Autoregulatory response and CO2 reactivity of the basilar artery.

Background And Purpose: Transcranial Doppler has been extensively used to measure cerebrovascular control mechanisms, including autoregulation in humans and in patients with cerebrovascular diseases. There have been sufficient reports on the measurement of normal autoregulatory response (AR) and CO2 reactivity (CR) of the middle cerebral artery (MCA) but few reports of these indices for the basilar artery (BA). We measured AR and CR in the BA in healthy volunteers to determine normal values and compared them with simultaneous measurements made in the MCA.

Methods: Sixteen normal subjects were enrolled. Time-averaged mean velocities of maximum blood flow in the BA and MCA were continuously and simultaneously monitored by using transcranial Doppler along with continuous measurement of mean arterial blood pressure (MABP). Values were obtained during rest, alterations of end-tidal PaCO2 (ETCO2), and acute decrease and recovery of MABP. AR was evaluated by using the thigh cuff method and graded by the standard dynamic autoregulatory index (ARI), with values between 0 and 9. CR was measured as percentage change in time-averaged mean velocity per mm Hg ETCO2.

Results: The mean age of 16 subjects was 27.38+/-8.50 years. Average baseline values for MABP and ETCO2 were 82.29+/-7.10 and 42.75+/-3.77 mm Hg, respectively. Mean ARI was 4.62+/-1.26 for the BA and was 4.77+/-1.23 for the MCA (n=15) (P=0.598). Average CR was 2.54+/-0.39%/mm Hg ETCO2 for the BA and 2.51+/-0.29%/mm Hg ETCO2 for the MCA (n=16) (P=0.686).

Conclusions: Our study demonstrates that ARI and CR values for the BA are similar to those for the MCA.