Objectiveor=to4) in critically ill children.DesignProspective sample.SettingPediatric intensive care unit in a tertiary care children's hospital.Patientsor=to24 hrs of scheduled intermittent intravenous ranitidine for stress ulcer prophylaxis were enrolled in the study. Patients with renal or hepatic dysfunction and those who received enteral nutrition through the nasogastric tube were excluded from enrollment.InterventionGastric pH was determined at the end of the ranitidine dosing interval, 1 hr after the dose, and at the midpoint between doses. All pH measurements were made from a sample of nasogastric aspirate, using pH sensitive paper. Gastric pH control with ranitidine was considered unsuccessful (poorly controlled) if the pH was <4 for any of the three measurements.Measurements and Main ResultsForty-five patients (median age 36 mos; range 2 wks to 264 mos) were included in the analysis. Eighty-two percent of the patients were mechanically ventilated, 16% were pharmacologically paralyzed, 18% required vasoactive infusions, 36% were nourished via transpyloric feeding tubes, and 7% received total parenteral nutrition.Gastric pH was poorly controlled in 36% of patients.Among these patients, the pH at the end of the dosing interval was significantly lower than the pH measured at 1 hr or at the midpoint between doses (p < .05)..05).Conclusionsor=to4. (Crit Care Med 1998; 26:1433-1436)