A New Protocol for Controlling and Targeting Reactive Hyperemia Shear Stress to Assess Endothelial Function

Abstract

Conduit artery endothelial function is most commonly assessed via measurement of flow mediated dilation (FMD) evoked by a transient post-limb occlusion increase in shear stress [reactive hyperemia (RH)]. A limitation of RH-FMD is the uncontrolled nature of the RH shear stress stimulus which produces RH-FMD response variability that is independent of conduit artery endothelial function. The purpose of this study was to develop a protocol to create a uniform, targeted, and transient (40 second) shear stress stimulus and compare it to the standard RH-FMD approach. We hypothesize that incorporating arterial compression in the controlled targeted RH (CTRH) protocol will allow us to achieve a uniform target shear rate (estimate of shear stress) within and between participants. Young, healthy men and women (females n = 4, age = 22 ± 1 years; males n = 2, age = 25 ± 3 years) underwent one trial of RH-FMD and two trials of CTRH-FMD with duplex ultrasound assessment. The CTRH protocol incorporated three-minutes of ischemic handgrip exercise at 30% of participants’ maximum voluntary contraction during a five-minute occlusion to create a large forearm vasodilation and hyperemic reserve. Upon cuff release, the ensuing hyperemia was controlled with manual brachial artery compression at the antecubital fossa. A live velocity readout permitted compression adjustments to achieve the velocity required to maintain a target shear rate of 120 s-1. The cuff was re-inflated after 40s of cuff release. The RH-FMD trial followed the standard five-minute forearm occlusion protocol. The 40s average shear rate was 116.0 ± 8.8s-1 in the CTRH test and 102.1 ± 15.4 s-1 in the RH test. The CTRH test shear rate was significantly closer to the 120s-1 target; calculated as the absolute distance from target in 3s time bins over the 40s period: CTRH = 19.0 ± 3.9s-1; RH = 36.5 ± 6.7 s-1 p = 0.002. The between participant coefficient of variation (CV) for the RH 40s average shear rate was lower in the CTRH test (CTRH CV = 8.1%; RH CV = 15.1%). The within subject trial to trial CV for the CTRH 40s average shear rate was 3.0 ± 4.6%. FMD responses were similar between tests (CTRH-FMD% = 8.3 ± 1.6%; RH-FMD% = 7.6 ± 4.8%, p=0.717). These preliminary findings demonstrate the capacity for RH stimulus control. CTRH-FMD is minimally impacted by stimulus variability and may provide a clearer reflection of brachial artery endothelial function.