Problems associated with perioperative morphine in horses: a retrospective case analysis.

Objective: To identify the incidence of adverse effects caused by morphine 100-170 micro g kg-1 administration during surgery in horses.

Design: Retrospective case record analysis (1996-2000).

Animals: Eighty-four healthy (ASA 1 or 2) horses, mean age 5.5 +/- 3.1 (SD) years (2 months to 16 years), mean weight 524 +/- 14 kg (100-950).

Methods: Physiological data and evidence of complications were collected from the anaesthetic records of all animals anaesthetized with romifidine, ketamine, diazepam and halothane and undergoing laryngeal surgery or orchiectomy at the Royal (Dick) School of Veterinary Studies. Cases were divided into those receiving (group M+; n = 18) and those not receiving morphine (M-; n = 29), and the data compared. Values for heart and respiratory rate and mean arterial pressure were compared at 15-minute intervals between 30 and 120 minutes after induction using anova for repeated measures. The incidence of intraoperative problems was compared using Fisher's exact test. Recovery scores were compared using Student's unpaired t-test. The records of a further 37 horses undergoing umbilical herniorrhaphy (n = 5), arthroscopy (n = 29) or tarsal arthrodesis (n = 3) were also studied but not analysed statistically due to disparate treatment distribution.

Results: There were no significant differences between the M+ and M- groups. The incidence of post-operative complications such as box-walking and colic were similar in each group.

Conclusions: Morphine doses of 100-170 micro g kg-1 do not increase the risk of problems when used to provide perioperative analgesia in horses anaesthetized with romifidine, ketamine, diazepam and halothane.

Clinical Relevance: Morphine provides an acceptable and relatively inexpensive way to provide perioperative analgesia in horses.