Postoperative respiratory effects of intravenous fentanyl compared to intravenous methadone in dogs following spinal surgery.

Objective To evaluate the 24-hour postoperative respiratory effects of either intravenous fentanyl administered as a constant rate infusion or boluses of methadone, in dogs following spinal surgery, assessed by serial arterial blood gas analyses. Study design Prospective, randomized clinical study. Animals Thirty-two healthy dogs (American Society of Anesthesiologists I/II) anaesthetized for elective caudal thoracic and/or lumbar decompression spinal surgery. Methods Dogs were assigned randomly to be administered a fentanyl constant rate infusion (5 mg kg(-1) hour(-1); group F, n = 14) or methadone boluses (0.2 mg kg(-1), every 4 hours; group M, n = 15) postoperatively for 24 hours. Each dog's anaesthesia protocol was customized. Arterial blood samples were collected from an arterial cannula, placed under anaesthesia, at 4, 8, 12 and 24 hours postextubation, while breathing room air. Cardiorespiratory variables, Glasgow composite pain scale (GCPS) and sedation (SED) scores were also recorded at these time points. Independent t tests, repeated measures ANOVA and Mann-Whitney U tests were used. Significance was defined as p < 0.05. Results There were no significant differences found between groups in any of the overall mean values or at any time point for values of partial pressure of oxygen [13.9 +/- 2.1 kPa (103.9 +/- 16.1 mmHg) and 12.6 +/- 2.0 kPa (94.7 +/- 15.2 mmHg)], partial pressure of carbon dioxide [4.8 +/- 0.6 kPa (36 +/- 4.2 mmHg) and 4.9 +/- 0.6 kPa (36.5 +/- 4.5 mmHg)], pH (7.38 +/- 0.03 and 7.40 +/- 0.03), bicarbonate (21.5 +/- 2.3 mM and 21.9 +/- 6.6 mM) and base excess (-3.4 +/- 2.6 mM and -2 +/- 3 mM) for groups F and M, respectively. Cardiorespiratory variables, GCPS and SED scores were also similar between groups. Conclusions and clinical relevance At the doses studied, neither fentanyl nor methadone caused respiratory depression postoperatively in dogs following caudal thoracic and/or lumbar spinal surgery.