumen may be regarded as a large fermentation chamber that provides a suitable environment for the continuous culture of a mixed population of bacteria and protozoa. The ingested ration is the substrate on which the microflora grow and produce fermentation end products, which are absorbed from the rumen as a primary energy source. Indeed, the rumen wall in effect serves as a semipermeable membrane separating rumen content and the extracellular fluid volume of the animal. The economy of the whole animal is ultimately linked to events in the rumen. Changes on one side of the rumen influence the other side. Consequently, when the animal's ration is abruptly changed or halted (for example, due to preanesthetic fast) or rumen motility is reduced or halted (for example, by certain preanesthetic and anesthetic drugs), the normal fermentation patterns change. With this change, rumen-content pH is influenced and may additionally depress rumen mobility and destroy large quantities of the normal ruminal microflora. Clinical experience suggests that under usual conditions, relatively healthy ruminants presented for short-term anesthesia cope successfully with these insults. However, when these systems are already compromised by diseases, circumstances related to anesthetic management may overwhelm usual compensatory mechanisms and add to postanesthetic morbidity. Body Position. In order to determine the nature and magnitude of effect of recumbency (sternal) on abdominal events and, in turn, respiratory system mechanics, Musewe studied healthy, awake 14 COWS. He found that when cattle position themselves in sternal recumbency, there is (1) a rise in intraruminal and intraperitoneal pressure; (2) evidence that the diaphragm is forced further into the chest cavity by a higher pressure on the peritoneal side than on the pleural
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side; (3) an increase in respiratory resistance, suggesting a reduction in the caliber of small or intermediate airways associated with a decrease in end-expired lung volume; and (4) a breathing pattern readjustment whereby inspiratory frequency is slowed and duration is prolonged, probably by an alteration in the contractile activity of inspiratory muscles to minimize the energy cost of breathing. Clinical experience and the recent work of Adetunji and colleagues at the Ontario Veterinary College suggest that dorsal (supine) posture produces even greater changes and respiratory embarrassment. 1 Stomach Distention. Food, water, or gas distention of the rumen may magnify the influence of body posture and further depress respiratory function. 14, 15, 23, 26 Sedative and anesthetic drugs also increase the severity of compromise by depressing homeostatic mechanisms that attempt to correct for insults and limit respiratory depression. For example, loss of eructation accompanies heavy sedation or general anesthesia. In the absence of eructation, ruminal gas accumulation relates directly to the rate of gas production. Ruminal bloat can be minimized by animal fasting or at least omission of green grass and other highly fermentable foodstuffs before anesthesia. Postprandial gas production of about 30 L per hour has been reported in cattle. 4 In the absence of an adequate fast, the magnitude of insult produced by gastrointestinal gas production can be decreased by shortening anesthesia and recumbency times. When included in the anesthetic management, nitrous oxide may further augment the expansion of the gastrointestinal tract. 18 Volume expansion occurs because the greater solubility of nitrous oxide in biologic solvents compared with that of gastrointestinal gases such as methane results in greater net transfer of nitrous oxide molecules into the gut. 7 , 21 Regurgitation and Aspiration. The danger of regurgitation and inhalation of stomach contents associated with general anesthesia is present with swine and ruminants. The incidence of pulmonary aspiration is minimized, especially with swine, by withholding water for 6 to 12 hours and food for 12 to 24 hours before induction of general anesthesia and recumbency. Despite a preanesthetic fast of up to 48 hours, regurgitation is especially common in ruminants and may occur during both light (active regurgitation, vomiting) and deep (passive or silent regurgitation) stages of general anesthesia. 22 , 23 Presumably, the active process requires a complicated and coordinated series of unsuppressed reflex mechanisms intent on rejecting unwanted material from the pharynx and other upper digestive tract structures. Passive regurgitation, on the other hand, is presumably the result of relaxed esophageal musculature and transruminal pressure gradients. Normally, nature prevents respiratory system aspiration of foreign material by a series of mechanisms that take advantage of an intact reflex arc and mechanical structures of the upper airway. Whenever sensors are desensitized, nervous pathways are blocked or weakened, or effector structures are weakened (as with sedative or
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anesthetic drugs), the risk of aspiration of foreign material into the airways increases. Experimental evidence, largely from monogastric animals, and clinical experience indicate that the consequences of asplfation of stomach contents depend on the amount and distrubution of inhaled material, its pH, and the presence or absence of food, particulate matter, and bacteria. 22 , 30 In nonruminant animals, perhaps including swine, gastric hydrochloric acid appears to be the major factor responsible for morbidity and mortality following lung aspiration. The toxic effect of acid in the lungs is frequently equated with chemical burn. After acid aspiration, there is immediate reflex airway closure and destruction of both type II alveolar cells, which produce surfactant, and the pulmonary capillary lining cells. The loss of alveolar and capillary integrity results in pulmonary edema and hemorrhage, which in turn cause hypoxemia and arterial hypotension. Under normal circumstances, rumen pH remains within the range of 5.5 to 7.0. During fasting, the ruminal pH of normal animals may rise to B.O. Therefore, presumably of greater importance in ruminants is the pulmonary damage caused by aspiration of solid food particles, which cause reflex airway constriction and mechanically plug airways, and bacteriologically active material, which contaminates lung regions with lethal results.
CARDIOPULMONARY SYSTEMS
Peripheral Circulation. Adult swine tend to be stocky in body build, with thick and often crusty skin. Accordingly, superficial sites for intravenous injection are difficult to locate even in healthy animals. Hypovolemia may further decrease accessibility of peripheral veins. Auricular veins on the external surface of the ear flap are usually most easily located. Respiratory System Lack of familiarity with species' differences in respiratory system anatomy and function has always complicated anesthetic management of animals. Upper Airway. Endotracheal intubation of swine and ruminants can be very difficult. Swine and ruminants have jaws that do not open very widely, and they have long narrow oral cavities and deeply set laryngeal openings. Even with available laryngoscopes, this anatomic arrangement makes direct visualization of the larynx difficult in swine and small ruminants and nearly impossible in adult cattle. Lung. Anatomy. There are major differences in lung anatomy among species. 13 These have recently been reviewed in depth with regard to their influence on differences in lung function and reaction to injury.16, 25 Because of problems associated with endotracheal in-
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tubation, one anatomic difference that deserves added emphasis is that of bronchial supply to lungs. The lungs of domestic animals are divided into lobes. In most animals, inspired air is supplied to the lobes via the trachea, which bifurcates into right and left mainstem bronchi. These principal bronchi then enter the right and left lung, respectively, and continue to branch. Ruminants and swine differ from other species in that the right cranial lobe bronchus arises directly from the trachea at about the level of the third rib rather than from the mainstem bronchus. 8 Function. General anesthesia and recumbency usually result in respiratory depression and impairment of arterial oxygenation. Ruminants and probably large swine, like horses, seem to be at even greater risk than smaller species for developing complications of respiratory function. Marked elevations in arterial carbon dioxide tension (PaC0 2) with an accompanying reduction in arterial pH are regularly observed in deeply sedated, recumbent or anesthetized, spontaneously breathing ruminants. Laboratory20 and clinical experience has been that the magnitude of PaC0 2 increase is generally much greater in ruminants than that found in similarly lightly anesthetized, spontaneously breathing small animals such as dogs 19 and at least young swine. 24 For example, surgically unstimulated healthy dogs lightly anesthetized with halothane in oxygen usually maintain a PaC0 2 below 50 mm Hg (normal awake value is 38 to 44 mm Hg). However, PaC0 2 values from similarly studied cattle are usually greatly in excess of 55 to 60 mm Hg. These findings suggest that in ruminants, anesthetics cause greater depression to the central nervous system medullary respiratory center's drive to breathe or to the diaphragm and intercostal muscles, thereby making the respiratory pump less effective at moving sufficient quantities of gas into and out of the lung. Thus, either anesthetics depress the recognition of a need to breathe, or the events related to general anesthesia reduce the ability to breathe, or both. Studies that further define specific anesthetic effects on respiration in food animals are presently lacking. A second major complication associated with general anesthesia and recumbency of food animals, especially ruminants, is an impairment in arterial oxygenation. Arterial oxygen tension (Pa02) is usually well below values predicted on the basis of inspired oxygen tension (P102) and is manifested as a large difference between alveolar P0 2 and Pa02 (that is, PA02 - Pa02). Indeed, some animals breathing pure oxygen may be hypoxemic (Pa02 < 80 mm Hg). The classic causes of deficiencies in oxygenation are insufficient movement of a volume of air per unit time into and out of the lung (that is, hypoventilation); reduced PI02 (for example, with altitude); diffusion limitation; arteriovenous blood shunt; and mismatching of ventilation and lung blood flow. Because I believe considerations related to hypoventilation and P102 are well understood, and diffusion limitation is not normally considered a problem in healthy animals, I
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will focus brief attention on the imbalance of ventilation (V) and perfusion (0). The primary function of the lung is gas exchaI1ge. The efficiency of gas exchang~ is related to how well fresh gas (V) is matched. with ~enous bloQd.(Q). An ideal situation is a near-perfect match of V and Q (that is, Y/Q = 0.8.-~.0). Regions of lung that are underventilated relative to Q have a V/Q ratio less than 0.8. A shunt is a circumstance in which some blood reaches the arterial system without passing through a ventilated lung unit. The blood does not participate in gas exchange. It is usually thought of as a direct anatomic venous:to.-arterial connection that bypasses functional alveoli; therefore, V/Q = O. Functionally, the same thing occurs when perfusion continues to collap~e9 alveoli (atelectflS!s). Obviously, as the quantity of lung with low V/Q « 0.8) or zero V/Q increases, aortic Pa02 decreases. More in-depth reviews are readily available. 28 , 29 The mechanisms that ut:lderlie respiratory dysfunction in ruminants relate heavily to V and Q inter.reJationships. For example, gravity causes a vertical stratification of V/Q in lungs of standing animals. Prolonged abnormal body positioning usually accompanies. geneql.l anesthesia. Thi~ places an unusual force of gravity on both ~ apd Q, but especially Q, and causes a substantial m~lc~istribution ofV/Q, frequently with resultant overall reduction in V/Q. Weights of tissues overlying the dependent lung (for example, chest wall, diaphragm, gastrointestinal tract and contents) are significant impediments to normal pulmonary function. Not uncommonly, food animals presented for anesthesia are pregnant. Further, changes that mechanically depress lung function are expected to increase with duration of pregnancy and increasing fetal size. These demands for increasing effort coupled with drug-induced depres~ion o( the respiratory pump further augment the maldistribution of V and Q. A common sequela in the more dependent lung areas is atelectasis. Inhaled anesthetics may additionally compromise gas exchange by suppressing the vital regulation of pulmonary circulation via pulmonary hypoxic vasoconstriction, and thereby prevent compensato~y ~hanges that act to minimize the deterioration of this matching of VI Q. 12
NEUROMUSCULAR SYSTEMS Ischemic Myopathy and Neuropathy It is a general clinical impression that improper body position (especially of limbs) and prolonged immobile recumbency may result in postanesthetic myopathy or neuropathy. It is also generally appreciated that the incidence and severity of pathology increase directly with the size of the patient. Therefore, adult cattle and large swine, like horses, are particularly at risk. 5 The exact cause of this syndrome, which manifests itself in the
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immediate postanesthetic period, is unknown and may include multiple etiologies. It is likely that regional ischemia plays an important role. Prolonged tissue compression secondary to immobilization, recumbency, and overlying skeletal and muscle mass result in diminished local blood flow and cellular necrosis. Diminished local blood flow may also result from external compression of major blood vessels or drug-induced reduction in cardiac output or alteration in regional blood flow and perfusion pressure.
Malignant Hyperthermia Malignant hyperthermia (MH) is a stress- or chemically induced, genetic myopathy characterized by a rapid and marked rise in body temperature in humans and animals, usually without sepsis. 2 , 3 The term malignant refers to the rapid progression of events leading to irreversibility and death. In those who are genetically susceptible, MH is an established complication of exposure to some anesthetic or anesthetic adjuvant drugs or to high circulating levels of catecholamines produced by severe stress (for example, porcine stress syndrome). Malignant hyperthermia was first described in human patients by Denborough and colleagues at the Australian National University in 1960 6 and later in swine by Hall and colleagues 9 and Jones and colleagues. Io Although the syndrome has been described in a number of species since then, only in swine does it occur with any regularity. Certain breeds of swine are particularly prone to the condition, including Poland China, Landrace, and Pietrain. The condition is less common in the Large White, Yorkshire, and Hampshire breeds. In addition to anesthetic considerations for the individual animal, MH (or a qualitatively similar syndrome) is of significant concern and economic impact to some commercial swine herds. The syndrome (under these circumstances, more commonly known as porcine stress syndrome) is important to the swine industry because stress initiation in awake susceptible swine causes accelerated metabolism, hyperthermia, and deterioration of muscle resulting in a pale, soft, exudative meat that is unfit for sale for human consumption. Susceptible swine usually show no prior evidence of myopathy but can be identified by exposure to halothane. 27
SUMMARY Successful anesthetic management of food animals depends on knowledge of basic principles and techniques of anesthesia common to most species. When specifically considering food animals, additional emphasis is directed toward animal size, temperament, and anatomy. Respiratory failure induced by a variety of mechanisms is a major complication of special importance in ruminants. Problems relate especially to difficulty of endotracheal intubation, inhalation of saliva and rumen contents, and reduced lung gas volume caused by
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abdominal organ (especially rumen)-induced cranial diaphragmatic displacement. When evaluating swine for anesthesia, specific additional management considerations include accessibility of peripheral blood vessels, ease of endotracheal intubation, and porcine malignant hyperthermia.
REFERENCES 1. Adetunji, A., McDonell, W. N., and Pascoe, P. J.: Cardiopulmonary effects of xylazine, acetylpromazine and chloral hydrate in supine cows. Presented at the 2nd International Congress of Veterinary Anesthesia, Sacramento, Oct. 7-10, 1985. 2. Aldrete, J. A., and Britt, B. A. (eds.): Second International Symposium on Malignant Hyperthermia. New York, Grune and Stratton, 1978. 3. Britt, B. A. (ed.): Malignant hyperthermia. Int. Anesthesiol. Clin., 17:1979. 4. Church, D. C.: Digestive Physiology and Nutrition of Ruminants. Edition 2. Corvallis, Oregon, D.C. Church, 1975. 5. Cox, V. S., McGrath, C. J., and Jorgenson, S. E.: The role of pressure damage in pathogenesis of the downer cow syndrome. Am. J. Vet. Res., 43:26-31, 1982. 6. Denborough, M. A., and Lovell, R R H.: Anaesthetic deaths in a family. Lancet, 2:45, 1960. 7. Eger, E. I., II: Pharmacokinetics. In Eger, E. I., II (ed.): Nitrous Oxide/N 2 0. New York, Elsevier, 1985, pp. 81-107. 8. Getty, R: Sisson and Grossman's The Anatomy of the Domestic Animals. Edition 5. Philadelphia, W.B. Saunders Co., 1975. 9. Hall, L. W., Woolf, N., Bradley, J. W. P., et al.: Unusual reaction to suxamethonium chloride. Br. Med. J., 2:1305, 1966. 10. Jones, E. W., Nelson, T. E., Anderson, I. L., et al.: Malignant hyperthermia of swine. Anesthesiology, 36:42-51, 1972. 11. Kay, R N. R: The rate of flow and composition of various salivary secretions in sheep and calves. J. Physiol., 150:515-537, 1960. 12. Marshall, B. E., and Marshall, C.: Anesthesia and pulmonary circulation. In Covino, B. G., Fuzzard, H. A., Rehder, K., et al. (eds.): Effects of Anesthesia. Bethesda, American Physiology Society, 1958, pp. 121-136. 13. McLaughlin, R F., Tyler, W. S., and Canada, R. 0.: A study of the subgross pulmonary anatomy in various mammals. Am. J. Anat., 108:149-165, 1961. 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. Musewe, V. 0., Gillespie, J. R, and Berry, J. D.: Influence of ruminal insufflation on pulmonary function and diaphragmatic electromyography in cattle. Am. J. Vet. Res., 40:26-31, 1979. 16. Robinson, N. E.: Some functional consequences of species differences in lung anatomy. Adv. Vet. Sci. Compo Med., 26: 1-33, 1982. 17. Somers, M.: Saliva secretion and its functions in ruminants. Aust. Vet. J., 33:297301, 1957. 18. Steffey, E. P., and Eger, E. I., II: Nitrous oxide in veterinary practice and animal research. In Eger, E. I., II (ed.): Nitrous Oxide/N 2 0. New York, Elsevier, 1985, pp. 305-312. 19. Steffey, E. P., Gillespie, J. R, Berry, J. D., et al.: Circulatory effects of halothane and halothane-nitrous oxide anesthesia in the dog: Spontaneous ventilation. Am. J. Vet. Res., 36:197-200, 1975. 20. Steffey, E. P., and Howland, D., Jr.: Halothane anesthesia in calves. Am. J. Vet. Res., 40:372-376, 1979. 21. Steffey, E. P., Johnson, B. H., Eger, E. I., II, et al.: Nitrous oxide: Effect on accumulation rate and uptake of bowel gases. Anesth. Analg., 58:405-408, 1979.
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22. Stewardson, R. H., and Nyhus, L. M.: Pulmonary aspiration: An update. Arch. Surg., 112:1192-1197, 1977. 23. 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., 1981, pp. 58-81. 24. 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. 25. Tyler, W. S.: Comparative subgross anatomy of lungs: Pleuras, interlobular septa and distal airways. Am. Rev. Respir. Dis., 128:S32-S36, 1983. 26. Ungerer, T., Orr, J. A., Bisgard, C. E., et al.: Cardiopulmonary effects of mechanical distension of the rumen in nonanesthetized sheep. Am. J. Vet. Res., 37:807-810, 1976. 27. Webb, A. J.: The halothane test: A practical method of eliminating porcine stress syndrome. Vet. Rec., 106:410-412, 1980. 28. West, J. B.: Ventilation/Blood Flow and Cas Exchange. Edition 2. Philadelphia, F. A. Davis Co., 1970. . 29. West, J. B.: Pulmonary Pathophysiology-The Essentials. Edition 2. Baltimore, Williams and Wilkins, 1982. 30. Wynne, J. W., and Modell, J. H.: Respiratory aspiration of stomach contents. Ann. Intern. Med. 87:466-474, 1977. Department of Surgery School of Veterinary Medicine University of California Davis, California 95616