Twenty patients undergoing clinically indicated elective colonoscopy were prospectively monitored noninvasively for alterations in cardiorespiratory function. Most of the patients were elderly and many had either cardiac or pulmonary disease. All subjects were premedicated with intramuscular meperidine and continuously monitored with ECG, blood pressure, earlobe pulse oximetry, nasal air flow by thermistor probe, and impedance pneumography. Any use of additional analgesic or sedative was determined by the endoscopist, who was blinded to the physiologic tracings, and dosages of medications given were titrated to each patient's tolerance of the procedures as assessed by the endoscopist. Seventeen patients (85%) required additional sedation with the benzodiazepine, midazolam. These patients exhibited frequent episodes of hypotension (reductions in mean arterial blood pressure of 23 +/- 12 mm Hg from baseline, means +/- SD) and respiratory depression (as noted by the greater number of apneas and arterial oxygen desaturation as low as 7.1 +/- 2% from baseline, means +/- SD). In addition, elderly patients and patients with an underlying history of cardiac or pulmonary disease had a greater incidence of potentially untoward cardiorespiratory events.