Transcranial Doppler blood velocity measurement—the effect of changes in velocity profile

Abstract

Transcranial Doppler (TCD) units measure blood velocity in the middle cerebral artery (MCA) and are used to examine the effects of pharmacological agents. The units actually measure the average of the maximum blood velocity envelope (aveV max) and it is assumed that changes in aveV max follow changes in the true mean velocity (aveV mean). This may not be true if there are changes in velocity profile. Results from previous TCD studies using acetazolamide (ACZ) and caffeine were examined for evidence for changes in velocity profile. ACZ increased aveV max by 21% (95% CI 13 to 29%) and aveV mean by 14% (95% CI 9 to 19%). Caffeine decreased aveV max by 8% (95% CI 4 to 12%) and aveV mean by 5% (95% CI 4% increase to 13% decrease). In both cases, the true change, measured using aveV mean was lower, indicating possible changes in velocity profile. We conclude that the possibility of changes in velocity profile must be considered when using TCD to quantify changes in blood velocity.