Doppler C02 and Diamox Test: Decreased Reliability by Changes of the Vessel Diameter?

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Abstract
Transcranial Doppler sonography measures cerebrovascular reserve capacity by comparing blood flow velocities in basal cerebral arteries before and after vasostimulation by different methods. This presupposes constancy of the vessel diameter insonated. To investigate this assumption 18 subjects underwent a CO2 (inhalation of 5% CO2 respectively hyperventilation) and a Diamox test (i.v. injection of 1 g acetazolamide). Since the diameter of the middle cerebral artery in humans cannot be assessed sonographically with sufficient accuracy, we used the diameter of the internal carotid artery as a measure and made a comparison of the flow velocities in both vessels. The relative changes in time-averaged maximum velocity in both vessels showed a close correlation with a regression coefficient of 0.96 in the CO2 test and 0.87 in the Diamox test. Since, on the other hand, no significant changes in the lumen 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 constant diameter of the middle cerebral artery during moderate hyper- respectively hypocapnia and after intravenous injection of 1 g acetazolamide.

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