Sustained Shear Stress Reveals Agreement Between Flow-Mediated Dilation and Flow-Mediated Slowing

BACKGROUND: Flow-mediated slowing (FMS), the decrease in pulse wave velocity (PWV) in response to elevated shear stress, is a simpler, operator-independent alternative to assessing arterial diameter (DIA) increase via flow-mediated dilation (FMD). Traditional flow mediated assessment uses reactive hyperaemia, yet sustained shear stress is a more ubiquitous in vivo physiological occurrence. This study aimed to determine the agreement between FMD and FMS in response to a sustained shear stress stimulus designed to enhance endothelial function. METHODS: In 19 healthy participants (12 male, 28±8 years), brachial artery diameter and brachial-radial PWV were measured in experimental and contralateral control arms. Following baseline, sustained stepwise increases in shear rate were induced using upper-limb heating (10 min) at 47.5°c and 52.5°c, and handgrip exercise (4 min) at 0.33Hz and 0.67Hz in the experimental arm. Agreement was tested by modelling the overall association using a linear mixed-effects models. RESULTS: The sustained shear protocol elicited a large increase in mean shear rate in the experimental arm (Δ 299 1/s, P <0.001). This resulted in a significant Time x Condition interaction in the experimental arm, with a moderate increase in DIA (Δ 0.04 cm, ∼13%; P <0.001) and a moderate decrease in PWV (Δ -1.7 m/s, ∼20%; P <0.001). There was a strong overall inverse association (ICC=0.88, 95%CI: 0.75, 0.96) between FMD and FMS in the experimental arm. CONCLUSION: FMS demonstrates a response consistent with FMD following a sustained shear stress experimental protocol. Sustained-shear FMS is a viable, operator-independent tool for assessing acute improvements in endothelial function.