Evaluation of postoperative respiratory function by serial blood gas analysis in dogs treated with transdermal fentanyl

Objective: To perform blood gas analysis of the respiratory response to transdermal fentanyl in dogs which have experienced an open‐chest surgical procedure. Design: Prospective trial. Setting: Veterinary Teaching Hospital Surgical Research and Student Laboratory. Intervention: Dogs were purchased for a surgical laboratory. Students performed a cranial abdominal exploratory and diaphragmatic hernia repair. Sixteen dogs were divided into 2 groups. Dogs received transdermal fentanyl (group F), using an average dose of 4.8 µg/kg/hr, applied to the caudal‐lateral abdomen 22 hours before surgery, or intravenous buprenorphine (group B; 0.02 mg/kg) given 1 hour prior to anesthetic induction and every 6 hours postoperatively. All dogs received intravenous acepromazine (0.05 mg/kg) preoperatively and every 6 hours postoperatively. Dogs were instrumented with carotid artery catheters. Measurements and main results: Arterial blood gas values were analyzed every 2 hours postoperatively. Plasma fentanyl levels were analyzed every 4 hours postoperatively. The mean carbon dioxide tension (PCO₂) did not exceed 45 mmHg in either group. The range in mean PCO₂ levels was 32.9 (± 3.4)?38.1 (± 3.9) in group B and 34.7 (± 3.25)?43.6 (± 5.5) in group F. At 2 time points, the mean PCO₂ was significantly lower in group B compared with normal levels in group F. Hypoxemia occurred in both the groups. The range in mean oxygen tension (PO₂) was 76.5 (± 18.3)?91.1 (± 16.3) in group B and 76.0 (± 10.8)?96.6 (± 7.6) in group F. There was no significant difference in PO₂ between groups. Levels of fentanyl considered to be analgesic were maintained for the postoperative period. Conclusions: The use of a relatively high dose of transdermal fentanyl did not induce postoperative hypoventilation as evidenced by serial arterial blood gas analysis in this model.