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It is not always possible to empty the stomach effectively prior to certain types of GI, emergency or obstetrical surgery, and with other high risk groups of patients. The major problem when effective stomach emptying does not occur is the risk of respiratory aspiration of stomach contents. The aspirate may be acidic or nonacidic. Gastric fluids with a pH below 2.5 may cause severe pulmonary injury. However, nonacidic fluids can also cause similar, but less serious, effects. Although the use of antacids does not alter the incidence of aspiration, it may prevent Mendelson's Syndrome, characterized by acid-aspiration pneumonitis, which can be fatal. Approximately 8 percent of all maternal deaths are due to this particular syndrome during anesthesia. Several antacids were compared in a study by Rossett and Flexner. They concluded that the antacid with the best neutralizing capacity would be the most beneficial. Other considerations are also important when administering antacids preoperatively; these include the volume and content of gastric fluid, as well as patient acceptance.· It has been established that the gastric pH must be increased above 3-3.5 to achieve a safe level of acidity.. Gastric pH values below 3 can be deleterious to lung tissue if aspiration occurs. As a prophylactic therapy, a regimen of magnesium trisilicate 15 ml every 2 hours for 2 to 3 doses during labor or pre-anesthesia has been tried with success. It has been suggested by Bisterman that the prophylactic use of cimetidine 300 mg may be superior to oral antacid use, considering this drug's rapid onset and long duration of action. Cimetidine warrants further study for the prevention of acid aspiration. Way has indicated that the standard practice at the UCSF Moffitt Hospital is to administer 45-60 ml of antacid (e.g., Maalox®) as a single dose, to neutralize the gastric contents. Pregnant patients routinely receive this antacid therapy prior to delivery, to prevent the aspiration effect on the pulmonary mucosa. The side effects from antacids are limited when they are administered in single doses. The use of antacids prior to surgery has been recommended in order to reduce the acidity of gastric contents, although no conclusive evidence of reduced morbidity or mortality has been produced. Some reports suggest that, while aspiration may still occur, fatalities associated with acid aspiration may be reduced by antacid administration. Magnesium trisilicate has been used successfully and may be useful, considering its rapid neutralizing capacity. Cimetidine may also be beneficial in preventing Mendelson's Syndrome. However, more study is needed.