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

Summary 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.