Diffusion hypoxia: a reappraisal using pulse oximetry.

Sixty healthy surgical patients were monitored during surgery with a pulse oximeter. At the completion of the operation, nitrous oxide and oxygen were discontinued abruptly in 50 of these patients. During air breathing, a small drop in arterial hemoglobin oxygen saturation (SaO2), to about 4% below preoperative values, was observed in all patients. In 10 patients, only oxygen was given before removal of the mask. There was no sudden drop in SaO2 in these patients, but by 5 minutes after discontinuation there was no difference between the two groups of patients--SaO2 was reduced 2 to 3% below preoperative values in both groups. For patients without cardiopulmonary disease, the phenomenon of "diffusion hypoxia" is a mild and transient event. Clinically significant hypoxemia (SaO2 less than 90%) after removal of nitrous oxide/oxygen at the completion of the anesthetic occurred in 3 patients (6%) and was associated with airway obstruction in each case.