pH-Stat Management Reduces the Cerebral Metabolic Rate for Oxygen during Profound Hypothermia (17 degrees Celsius): A Study during Cardiopulmonary Bypass in Rabbits

Background: Greater cerebral metabolic suppression may increase the brain's tolerance to ischemia. Previous studies examining the magnitude of metabolic suppression afforded by profound hypothermia suggest that the greater arterial carbon dioxide tension of pH-stat management may increase metabolic suppression when compared with alpha-stat management.

Methods: New Zealand White rabbits, anesthetized with fentanyl and diazepam, were maintained during cardiopulmonary bypass (CPB) at a brain temperature of 17 degrees Celsius with alpha-stat (group A, n = 9) or pH-stat (group B, n = 9) management. Measurements of brain temperature, systemic hemodynamics, arterial and cerebral venous blood gases and oxygen content, cerebral blood flow (CBF) (radiolabeled microspheres), and cerebral metabolic rate for oxygen (CMRO2) (Fick) were made in each animal at 65 and 95 min of CPB. To control for arterial pressure and CBF differences between techniques, additional rabbits underwent CPB at 17 degrees Celsius. In group C (alpha-stat, n = 8), arterial pressure was decreased with nitroglycerin to values observed with pH-stat management. In group D (pH-stat, n = 8), arterial pressure was increased with angiotensin II to values observed with alpha-stat management. In groups C and D, CBF and CMRO2 were determined before (65 min of CPB) and after (95 min of CPB) arterial pressure manipulation.

Results: In groups A (alpha-stat) and B (pH-stat), arterial pressure; hemispheric CBF (44 plus/minus 17 vs. 21 plus/minus 4 ml *symbol* 100 g sup -1 *symbol* min sup -1 median plus/minus quartile deviation]; P = 0.017); and CMRO2 (0.54 plus/minus 0.13 vs. 0.32 plus/minus 0.10 ml Oxygen2 *symbol* 100 g sup -1 *symbol* min sup -1; P = 0.0015) were greater in alpha-stat than in pH-stat animals, respectively. As a result of arterial pressure manipulation, in groups C (alpha-stat) and D (pH-stat) neither arterial pressure (75 plus/minus 2 vs. 78 plus/minus 2 mm Hg) nor hemispheric CBF (40 plus/minus 10 vs. 48 plus/minus 6 ml *symbol* 100 g sup -1 *symbol* min sup -1; P = 0.21) differed between alpha-stat and pH-stat management, respectively. Nevertheless, CMRO2 was greater in alpha-stat than in pH-stat animals (0.71 plus/minus 0.10 vs. 0.45 plus/minus 0.10 ml Oxygen2 *symbol* 100 g sup -1 *symbol* min sup -1, respectively; P = 0.002).