Many anesthetic drugs, particularly the inhaled anesthetics halothane, isoflurane, and sevoflurane, tend to reduce arterial blood pressure as a result of decreased cardiac contractility and vasodilation.⁸ So anesthetizing animals incurs the potential risk of hypotension, which could damage the kidneys or other vital organs. Kidney disease is often cited as a leading cause of death in dogs,^10,11 and we think that hypotension during anesthesia could be a contributing factor. At least one expert on renal disease in animals has suggested that measuring arterial blood pressure during anesthesia (and, presumably, treating hypotension) would help reduce the likelihood of renal ischemia.¹² Moreover, a recent study of anesthetic management in people concluded that intraoperative hypotension was a significant predictor of increased mortality during the year after an anesthetic episode, suggesting that "intraoperative anesthetic management may affect outcomes over longer time periods than previously appreciated."¹³

A survey of anesthetic records at the Colorado State University Veterinary Teaching Hospital indicated that 32% of dogs were hypotensive at some point during anesthesia.¹⁴ Previously, two other published studies have reported the incidence of hypotension during anesthesia in dogs: 7% in one study¹ and 17.9% in the other.² The report with the lowest incidence of hypotension was a retrospective review of anesthesia records that relied on the anesthetists' checking a box to indicate that hypotension had occurred.¹ It is possible that the true incidence of hypotension was actually higher, as not all anesthetists may have remembered to check the box when completing the record at the end of a case. In the latter report, the criteria for defining hypotension were different (the hypotension had to be more severe) than in the study reported here: systolic arterial blood pressure less than or equal to 70 mm Hg or mean arterial blood pressure less than or equal to 50 mm Hg.² Thus, animals with systolic arterial blood pressures less than 90 mm Hg but greater than 70 mm Hg were not counted as hypotensive.

In all of these reports, data were gathered from dogs anesthetized in a university veterinary teaching hospital setting, which might imply an older or more critically ill patient population and, thus, one that is more susceptible to anesthetic-induced hypotension than is encountered in many private veterinary practices. However, a separate survey of one month's anesthesia records from dogs anesthetized for ovariohysterectomy at the Colorado State University Veterinary Teaching Hospital indicated an incidence of hypotension of 28% even in those routine cases.¹⁴

Anesthetic-related hypotension can usually be corrected. Fewer than half (six of 14) of the hypotensive dogs and cats in the current study responded (exhibited increased blood pressure) when anesthetic delivery was reduced and additional intravenous fluids were administered. For anesthetized dogs undergoing ovariohysterectomy or castration at the Colorado State University Veterinary Teaching Hospital, decreasing the anesthetic vaporizer setting and administering additional intravenous fluids resulted in improved blood pressure in 57% of hypotensive animals (Colorado State University Teaching Hospital, Fort Collins, Colo: Unpublished data). For the remaining 43% of hypotensive dogs, administering inotropic drugs, generally ephedrine or dobutamine, was required to increase blood pressure to a satisfactory level. It could be concluded that inotropic drugs should be available for use in any veterinary practice that anesthetizes animals. Recommendations for the appropriate use of inotropic drugs have been previously published.^4,15

While treatments for anesthetic-related hypotension in the above-mentioned surveys were not difficult to implement, it is unlikely that the anesthetists would have used them if blood pressure had not been measured and hypotension had not been detected. It appears that the reason many veterinarians do not worry about hypotension is that they do not recognize that it is occurring since they do not measure blood pressure during anesthesia.³ The results of this retrospective clinical study, indicating a 26.7% occurrence of anesthetic-related hypotension in this population of dogs and cats, should encourage veterinarians to consider monitoring arterial blood pressure in all anesthetized animals.

Andrea M. Gordon, CVT, BS
Westarbor Animal Hospital
6011 Jackson Road
Ann Arbor, MI 48103

Ann E. Wagner, DVM, MS, DACVP, DACVA
Department of Clinical Sciences
College of Veterinary Medicine & Biomedical Sciences
Colorado State University
Fort Collins, CO 80523