Validity of cerebral arterial blood flow calculations from velocity measurements.

It is obvious that a noninvasive technique that can provide continuous measurements of blood flow through a cerebral artery would be very valuable for clinical and investigative purposes. The most promising approach to the development of such a technique involves the use of ultrasound for the measurement of blood flow velocity. Since blood flow through a vessel is equal to the instantaneous average velocity times the cross-sectional area of the vessel, monitoring of velocity and vessel caliber provides the essential measurements that are needed to estimate blood flow. Busija et al showed that such a method is both accurate and useful. They measured blood flow velocity in a cerebral artery in cats and dogs with an ultrasonic Doppler probe placed around the artery, and simultaneously they measured vessel diameter by direct visualization. They found that blood flow calculated from the velocity and vessel caliber correlated exceedingly well with blood flow measured directly with radioactive microspheres. In this issue of Stroke, Aaslid and his colleagues measured blood flow velocity in the middle cerebral artery of human volunteers to investigate the responses of cerebral blood flow to rapid decreases in arterial blood pressure. The approach developed by these authors involves certain important assumptions that can have a critical influence on the validity of the results. 1 examine the soundness of these assumptions and their potential consequences on the usefulness of the technique. Two important features of the method used by Aaslid et al should be borne in mind. First, these authors did not measure vessel caliber, and they assumed that the middle cerebral artery had a constant cross-sectional area throughout the interventions they used. Second, they did not measure average velocity through the middle cerebral artery but maximum velocity. The authors justified their assumption of an unchanging vessel caliber in the