Cardiopulmonary bypass: perioperative cerebral blood flow and postoperative cognitive deficit

Increased cerebral blood flow occurring during cardiopulmonary bypass as a result of changes in arterial carbon dioxide tension during acid-base regulation is thought to increase postoperative cognitive dysfunction. We studied 70 patients undergoing coronary artery bypass procedures who were randomized to two different acid-base protocols: pH-stat or alpha-stat regulation. Cerebral blood flow, cerebral blood flow velocity, and cerebral oxygen metabolism were measured before bypass, during bypass (hypothermic [28 degrees C] and normothermic phases), and after bypass. Detailed cognitive tests were conducted before operation and 6 weeks after operation. During 28 degrees C bypass, cerebral blood flow was significantly (p < 0.05) higher in the pH-stat group than in the alpha-stat group (41 +/- 2 versus 24 +/- 2 mL.100 g-1.min-1), and cerebral blood flow velocity was significantly increased in the pH-stat group and significantly decreased in the alpha-stat group (152% +/- 10% versus 78% +/- 7%). Cerebral extraction ratio of oxygen demonstrated a relatively greater disruption of autoregulation in the pH-stat group than in the alpha-stat group with relative hyperemia of 0.12 +/- 0.02 versus 0.26 +/- 0.03, respectively, during 28 degrees C bypass. Using the criterion of deterioration in three or more neuropsychologic tests, a significantly higher proportion of patients in the pH-stat group fared less well than in the alpha-stat group (49% +/- 17% versus 20% +/- 13%). Patients in the alpha-stat group experienced less disruption of cerebral autoregulation during hypothermic cardiopulmonary bypass, and this was accompanied by a reduction in postoperative cognitive dysfunction.