[Evaluation of cerebral circulation and cerebrovascular CO2 reactivity with transcranial Doppler ultrasonography].

We examined the cerebrovascular CO2 reactivity (K) determined by measurements of the middle cerebral artery blood flow velocity (Vm) with transcranial Doppler ultrasonography (TCD) in 24 anesthetized patients. The relationship between Vm and mean arterial blood pressure (mBP) showed a linear regression in each constant level of PaCO2. The exponential curve with an exponent of 0.0399 mmHg-1 was found to be the best fit to the plotted data of Vm against PaCO2 during normotension of 90 mmHg. The following equation was used to evaluate K from the two flow values, because the calculated K value would be logically constant, i.e., an exponent of Vm/PaCO2 exponential curve: K (mmHg-1) = delta lnVm/delta PaCO2 = (lnVm(1) - lnVm(2)/(PaCO2(1) - PaCO2(2)). The validity of K values could be enhanced under conditions of the delta PaCO2 > or = 12.5 mmHg and the variation in arranged mBP < or = 30 mmHg. This methodology with TCD can be used to correlate changes in cerebral blood flow and provide much benefit for the evaluation of cerebrovascular CO2 reactivity.