Does arterial shear explain the magnitude of flow-mediated dilation?: a comparison between young and older humans

Abstract

Flow-mediated dilatation (FMD) has become a commonly applied approach for the assessment of vascular function and health in humans. Recent studies emphasize the importance of normalizing the magnitude of FMD to its apparent eliciting stimulus, the postdeflation arterial shear. However, the relationship between shear stress and the magnitude of FMD may differ between groups. The aim of this study was to examine the relationship between the brachial FMD and four different indexes of postdeflation shear rate (SR) in healthy children ( n = 51, 10 ± 1 yr) and young ( n = 57, 27 ± 6 yr) and older ( n = 27, 58 ± 4 yr) adults. SR was calculated from deflation ( time 0) until 9 s (peak), 30 s (0–30), 60 s (0–60), or until the time-to-peak diameter in each individual (0-ttp). Edge detection and wall tracking of high resolution B-mode arterial ultrasound images were used to calculate the conduit artery diameter. In young adults, the brachial artery FMD demonstrated a significant correlation with the area under the SR curve (SR_AUC) 0–30 s ( r² = 0.12, P = 0.009), 0–60 s ( r² = 0.14, P = 0.005), and 0-ttp ( r² = 0.14, P = 0.005) but not for the peak SR_AUC 0–9 s ( r² = 0.04, P = 0.12). In children and older adults, the magnitude of the brachial artery FMD did not correlate with any of the four SR_AUC stimuli. These findings suggest that in young subjects, postdeflation SR_AUC correlates moderately with the magnitude of the FMD response. However, the relationship between FMD and postdeflation shear appears to be age dependent, with less evidence for an association in younger and older subjects. Therefore, we support presenting SR_AUC stimuli but not normalizing FMD responses for the SR_AUC when using this technique.