Efficacy of antacid therapy.

Sir,—The recent letter from White, Clark and Stanley-Jones (1976) prompted us to review our own results on the efficacy of antacid therapy. In our maternity unit patients in labour receive magnesium trisilicate 20 ml orally at 2-h intervals. Immediately before induction of anaesthesia an additional dose of 20 ml is given irrespective of the time of the preceding dose. In many patients apomorphine is used to empty the stomach before induction of anaesthesia and these patients receive the pre-induction dose of antacid following the apomorphine. Patients scheduled for elective surgery receive one dose of magnesium trisilicate 20 ml 1 h before, and a second dose immediately before induction of anaesthesia. In all instances in which the pre-induction dose only is given, the volume of antacid is doubled. Aspiration of stomach contents was performed aC a convenient time during anaesthesia. In patients undergoing Caesarean section this was after delivery of the infant at a time up to 30 min after the pre-induction dose of magnesium trisilicate. The pH of the gastric aspirate was measured with M & B indicator paper (table I). In all but two patients, the pH values were greater than 4.5 units. Patient (a) had a gastric pH of 3.0 units. However, the last dose of magnesium trisilicate was given 125 min before induction of anaesthesia and the pre-induction dose had been omitted by mistake. The results in patient (b) cannot be explained, although a similar cause is possible. We believe that the pre-induction dose of magnesium trisilicate is essential, and probably accounts for the difference between our findings and those of White, Clark and Stanley-Jones (1976). As a result of these findings we believe that our regime provides almost totally effective antacid prophylaxis.