During cataract surgery, both the surgeon and the anesthesiologist need access to the patient's face. At our institution we achieved a working compromise by using an oxygen insufflating hoop, which allowed the surgeon access to the eye and a sterile field. The patient's airway was kept free by the hoop, and the patient breathed a high inspired oxygen fraction. We measured the partial pressure of carbon dioxide (PCO2) of the gas mixture under the surgeon's drapes because they form a semiclosed breathing system for the patient. Accumulation of CO2 occurred in all patients (mean +/- SD, 6.1 +/- 3.1 mmHg), but an oxygen flow of 10 L/min through the hoop prevented an additional rise of CO2 levels during the operation. Reducing the oxygen flow below 10 L/min led to increased retention of CO2 under the drapes. Paper drapes are permeable to CO2, but plastic drapes are impermeable. We did not measure the arterial partial pressure of CO2, and so we do not know whether CO2 accumulation was accompanied by respiratory acidosis.