This prospective study evaluated supplemental oxygen as an effective means of preventing hypoxemia and electrocardiographic changes in sedated patients undergoing endoscopic procedures. Ninety-five patients with documented ischemic heart disease and 106 patients without heart or lung disease were studied. Patients within each group received either supplemental oxygen (2 L/minute) via nasal cannula or room air. Hypoxemia (oxygen saturation below 90%) and ST-segment deviations occurred in cardiac and control groups. The combination of midazolam and meperidine significantly increased the risk for hypoxemia as compared to midazolam alone (44% versus 8%). Supplemental oxygen significantly decreased, but did not entirely prevent, hypoxemia in cardiac and control groups. The incidence of ST-segment deviation in cardiac patients who were hypoxic was significantly lower in the group receiving oxygen (p = .0015). Supplemental oxygen did not affect the incidence of arrhythmias. Prophylactic supplemental oxygen should be administered to patients with ischemic heart disease who undergo conscious sedation for upper and lower gastrointestinal endoscopic procedures.