Anesthesia-related hypotension in a small-animal practice - Veterinary Medicine
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Anesthesia-related hypotension in a small-animal practice
This retrospective study examines the occurrence of hypotension in dogs and cats undergoing elective proceduresand should help you decide whether to routinely monitor arterial blood pressure in all anesthetized animals.


VETERINARY MEDICINE


Blood pressure measurement

Baseline blood pressure measurements were not obtained in awake animals. During anesthesia, systolic arterial blood pressure was measured with an oscillometric blood pressure monitor (Pulse Ox/NIBP 6004—SurgiVet) in about 75% of the animals and a Doppler ultrasonic flow detector with sphygmomanometer (Hadeco Mini Dop ES-100 VX—Jorgensen Laboratories) in about 25% of the animals. In each case, the blood pressure monitor was applied as described in the literature, using a cuff width as close to 40% of the limb circumference as possible.4 Measurements were performed at least every 10 minutes or more often if low readings were obtained. A systolic blood pressure measurement less than 90 mm Hg was considered to indicate hypotension. For this study, the occurrence of hypotension was calculated as the percentage of the 101 anesthetized animals that had at least one systolic arterial blood pressure measurement of less than 90 mm Hg.

When hypotension was detected, the treatment depended on which anesthetist was in charge of the case: one anesthetist always attempted to correct hypotension, while the other anesthetist did not have the experience to do so. At the time of the study, the practice lacked a standard protocol for responding to a hypotensive incident. When attempted, treatment for hypotension included decreasing the isoflurane or sevoflurane vaporizer setting by 0.5% to 1% and administering an intravenous fluid bolus of 5 to 10 ml/kg. If blood pressure did not improve within five to 10 minutes, a second fluid bolus of 5 ml/kg was administered. Inotropic and pressor drugs were not available in the practice at the time these animals were anesthetized.

Statistical analysis was not performed on the blood pressure measurements.

RESULTS

Of the 59 dogs anesthetized during the study period, 13 (22%) met the criteria for hypotension. Of the 42 anesthetized cats, 14 (33%) met the criteria for hypotension. The overall occurrence of hypotension in this study population was 26.7%.

Of the 27 animals that were hypotensive, 13 received no treatment and 14 were treated by reducing anesthetic delivery and administering intravenous fluids. Six of the treated animals responded with an increased systolic arterial blood pressure measurement of 90 mm Hg or above within five to 15 minutes after treatment was initiated; eight treated animals showed no improvement.

DISCUSSION

The results of this retrospective clinical survey indicate that hypotension may occur during anesthesia in relatively healthy dogs and cats anesthetized for elective procedures. Hypotension was detected in at least one animal in every anesthetic group, indicating that hypotension can occur in any anesthetized animal, no matter what drugs are used. Because of the large variation in sample size among anesthetic regimens and our desire not to overinterpret the data, no statistical analysis was performed on the blood pressure measurements reported in this study.

The noninvasive methods used to measure systolic arterial blood pressure in this study are convenient and expeditious, although not as accurate as direct blood pressure measurement.4 In particular, the Doppler method tends to underestimate systolic blood pressure in cats by about 10 to 14 mm Hg.6 So it is possible that some of the animals counted as hypotensive in the current survey were not truly hypotensive. Nevertheless, it seems more prudent to treat animals whose measured blood pressures may be falsely low than to ignore the possibility of hypotension in all anesthetized animals. The risk of complications from an unnecessary bolus of intravenous fluid in an otherwise healthy animal is probably low, as it has been reported that administering intravenous fluids at rates of up to 90 ml/kg/hr does not cause pulmonary edema in normal dogs and cats.7

As the goal of this retrospective review was to document whether hypotension occurred during anesthesia, no attempt was made to define specific causes of hypotension. Because no baseline blood pressure measurements were obtained in awake animals, it is uncertain whether these dogs and cats had normal blood pressure before anesthesia. However, as they were considered to be otherwise healthy, and intraoperative hemorrhage was minimal, the most likely reason for hypotension during anesthesia was the anesthetic drugs.8

Vital organs such as the brain and kidneys have the ability to adjust blood supply for their metabolic needs through autoregulation of their vascular beds. This autoregulation is effective only if the mean arterial blood pressure is between 60 and 160 mm Hg.9 Mean arterial pressure is generally 20 to 30 mm Hg less than systolic arterial blood pressure, suggesting that adequate perfusion could be maintained at systolic arterial blood pressure between 80 to 90 mm Hg and 180 to 190 mm Hg.


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Source: VETERINARY MEDICINE,
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