Effects of preanesthetic administration of morphine on gastroesophageal reflux and regurgitation during anesthesia in dogs

Objective-To determine the effect of morphine administered prior to anesthesia on the incidence of gastroesophageal reflux (GER) in dogs during the subsequent anesthetic episode. Animals-90 dogs (30 dogs/group). Procedure-The randomized prospective clinical study included healthy dogs with no history of vomiting. Dogs were scheduled to undergo elective orthopedic surgery. Food was withheld for (mean +/- SD) 178 +/- 4.1 hours prior to induction of anesthesia. The anesthetic protocol included acepromazine maleate, thiopental, and isoflurane. Dogs were randomly selected to receive morphine at various dosages (0, 0.22, or 1.10 mg/kg, IM) concurrent with acepromazine administration prior to induction of anesthesia. A sensor-tipped catheter was used to measure esophageal pH, and GER was defined as a decrease in pH to < 4 or an increase to > 75. Results-40 dogs had acidic reflux, and 1 had biliary reflux. Proportions of dogs with GER were 8 of 30 (27%), 15 of 30 (50%), and 18 of 30 (60%) for morphine dosages of 0, 0.22, and 1.10 mg/kg, respectively. Mean duration of GER was 91.4 +/- 56.8 minutes. There was no significant association between GER and age, weight, vomiting after preanesthetic medication, administration of antimicrobials, or start of surgery. Conclusions and Clinical Relevance-Most healthy dogs vomit after a large dose of morphine, but vomiting does not increase the likelihood of GER during the subsequent anesthetic episode. Administration of morphine prior to anesthesia substantially increases the incidence of GER during the subsequent anesthetic episode.