Background. Postoperative confusion is a well-known complication, but the pathogenetic mechanisms, of which hypoxemia may be one, are not completely understood. Methods. Thirty otherwise healthy patients undergoing major abdominal surgery and ten patients undergoing middle ear minor surgery, all under general anesthesia, were monitored for oxygen saturation (SpO2) with a pulse oximeter on a preoperative night and on the second night after operation (11:00 PM to 7.00 AM). All patients were tested with a mental test questionnaire (the Hodkinson modification of the original Roth-Hopkins test) before operation and on the third and seventh day after operation. Results. Postoperative hypoxemia did not develop in patients undergoing minor surgery, whereas major surgery was associated with severe constant and episodic hypoxemia on the second postoperative night (p < 0.05). The patients undergoing major surgery had decreased mental function on the third day (p < 0.05) but not on the seventh day after operation, whereas minor surgery did not impair mental function. There was a significant correlation between mental function on the third day after operation and mean SpO2 on the second postoperative night after major surgery (r(s) = 0.53, p < 0.005) and minor surgery (r(s) = 0.74, p < 0.03). After major surgery there was also a correlation between decrease (preoperative to postoperative) in mental function on the third postoperative day and mean SpO2 on the second postoperative night (r(s) = -0.40, p < 0.04). During stepwise multiple regression analysis in patients undergoing major surgery a significant relationship was found between postoperative mental test score and SpO2 (p < 0.05) but not between mental function and other perioperative variables (age, premedication dose, duration of operation, and intraoperative and postoperative opioid dose). Conclusions. These results suggest postoperative hypoxemia to be an important pathogenetic factor in postoperative mental dysfunction.
