Internal carotid artery: relationship among arterial caliber, haematocrit and cerebral vascular resistance, investigation in eighty healthy men by echodoppler.

Eighty healthy male subjects (age range: 47 to 87) with hematocrits of 48% or greater and no vascular or hematologic pathology were studied. In each subject, the following values were determined: hematocrit, blood pressure (measured in both arms by sphygmomanometry), and the calibers and blood flow velocities of both internal carotid arteries. For the last 2 measurements, doppler ultrasound was used to examine the extracranial portion of the arteries before, 20 minutes after, and 7 days after euvolemic hemodilution. Following hemodilution, all the subjects demonstrated a statistically significant increase of the diastolic blood flow velocity of the two carotid arteries (p <0.01). This corresponded to a statistically significant reduction of the hematocrit (p <0.01). When the data were compared, no statistically significant relationships were observed among arterial caliber, side (left or right), blood pressure, and age. After 7 days, the results were practically unchanged. The precise correlation between the decrease of the hematocrit and the increase of the diastolic blood flow velocity of the internal carotid artery indicates that the latter is a valid index of cerebral vascular resistance. Thus, measurement of the diastolic blood flow velocity is a valid method for the assessment of cerebral blood flow, since the hematocrit is the single most important factor in the determination of blood viscosity.