Transcranial Doppler measurements of the middle cerebral artery. Effect of hematocrit.

We evaluated the effects of hematocrit on blood velocity in the middle cerebral artery measured by transcranial Doppler ultrasonography in 45 patients. Hematocrits ranged from 16% to 46%, and systolic blood velocities ranged from 70 to 190 (mean 40 to 140) cm/sec. Decreases in hematocrit correlated with increases in blood velocity in the middle cerebral artery. These results suggest that transcranial Doppler measurements made in the presence of anemia may need correction.