A new two-breath technique for extracting the cerebrovascular response to arterial carbon dioxide

Cerebrovascular autoregulation is evaluated from spontaneous fluctuations in mean flow velocity (MFV) by transcranial Doppler ultrasound of the middle cerebral artery (MCA) with respect to changes in arterial blood pressure (BPMCA), but the effects of spontaneous fluctuations in arterial PCO2 on MFV have been largely ignored. Autoregressive moving average analysis (ARMA), a closed-loop system identification technique, was applied to data from nine healthy subjects during spontaneous breathing, during inspiration of 10% CO2 for two breaths once per minute for 4 min, and during sustained breathing of 7% CO2. Cerebrovascular resistance index (CVRi) was calculated (CVRi = BPMCA/ MFV). Reliable estimates of gain for BPMCA 3 MFV were obtained for spontaneous breathing and the two-breath method. In contrast, reliable gain estimates for PCO2 --> MFV or PCO2 --> CVRi were achieved only under the two-breath method. PCO2 --> MFV gain was smaller with the two-breath method than during sustained 7% CO2 (P < 0.05). BPMCA was elevated by 7% CO2 but not by the two-breath method. The closed-loop model provides insight into interactions between BPMCA and PCO2 on cerebrovascular control, but reliable solutions for PCO2 effects with ARMA analysis require perturbation by the two-breath method.