Comparison of five preanesthetic medicaments in thiopental-anesthetized cats: Antagonism by selected compounds
Authors: Hatch Roger C. , Kitzman Joseph V. , Zahner Joanne M. , Clark J. Derrell 
Published: February 2025

Effects of im injections of saline solution (groups 1, 2, 3, and 4), xylazine (2.2 mg/kg of body weight, groups 5 and 6), acepromazine (0.11 mg/kg, groups 7 and 8), ketamine (11 mg/kg, groups 9 and 10), meperidine (4.4 mg/kg, groups 11 and 12), and diazepam (1 mg/kg, groups 13 and 14) were compared in atropinized cats. Treated cats were anesthetized to loss of palpebral reflex with thiopental, iv. Within 2 minutes, the cats were given iv injections of 0.15 mg of 4-aminopyridine (4-ap) with 0.125 mg of yohimbine/kg (groups 2, 6, 8, and 10), 0.04 mg of naloxone/kg (groups 3 and 12), or 5 mg of the benzodiazepine antagonist Ro 15-1788/kg (groups 4 and 14). Groups 1, 5, 7, 9, 11, and 13 were given saline solution instead of the test antagonists. Required doses of thiopental, arousal time, walk time (measured from injection of antagonists), respiratory rate, and heart rate were recorded. Induction phenomena were also recorded. Emergence was graded as smooth, fairly smooth, fairly smooth in some cats to fairly rough in other cats, rough, or very rough.

In group 1 cats, mean arousal time (mat) was 20.1 minutes, mean walk time (mwt) was 50 minutes, and emergence was rough. In groups given saline solution as the antagonist, the mat, mwt (both expressed in minutes), and emergence, respectively, were; group 5 = 52.5, 65.5, smooth; group 7 = 15.6, 36.2, fairly smooth; group 9 = 22.5, 58.1, rough; group 11 = 31.3, 52.7, fairly smooth to fairly rough; and group 13 = 91.8, 427, very rough. In groups given 4-ap with yohimbine, mat, mwt, and emergence were; group 6 = 6.2, 12.8, smooth; group 8 = 9.5, 20.3, fairly smooth to fairly rough; and group 10 = 16.7, 64.8, rough, respectively. In group 12 (naloxone), the values were 18.3, 40.1, fairly smooth to fairly rough, respectively. In group 14 (Ro 15-1788), the values were 31.8, 63.3, fairly smooth, respectively.

Neither meperidine nor diazepam caused sedation or significant decrease in the dose of thiopental; extra restraint was necessary, particularly for diazepam-treated cats, which manifested fear, rage, vocalization, biting, clawing, and attempts to escape when handled. Xylazine, acepromazine, and ketamine made handling of cats easy and decreased the dose of thiopental 81.2%, 28.9%, and 53.2%, respectively. During induction, marked respiratory depression occurred after incremental doses of thiopental in cats treated with ketamine or meperidine, whereas xylazine rendered cats refractory to this type of respiratory depression.

Thiopental anesthesia was reversed rapidly, safely, smoothly, and permanently in atropinized cats premedicated with xylazine and given 4-ap with yohimbine iv. Acepromazine-thiopental anesthesia was shortened by these antagonists. Ketamine, meperidine, and diazepam were not satisfactory premedicants for thiopental anesthesia in cats. The results provided a basis for dose-response studies using 4-ap with yohimbine as antagonists of xylazine-barbiturate or acepromazine-barbiturate anesthesia in cats.