DOPPLER CO2 AND DIAMOX TEST - DECREASED RELIABILITY BY CHANGES OF THEVESSEL DIAMETER

Transcranial Doppler sonography measures cerebrovascular reserve capac ity by comparing blood flow velocities in basal cerebral arteries befo re and after vasostimulation by different methods. This presupposes co nstancy of the vessel diameter insonated. To investigate this assumpti on 18 subjects underwent a CO2, (inhalation of 5 % CO2 respectively hy perventilation) and a Diamox test (i.v. injection of 1 g acetazolamide ). Since the diameter of the middle cerebral artery in humans cannot b e assessed sonographically with sufficient accuracy, we used the diame ter of the internal carotid artery as a measure and made a comparison of the flow velocities in both vessels. The relative changes in time-a veraged maximum velocity in both vessels showed a close correlation wi th a regression coefficient of 0.96 in the CO2 test and 0.87 in the Di amox test. Since, on the other hand, no significant changes in the lum en of the internal carotid artery could be found during the CO2 test ( r = 0.99), our results support the assumption of at least a fairly con stant diameter of the middle cerebral artery during moderate hyper- re spectively hypocapnia and after intravenous injection of 1 g acetazola mide.