The Relationship between Velocity and Cerebral Resistance during Vasomotor Reactivity Testing: Should We Report a Different Measurement?

Abstract

Introduction and Objective Changes in arterial resistance are an important aspect in the regulation of cerebral blood flow. Vasomotor reactivity testing with transcranial Doppler (TCD) is used commonly to report findings in terms of mean flow velocity changes. Yet, the pertinent clinical question is about resistance changes. How effectively do the cerebral resistance vessels dilate and constrict, or what is their vasomotor capacity? In this study, we examine the relationship between cerebral flow velocity and resistance and propose a new measurement for vasomotor reactivity studies. Methods We recruited 35 healthy subjects and measured TCD mean flow velocities in the middle cerebral artery during vasoreactivity testing using 6% CO2 inhalation (hypercapnia) to produce cerebral vasodilatation, and controlled hyperventilation (hypocapnia) to produce vasoconstriction. Brachial blood pressure was also recorded. Mean Resistance (mean blood pressure/mean flow velocity) was calculated at baseline, hypercapnia, and hypocapnia. We compared the % change from baseline in both mean flow velocity (cm/s) and Mean Resistance (mmHg s/cm), in relation to gender and age. Results For the group overall, hypercapnia produced a 39.3 ± 19.9% increase in mean flow velocity; however, this resulted in only a 21.9 ± 9.9% decrease in Mean Resistance. Whereas during hypocapnia, a 32.4 ± 9.3% decrease in velocity produced a much larger increase in Mean Resistance (48.9 ± 22.2%). Young women had the greatest capacity to vasodilate, showing a 26.1 ± 9.2% decrease in Mean Resistance (46.0 ± 14.6% increase in velocity). However, young men showed the largest change during vasoconstriction; a 59.5 ± 31.6% increase in Mean Resistance (35.3 ± 9.5% decrease in velocity). Conclusions Large velocity increases during hypercapnia do not represent large changes in resistance. Whereas small velocity decreases during hypocapnia actually represent much larger changes in resistance, the calculation of Mean Resistance is a more clinically relevant measurement to report than velocity alone, and should be incorporated into TCD studies in the future.