Sustained hyperaemia stimulus is necessary to induce flow-mediated dilation of the human brachial artery

We studied the relative importance of the magnitude and duration of the shear stimulus to induce flow‐mediated dilation (FMD) in the brachial artery of 10 healthy men by ultrasound imaging. The shear stress stimulus was induced by different durations of reactive hyperaemia following 15‐min forearm occlusion. The control condition of continuous postocclusion hyperaemia was compared to 20, 40 and 60 s of reactive hyperaemia followed by reapplication of circulatory arrest for 2 min and a second cuff release. In response to the first cuff release, peak shear rate was not different between conditions; total shear during the first minute was reduced in the 40 s and further reduced in the 20 s conditions. FMD in control (10·0 ± 3·0%), 60 s (10·5 ± 3·2%) and 40 s (7·8 ± 3·6%) were greater than the 20‐s condition (2·9 ± 2·8%). At second cuff release, peak shear of the 20‐s condition was slightly reduced from the first release, but 40 and 60‐s conditions were progressively reduced. Total shear to peak dilation was reduced after the second cuff release for the 20 and 40‐s conditions and further after the 60‐s condition. FMD was maintained in the 20‐s condition (8·3 ± 3·7%) but reduced in the 40‐s (3·7 ± 1·7%) and 60‐s conditions (1·5 ± 2·6%). FMD was not related to peak shear rate after the first occlusion (r = 0·003) but was after the second cuff release (r = 0·32, P = 0·004). The FMD response was correlated with the total shear to time of peak diameter after the first (r = 0·35, P<0·001) and the second (r = 0·25, P = 0·009) cuff release.