8.10 BRACHIAL ARTERY FLOW-MEDIATED DILATATION: DIFFERENT PATTERNS OF WALL SHEAR RATE INCREASE DURING REACTIVE HYPERAEMIA

Abstract

Background: Wall shear rate (WSR) is considered an important stimulus for flow-mediated dilatation (FMD). However, its estimation by conventional ultrasound is challenging due to inherent difficulties of velocity estimation near the arterial wall. To evaluate how WSR influences brachial artery FMD, we used a prototype Doppler ultrasound system which provides simultaneous estimates of WSR at near and far walls and continuous arterial diameter tracking. Methods: Data from 33 young healthy individuals (27.5±4.9yrs, 19F) were analysed. FMD was assessed with a conventional reactive hyperaemia technique using Ultrasound Advanced Open Platform (ULA-OP). All acquired raw data were post-processed using custom-designed software to obtain WSR and diameter parameters. Results: Baseline diameter and FMD were 3.29±0.45 mm and 6.54±3.54 %, respectively. During hyperaemia, we observed two distinct patterns of increased WSR: monophasic (MOP, n=15 fast increase reaching peak WSR at once) and biphasic (BIP, n=18 fast followed by slow increase before reaching peak WSR). In BIP, peak WSR (657±153 sec-1 vs 522±132 sec-1) and WSR area under the curve until peak dilation (20398±6265 au vs 13530±5592 au) were significantly greater than in MOP (both p<0.05). Absolute diameter increase was significantly greater in BIP (0.24±0.10 mm) than in MOP (0.15±0.09 mm, p<0.05). Percentage diameter increase tended to be greater in BIP (7.6±3.3 %) than MOP (5.3±3.5 %, p=0.08). Conclusions: These results demonstrate that there are distinct WSR increase patterns during hyperaemia, and that these patterns are associated with differences in the magnitude of hyperaemic WSR. Our observations suggest that these WSR increase patterns may be associated with the subsequent brachial artery FMD response.