Lower oesophageal pH was monitored in 240 anaesthetised dogs. The incidence of gastro-oesophageal reflux was 16.3 per cent and most of the reflux episodes occurred shortly after the induction of anaesthesia. The refluxate was nearly always acid (pH < 4.0), but in 10.3 per cent of the cases it was alkaline (pH > 7.5); gastric contents of pH below 2.5 were refluxed on 19 occasions (7.9 per cent). Regurgitation occurred in only one dog. Prolonging preoperative fasting was associated with an increased incidence of reflux and increased gastric acidity. Premedication with diazepam was associated with fewer reflux episodes than premedication with atropine and propionylpromazine.

Lower oesophageal pH was monitored in 270 dogs under anaesthesia. There were 47 episodes of gastro-oesophageal reflux (17.4 per cent), most of which occurred shortly after the induction of anaesthesia. The refluxate was usually acid (pH < 4.0), but in four of the episodes (8.5 per cent) it was alkaline (pH > 7.5). Gastric contents with a pH below 2.5 were refluxed on 27 occasions (10 per cent) for an average period of about 44 minutes. Regurgitation occurred in two of the dogs. Increased age seemed to be associated with an increased incidence of reflux and an increased gastric acidity. Body position (sternal, dorsal and left or right lateral) and the tilt of the body during surgery (horizontal or tilted to an 8 degrees head-up or head-down position) had no influence on the incidence of gastro-oesophageal reflux. Dogs undergoing intra-abdominal surgery had significantly more reflux episodes than dogs undergoing non-abdominal surgery.

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Lower oesophageal pH was monitored in 50 cats anaesthetized with either thiopentone or propofol. Gastro-oesophageal reflux, as evidenced by a decrease in lower oesophageal pH to less than 4.0 or an increase to more than 7.5, occurred in 16% (4/25) and 12% (3/25) of the cats anaesthetized with thiopentone and propofol, respectively, the difference between the two groups being non-significant. Reflux usually occurred shortly after the induction of anaesthesia and had a mean duration of about 23 min. The refluxate was always acidic (pH < 4.0). Gastric contents of pH below 2.5 were refluxed on three occasions, two in the thiopentone group and one in the propofol group. Regurgitation and flow of gastric contents from the mouth occurred in only one cat anaesthetized with propofol. None of the cats that exhibited reflux developed any signs of postanaesthetic oesophagitis or stricture formation.