Bi‐phasic pattern of brachial artery flow mediated dilation in response to a 20min step increase in shear rate

OBJECTIVE‐ to characterize brachial artery (BA) flow mediated dilation (FMD) in response to a 20min step increase in shear stress. BA diameter and mean blood velocity (MBV) were measured with Doppler ultrasound in 16 healthy subjects. A maintained target MBV was created by controlled release of BA compression upstream of heat‐induced forearm vasodilation. Two stimulus magnitudes were investigated; Large (L)‐ MBV increased to the maximum maintainable level. Small (S)‐ MBV targeted at 50% of the large step magnitude. RESULTS: The average increase in shear rate ±SD was L‐75.56±15.91 s −1 and S‐41.08±8.47 s −1 . The peak percent change in diameter was L‐14.28±3.69% and S‐5.53±2.19% (P<0.001). Both the L and the S step trials elicited a biphasic diameter adaptation with a fast first phase followed by a slower second phase. The first phase of the FMD was faster in the L vs. the S step trial (tau (time to 63% of maximal response) 27.40±12.06s vs. 42.94±31.55s P=0.026). The tau of the final phase of FMD was not different between trials (L‐ 467.56±375.16s vs. S‐ 394.87±624.24s P=0.598). CONCLUSION: A prolonged increase in BA shear rate results in a biphasic FMD suggesting that two distinct mechanisms, or sets of mechanisms are involved. A slower first phase FMD in the S trial suggests that stimulus magnitude impacts early FMD response mechanisms. Supported by NSERC, ACSM and HSFC.