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.

Hypotension, usually defined as mean arterial blood pressure less than 60 mm Hg or systolic arterial blood pressure less than 90 mm Hg, is reportedly one of the most common complications associated with general anesthesia in dogs and cats.1,2 Although most university veterinary teaching hospitals and some private practices routinely measure arterial blood pressure in anesthetized animals, many private veterinary practices unfortunately do not.3 Pulse strength or quality, which is commonly assessed by manual palpation, depends on the difference between systolic and diastolic arterial blood pressure and is not necessarily indicative of mean arterial blood pressure or perfusion pressure.4 So the incidence of anesthetic-related hypotension in healthy dogs and cats undergoing elective surgeries in the typical private practice setting is unknown since most anesthetists only assess pulse strength or quality.3 If blood pressure is not measured, hypotension cannot be recognized and corrected.

The purpose of this study was to document the occurrence of hypotension in dogs and cats anesthetized in a private veterinary practice. Our hypothesis was that hypotension is a frequent complication during anesthesia, even in relatively healthy animals undergoing elective procedures.

MATERIALS AND METHODS

Patient selection

This study involved a retrospective review of anesthesia records of healthy animals anesthetized in a private veterinary practice for elective surgery. As such, owner consent for this retrospective study was not sought. Animals were cared for according to the practice's routine protocols. The records of all dogs and cats that were anesthetized for elective procedures during a three-month period by one of the authors (Andrea M. Gordon) or another technician colleague were reviewed. This patient group comprised 101 client-owned animals (42 cats, 59 dogs). The procedures for which anesthesia was indicated included ovariohysterectomy, castration, onychectomy, and dental prophylaxis. All the patients were considered to be physical status Class I (a normal patient with no organic disease) or Class II (a patient with mild systemic disease).5

Anesthetic protocols

Anesthetic drugs were selected based on the patient's species, physical status, and temperament and the attending veterinarian's preference. The dogs were either not premedicated or were premedicated with atropine (0.03 mg/kg) and morphine (0.2 mg/kg) administered intramuscularly. Fifteen of the premedicated dogs also received acepromazine (0.01 mg/kg) intramuscularly to provide additional sedation. Some of the cats received no premedication or atropine (0.03 mg/kg). Others received acepromazine (0.01 mg/kg), and butorphanol (0.22 mg/kg) intramuscularly. Alternatively, six cats undergoing onychectomy were premedicated with ketamine (22 mg/kg) and acepromazine (0.2 mg/kg) intramuscularly.

Anesthetic induction was achieved in all the patients by administering ketamine (5.6 mg/kg) and diazepam (0.28 mg/kg) intravenously or with isoflurane or sevoflurane in 50% oxygen, with 50% nitrous oxide, delivered by mask. The patients were intubated with appropriately sized endotracheal tubes, at which time nitrous oxide administration was discontinued and either isoflurane or sevoflurane, delivered through a circle breathing system in 100% oxygen, was used to maintain anesthesia. Vaporizer settings during maintenance were 1.5% to 2% for isoflurane and 2% to 3% for sevoflurane. Oxygen flow rates were 800 ml/min for animals less than 20 lb (9.1 kg), 900 ml/min for animals 20 to 40 lb, and 1 L/min for animals greater than 40 lb (18.2 kg). All animals except for cats undergoing castration only had an intravenous catheter in place and were given lactated Ringer's solution at 5 ml/kg/hr intravenously.