Gastroesophageal reflux associated with preanesthetic morphine

Clinicians have long realized that gastroesophageal reflux (GER) may occur during anesthesia, and that the sequella of esophagitis or esophageal stricture may result following this phenomenon (Pearson H. et al, Reflux oesophagitis and stricture formation after anesthesia; a review of seven cases in dogs and cats, The Journal of Small Animal Practice, 1978; 19:507-519). A new study by Wilson et al in the AmericanJournal of Veterinary Research (Vol. 66, No. 3, March 2005, pages 386-389) examines the effect of morphine administered prior to anesthesia on the incidence of GER. In their introduction they note the rising prevalence of this drug as a preanesthetic, pre-emptive analgesic in small animal medicine.

Table 1

Case in point

These investigators studied 90 dogs undergoing elective orthopedic surgery. The patients were randomly given 0, 0.22, or 1.10 mg/kg IM dosages of morphine (30 dogs per dosage selection) as a preanesthetic along with acepromazine. Thiopental was used for induction and intubation, and isoflourane was the maintenance anesthetic agent. They were positioned on their backs, and cephalozin was given IV every two hours during the anesthetic/surgical procedures. The determination of GER was made by measuring the pH of the esophagus just rostral to the gastroesophageal junction with a sensor probe. Esophageal pH of <4 (typical acidic GER) or >7.5 (unusual biliary GER) defined cases of GER in this study.

The investigators found (Table 1) that the incidence of GER increased along with a rising dosage of preanesthetic morphine, and that when GER did occur it often commenced soon after anesthetic induction, with median durations well over one hour in all groups.

Incidental observations in this group included a finding that 24 of 30 dogs given the highest dose of morphine vomited prior to induction, and nine of 30 given the lower dose of morphine vomited. Vomiting was not observed in those dogs receiving 0 morphine as a preanesthetic. It was also of interest to note that four of the 60 dogs given morphine and one out of 30 dogs not given morphine were observed to passively discharge liquid from the nose or mouth during anesthesia (regurgitation).

Consensus

This study did not report or address whether these patients ever developed signs of esophageal disease after the anesthesia. In addition, other variables such as duration of fasting may increase the incidence of GER (Galatos et al, Gastroesophageal reflux during anesthesia in the dog: the effect of preoperative fasting and premedication. Vet Rec 1995; 137:479-483). Nevertheless, the clinician should consider the increased incidence of GER as well as the potential for respiratory depression when deciding on the dosage and timing of the very potent and effective analgesic, morphine.

Ronald Lyman DVM, Dipl. ACVIM is a graduate of The Ohio State University College of Veterinary Medicine. He completed a formal internship at the Animal Medical Center in New York City. Lyman is a co-author of chapters in the 2000 editions of Kirk’s Current Veterinary Therapy XIII and Quick Reference to Veterinary Medicine.