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

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 x 10(-4) +/- 1.6 x 10(-4)% Delta/s(-1).s; BA: 2.5 x 10(-4) +/- 0.4 x 10(-4)% Delta/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.