Milk of magnesia is an effective preinduction antacid in obstetric anesthesia.

Abstract

The effects on gastric fluid pH of a single dose of milk of magnesia given at various intervals prior to induction of general anesthesia in 115 parturients needing emergency operative delivery with general anesthesia were studied. In addition, the responses of rat lungs to tracheally injected gastric contents from parturients treated with orally administered milk of magnesia and untreated were compared. Stomach contents from parturients were removed by orogastric sump tube as soon as possible after induction of anesthesia. Single doses of milk of magnesia were given at various intervals prior to induction to 99 patients. No patient who received milk of magnesia 10 to 80 min prior to sampling was found to have a gastric-fluid pH of less than 2.5 and volume of more than 25 ml. Five parturients who had taken milk of magnesia and had gastric-fluid pH values of less than 2.5 had received the antacid more than 80 min prior to sampling of gastric contents. Of 16 patients who did not receive an antacid, five had gastric-fluid pH values of less than 2.5 and volumes of more than 25 ml. Lung-weight-to-body-weight (LW/BW) ratios were significantly different for the three groups of rats studied. The LW/BW ratio was 8.0 x 10-3 for rats that received tracheal instillations of gastric fluid from one parturient who was not treated with milk of magnesia and whose gastric-fluid pH was 1.4. The LW/BW ratio was 6.4 x 10-3 in 20 rats that received intratracheal instillations of gastric fluid from one parturient who was treated with milk of magnesia and whose gastric-fluid pH was 8.7. The LW/BW ratio of sham-injected rats was 5.3 x 10-3. The authors conclude that milk of magnesia is an effective preinduction antacid in emergency obstetric anesthesia; that a single dose of 30 ml prior to induction will increase the pH values of gastric contents to more than 2.5 in all patients in the interval from 10 to 80 min after ingestion; and that milk of magnesia-neutralized gastric contents resulted in less severe induced aspiration pneumonitis in rats than did acidic gastric contents.