Limb-specific differences in flow-mediated dilation: the role of shear rate

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Abstract
We sought to examine flow-mediated vasodilation (FMD) in both the arm [brachial artery (BA)] and lower leg [popliteal artery (PA)] of 12 young, healthy subjects. Vessel diameter, blood velocity, and calculated shear rate were determined with ultrasound Doppler following a suprasystolic cuff occlusion (5 min) in both the BA and PA and an additional reduced occlusion period (30–120 s) in the BA to more closely equate the shear stimulus observed in the PA. The BA revealed a smaller diameter and larger postischemic cumulative blood velocity [area under curve (AUC)] than the PA, a combination that resulted in an elevated postcuff cumulative shear rate (AUC) in the BA (BA: 25,419 ± 2,896 s−1·s, PA 8,089 ± 1,048 s−1·s; P < 0.05). Thus, when expressed in traditional terms, there was a tendency for the BA to have a greater FMD than the PA (6.5 ± 1.0 and 4.5 ± 0.8%, respectively; P = 0.1). However, when shear rate was experimentally matched (PA: 4.5 ± 0.8%; BA: −0.4 ± 0.4%) or mathematically normalized (PA: 6.8 × 10−4± 1.6 × 10−4%Δ/s−1·s; BA: 2.5 × 10−4± 0.4 × 10−4%Δ/s−1·s), the PA revealed a greater FMD per unit of shear rate than the BA ( P < 0.05). These data highlight the importance of assessing the shear stimulus to which each vessel is exposed and reveal limb-specific differences in flow-mediated dilation.