To determine the effect of morphine administration on commonly monitored cardio-respiratory
variables and recovery quality in horses undergoing anaesthesia and surgery.
Prospective, randomized clinical study.
Thirty-eight thoroughbred horses, 32 geldings and six mares, 3–13 years old, weighing
Materials and methods
A standard anaesthetic technique was used. Twenty minutes after induction of anaesthesia
horses received 0.1 mg kg−1 (0.1 m) or 0.2 mg kg−1 (0.2 m) morphine by intravenous injection. A control group did not receive morphine. Heart
rate, respiratory rate (fr), mean arterial pressure (MAP) and blood gases were measured before morphine administration
and every 10 minutes thereafter. Horses were positioned for 35 minutes in right lateral
recumbency for tension palatoplasty by cautery and were then moved into dorsal recumbency
for additional intraluminal surgery comprising one or more of aryepiglottic fold resection,
sub-epiglottal mucosal resection, ventriculectomy and cordectomy. A subjective recovery
score from 0 (worst) to 5 (best) was assigned by a single observer who was unaware
of treatment group. Two-way repeated measures anova, one-way anova, Kruskal–Wallis test, Mann–Whitney test, Pearson and Spearman correlation coefficients,
and chi-squared tests were used to analyse the data where appropriate.
Arterial partial pressure of oxygen (PaO2) decreased significantly over time and was significantly lower in horses that received
morphine. One horse in the control group and two horses in each of the morphine groups
had a PaO2 <13 kPa. No other significant cardiopulmonary effects were detected. Recovery scores
[median (range)] were higher in morphine recipients: 4 (2–5) in 0.1 m, 4 (3–5) in 0.2 m compared with 3 (2–4) in the control group.
Conclusions and Clinical relevance
The lower PaO2 in morphine recipients did not appear to be of clinical significance in healthy horses
because the number of horses with a low PaO2 was similar between groups. The quality of recovery was significantly better in morphine
recipients. These results indicate that morphine may be considered for use in clinical
cases although further work is required to assess the analgesic properties of the
drug in this species.