The relationship between cerebral blood flow (CBF) and the cerebral blood flow velocity (CBFV):: Influence of halothane and cerebral CO2 reactivity.

Objective: This controlled study was designed to investigate 1.) the effects of 0,8% halothane and 2.) the effects of a variation in PaCO2 on the relationship between global cerebral blood flow (CBF) and middle cerebral artery flow velocity (CBFVMCA). Method: With ethical committee approval and informed patient consent we investigated 10 patients undergoing coronary artery bypass surgery. Measurements were performed under fentanyl/midazolam anaesthesia prior to the start of surgery. First, during a baseline period, ventilation was changed in a random sequence to achieve two different levels of arterial PCO2. (30 and 50 mmHg, respectively). Consequently, measurements were repeated during application of 0.8% halothane at identical PaCO2 levels. Measurements of global CBF were performed by the Kety-Schmidt-technique with argon as an indicator. Simultaneously, CBFVMCA was recorded by use of a 2-MHz transcranial Doppler system. Results: Application of 0.8% halothane caused a significant decrease in cerebrovascular resistance (CVR) both at hypocapnia and hypercapnia by 36 and 23%, respectively. Because of a concomitant reduction in cerebral perfusion pressure (CPP), however, CBF remained unchanged during application of halothane. The relationship between CBF and CBFVMCA was not altered when compared to the baseline period. Similarly, CO2 reactivity of CBF and CBFVMCA remained unchanged. CO2 reactivity of CBF significantly exceeded CO2 reactivity of CBFVMCA. Conclusion: The results of this clinical study demonstrate that Doppler-sonographic estimation of relative changes in CBF is not altered by application of 1 MAC halothane indicating that the decrease in CVR is not associated with a vasodilation of the proximal segments of basal cerebral arteries. The difference between CO2 reactivity of CBF and CBFVMCA, however, suggests that CO2-induced changes in CBF are slightly underestimated by TCD monitoring of CBFVMCA.