Dose-Response Effects of Intravenous Ranitidine on Gastric pH and Volume in Outpatients

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Abstract
The dose-response effects of intravenous ranitidine given 45 min to 5 h earlier on gastric pH and volume were evaluated in six groups of 25 outpatients, each undergoing elective surgery under general anesthesia. Patients in Group 1 received no ranitidune and served as controls. Patients in Groups 2-6 received ranitidine intravenously in incremental doses of 0.5 mg .cntdot. kg-1 body weight from 0.5 mg to 2.5 mg (Group 2, 0.5 mg; Group 3, 1.0 mg; Group 4, 1.5 mg; Group 5, 2.0 mg; and Group 6, 2.5 mg). Ninety-six percent of patients in the control group (Group 1) had gastric pH .ltoreq. 2.5 while 36% of the patients had gastric content volumes .gtoreq. 25 ml with pH .ltoreq. 2.5. Ranitidine, in incremental doses of 05.-2.5 mg .cntdot. kg-1 body weight, caused a significant reduction of gastric acidity and volume. The ED50 of ranitidine producing a gastric pH > 2.5 was 0.36 mg .cntdot. kg-1, and the ED95 was 0.98 mg .cntdot. kg-1 body weight. The ED95 of ranitidine producing a gastric volume < 25 ml was 1.96 mg .cntdot. kg-1. At the dose of 1.5 mg .cntdot. kg-1 of ranitidine, 100% of the patients had gastric contents with pH > 2.5. The proportion of patients with volume < 25 ml was 68% with ranitidine, 0.5 mg .cntdot. kg-1, and gradually increased to 100% with 2.5 mg .cntdot. kg-1 body weight. It is concluded that a significant number of outpatients are at risk for aspiration of acid gastric contents and that this risk is lowered by preoperative administration of ranitidine.

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