Effects of bupivacaine and morphine on the electroencephalogram and postoperative pain in castrated dogs

Abstract

Objective: To compare the effects of locally administered bupivacaine and systemic morphine, separately or in combination, on the electroencephalogram (EEG) and postoperative pain in castrated dogs.

Study design: Randomized blinded clinical study.

Animals: A group of 21 healthy mixed-breed dogs.

Methods: Dogs were randomly assigned to one of three groups (n = 7 per group): morphine (0.5 mg kg^-1), bupivacaine (2 mg kg^-1) or a combination of both. Premedication consisted of acepromazine (0.05 mg kg^-1) (all groups) and morphine (morphine and combination groups only) administered subcutaneously. Anaesthesia was induced with intravenous propofol to effect and maintained with halothane in oxygen. Bupivacaine was infiltrated at the incision site 30 minutes prior to surgery in bupivacaine and combination groups. Systolic arterial blood pressure (SAP), heart rate (HR) and haemoglobin oxygen saturation (SpO₂) were recorded. All dogs underwent an open castration via a routine prescrotal approach. EEG was recorded throughout anaesthesia. Median frequency (F₅₀), 95% spectral edge frequency (F₉₅) and total power (P_tot) were derived from raw EEG traces. Postoperative pain was assessed at 1, 3, 6 and 9 hours using a short form of the Glasgow Composite Measure Pain Scale. EEG and pain scores were compared among groups using a generalized linear mixed model and Wilcoxon-Mann-Whitney odds macro, respectively. Data were presented as mean ± standard deviation (p < 0.05).

Results: The morphine and bupivacaine groups had a significantly higher F₅₀ and F₉₅ and a lower P_tot than the combination group during castration. Postoperative pain scores did not significantly differ between bupivacaine and combination groups; both groups had significantly lower pain scores than the morphine group. SAP, HR and SpO₂ did not differ significantly among groups.

Conclusions and clinical relevance: Presurgical incisional infiltration of bupivacaine, alone or with a systemic opioid, reduces intraoperative nociception and postoperative pain in dogs undergoing castration.

Keywords: bupivacaine; canine castration; electroencephalogram; local analgesia; morphine.