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1 School of Physical and Health Education, Queen's University, Kingston, Ontario, Canada
* To whom correspondence should be addressed. E-mail: mt29{at}post.queensu.ca.
The reactive hyperemia test for the assessment of vascular endothelial function evokes an uncontrolled, transient increase in shear stress as a stimulus for forearm conduit vessel flow mediated vasodilation (EDFMD). We developed a non-invasive method to create controlled elevations in brachial artery shear rate (SR; estimate of shear stress, CHtest; controlled hyperemia test) and assessed the impact of this vs. the reactive hyperemia (RHtest) approach on EDFMD responses. 8 healthy young subjects (age range 22-37 yrs) participated in 2 trials of each hyperemia test on three separate days. For the CHtest: step increases in SR from 8-1 to a target SR of 50-1 were achieved by controlled release of brachial artery compression during forearm heating. For the RHtest: transient increases in SR were achieved following 5 min of forearm occlusion. Brachial artery diameter and blood flow velocity (Echo and Doppler ultrasound) were measured upstream of the compression and occlusion sites. Both tests elicited a significant change in diameter (RHtest 6.33 ±3.12% ; CHtest 3.00 ±1.05%)The CHtest approach resulted in 1) substantially reduced between-subjects SR and EDFMD variability vs. the RHtest (SR Coefficient of Variation (CV); CHtest 4.9% vs. RHtest 36.6%; EDFMD CV 36.16% vs. RHtest 51.80%), 2) virtual elimination of the impact of brachial artery diameter on the magnitude of the peak EDFMD response observed with the RHtest (regression of peak EDFMD vs. Baseline Diameter; RHtest slope = -4.75, r2 = 0.64, P<0.01, vs. CHtest slope = -0.77, r2 = 0.14, P<0.01). Normalization of the RHtest EDFMD response to the magnitude of the SR stimulus eliminated test differences in between subjects response variability. Reductions in trial-to-trial SR variability with the CH test did not result in a statistically significant improvement in trial-to-trial Peak EDFMD variability (CV; CHtest 8.32 ± 7.05% vs. RHtest 14.67 ± 15.03%, P=0.16). Reductions in day-to-day SR variability also did not improve day-to-day EDFMD variability. These data identify a between-subjects SR variability contribution to EDFMD variability with the RHtest, and indicate that SR control with the CHtest or RHtest response normalization are essential for examining EDFMD between groups differing in baseline arterial diameter.
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