FRANK J. MILNE STATE-OF-THE-ART LECTURE


animal anesthetic machines have 10 L/min flow capacity. The development of specialized anesthetic ma-


chines and ventilators and the increased clinical availability of arterial blood gas analysis to adjust ventilatory variables facilitated the completion of longer, more complex surgical procedures. Al- though, it was suspected from laboratory studies, the increased use of arterial blood gas analysis con- firmed that a small, but significant number of anes- thetized horses breathing oxygen concentrations in excess of 90%, particularly those placed in dorsal recumbency, did not maintain expected levels of ar- terial oxygenation even when they were ventilated to produce arterial partial pressure of carbon dioxide


(PaCO2) levels in the normal range. Ideally, horses breathing oxygen concentrations in excess of 90% should have arterial partial pressures of oxygen


(PaO2) in excess of 400 mm Hg, but this is infre- quent. The PaO2 in some cases falls below those levels seen in standing healthy horses breathing


room air (approximately 90 to 100 mmHg). This “relative hypoxemia” has been the subject of over 90 scientific publications since the advent of convenient arterial blood gas analysis in the 1970s. Current thought is that the majority of the impairment of oxygenation occurs because of physiologic shunts or blood flow through the lung that does not pass by alveoli that are being appropriately ventilated.47 The alveoli that are not being appropriately venti- lated appear shrunken or atelectic, due to a combi- nation of compression and absorption of the gases contained in the alveolar space. One of the early, more interesting papers on the subject appeared in 1987. Nyman placed small-bore, extended-length endotracheal tubes into the bronchi leading to the diaphragmatic lung lobes of adult horses.48 This al- lowed for selective ventilation of the diaphragmatic lobes with the application of positive end expiratory pressure (PEEP). Arterial partial pressures of ox- ygen increased 3 to 3.5 times with the application of selective mechanical ventilation with PEEP to the diaphragmatic lobes alone. These results sug- gested that the diaphragmatic lung lobes had collapsed or were occluded prior to selective ventila- tion. While the technique is impractical for clinical use, it established that reduced ventilation of the diaphragmatic lung lobes is a major contributor to differences between alveolar and arterial oxygen tension differences.


7. Monitoring and Complications of Anesthesia


Post-operative myopathy, rhabdomyolysis, or tying- up syndrome is a significant complication of equine anesthesia but its incidence has been greatly re- duced based on the evidence found and the recom- mendations made in a series of papers. The topic was the subject of a session at the 1978 annual meeting of the American Association of Equine Prac- titioners. As part of the session, Klein49 presented a review of 50 cases of post-operative myopathy in


the horse and discussed intrinsic and management factors affecting risk. Between 3% and 4% of horses anesthetized over a 2-year period in the study had neurologic or muscular deficits (localized and generalized) following anesthesia. No direct causes were established although 46% of myopathic horses had mean arterial blood pressures below 55 mm Hg for 30 minutes or longer and duration of anesthesia (2.9 hours) was longer than in a compar- ison group. Cited potential contributors to myop- athy included difficulties in maintaining anesthesia including movement, pronounced cardiopulmonary depression, and increases in body temperature. This paper supported the growing recognition of post-anesthetic complications and suggested that hypotension was a potential contributor. The significance of arterial hypotension as a factor


in postoperative myopathy was established by a pa- per from Grandy et al in 1987.50 Six horses were anesthetized for 3 hours on 2 occasions, once with mean arterial blood pressures maintained in the range of 85 to 95 mm Hg (normotensive) and once with mean arterial blood pressures maintained in the range of 55 to 65 mm Hg (hypotensive). There were no significant complications after the normoten- sive anesthesia but every horse in the hypotensive group had muscle dysfunction with increases in serum enzymes indicative of muscle damage. Three of six animals were euthanized due to their condition. This study provided dramatic evidence of the association between arterial hypotension and post-anesthetic my- opathy. Its conclusions have been confirmed by a number of subsequent retrospective studies and led to a number of additional investigations looking at meth- ods of monitoring and manipulating arterial blood pressure under anesthesia. Monitoring arterial blood pressure in anesthe-


tized horses and striving to maintain the mean ar- terial blood pressure in excess of 60 to 70mmHg are arguably the most important components of safe anesthetic practice, particularly when inhalant an- esthetics are employed. A number of methods have been and are being employed in support of blood pressure including the administration of large volumes of isotonic fluids, calcium solutions, inhalant-sparing anesthetic adjuncts, vasopressors and drugs used to increase myocardial contractility. The most consistently useful drug for increasing arterial blood pressure in the anesthetized horse is dobutamine. Dobutamine was compared to a sim- ilar agent, dopamine, in a paper by Swanson et al in 1985.51 Horses were anesthetized in two experi- mental groups and received dopamine (3, 5, 10 ug/ kg/min, IV) on one occasion and dobutamine (3, 5, 10 ug/kg/min,IV) on another. Both drugs produced positive inotropic effects at doses that increased con- tractility without causing tachycardia. This study demonstrated that dobutamine was more predict- able than dopamine for support of arterial blood pressure in horses anesthetized with halothane.


AAEP PROCEEDINGS  Vol. 66  2020 157


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