Coupling of cerebral blood flow and oxygen metabolism in infant pigs during selective brain hypothermia

Studies documenting the cerebral hemodynamic consequences of selective brain hypothermia (SBH) have yielded conflicting data. Therefore, the authors have studied the effect of SBH on the relation of cerebral blood flow (CBF) and CMRO2 in the forebrain of pigs. Selective brain hypothermia was induced in seven juvenile pigs by bicarotid perfusion of the head with extracorporally cooled blood. Cooling and stepwise rewarming of the brain to a Tbrain of 38 degrees C, 25 degrees C, 30 degrees C, and 38 degrees C at normothermic Ttrunk (38 degrees C) decreased CBF from 71 + 12 mL 100 g(-1) min(-1) at normothermia to 26+/-3 mL 100 g(-1) min(-1) and 40+/-12 mL 100 g(-1) min(-1) at a Tbrain of 25 degrees C and 30 degrees C, respectively. The decrease of CMRO2 during cooling of the brain to a Tbrain of 25 degrees C resulted in a mean Q10 of 2.8. The ratio between CBF and CMRO2 was increased at a Tbrain of 25 degrees C indicating a change in coupling of flow and metabolism. Despite this change, regional perfusion remained coupled to regional temperatures during deep cerebral hypothermia. The data demonstrate that SBH decreases CBF and oxygen metabolism to a degree comparable with the cerebrovascular and metabolic effects of systemic hypothermia. The authors conclude that, irrespective of a change in coupling of blood flow and metabolism during deep cerebral hypothermia, cerebral metabolism is a main determinant of CBF during SBH.