Local Shear Stress and Brachial Artery Flow-Mediated Dilation

Endothelium-dependent flow-mediated dilation is a homeostatic response to short-term increases in local shear stress. Flow-mediated dilation of the brachial artery in response to postischemic reactive hyperemia is impaired in patients with cardiovascular disease risk factors and may reflect local endothelial dysfunction in the brachial artery. However, previous studies have largely neglected the effect of risk factors on evoked shear stress, which is the stimulus for dilation. We evaluated brachial artery percent dilation and evoked diastolic shear stress during reactive hyperemia using high-resolution ultrasound and Doppler in 2045 participants (1107 women, mean age 61 years) in the Framingham Offspring Study. In age- and sex-adjusted models, baseline and hyperemic shear stress were related to brachial artery percent dilation. In stepwise multivariable analyses examining clinical correlates of percent dilation (without shear stress in the model), age, sex, mean arterial pressure, pulse pressure, heart rate, body mass index, lipid medication use, and hormone replacement therapy were related to percent dilation (R2=0.189; P<0.001). When hyperemic shear stress was incorporated, the overall R2 improved (R2=0.335; P<0.001), but relationships between risk factors and percent dilation were attenuated (age and mean arterial pressure) or no longer significant (all others). In contrast, risk factors were related to baseline and hyperemic shear stress in multivariable analyses. Evoked hyperemic shear stress is a major correlate of brachial artery flow-mediated dilation. The associations between many risk factors and brachial artery flow-mediated dilation may be attributable to reduced stimulus for dilation rather than impaired local conduit artery response during hyperemia.