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 (r(2) = 0.12, P = 0.009), 0-60 s (r(2) = 0.14, P = 0.005), and 0-ttp (r(2) = 0.14, P = 0.005) but not for the peak SR(AUC) 0-9 s (r(2) = 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.

Flow-mediated dilation (FMD) has become a commonly applied approach for the assessment of vascular function and health, but methods used to calculate FMD differ between studies. For example, the baseline diameter used as a benchmark is sometimes assessed before cuff inflation, whereas others use the diameter during cuff inflation. Therefore, we compared the brachial artery diameter before and during cuff inflation and calculated the resulting FMD in healthy children (n ϭ 45; 10 Ϯ 1 yr), adults (n ϭ 31; 28 Ϯ 6 yr), and older subjects (n ϭ 22; 58 Ϯ 5 yr). Brachial artery FMD was examined after 5 min of distal ischemia. Diameter was determined from either 30 s before cuff inflation or from the last 30 s during cuff inflation. Edge detection and wall tracking of high resolution B-mode arterial ultrasound images was used to calculate conduit artery diameter. Brachial artery diameter during cuff inflation was significantly larger than before inflation in children (P ϭ 0.02) and adults (P Ͻ 0.001) but not in older subjects (P ϭ 0.59). Accordingly, FMD values significantly differed in children (11.2 Ϯ 5.1% vs. 9.4 Ϯ 5.2%; P ϭ 0.02) and adults (7.3 Ϯ 3.2% vs. 4.6 Ϯ 3.3%; P Ͻ 0.001) but not in older subjects (6.3 Ϯ 3.4% vs. 6.0 Ϯ 4.2%; P ϭ 0.77). When the diameter before cuff inflation was used, an age-dependent decline was evident in FMD, whereas FMD calculated using the diameter during inflation was associated with higher FMD values in older than younger adults. In summary, the inflation of the cuff significantly increases brachial artery diameter, which results in a lower FMD response. This effect was found to be age dependent, which emphasizes the importance of using appropriate methodology to calculate the FMD. methodology FLOW-MEDIATED DILATION (FMD) describes the vasodilatation of a conduit artery following an increase in shear stress, typically induced by a 5-min period of ischemia induced by a limb cuff placed distal to the recording probe (3). With the assumption that appropriate methodology is utilized (12, 36), the FMD response is largely nitric oxide (NO) mediated (13,19) and provides information about the integrity of the endothelium (38). Since impaired endothelial vasodilator function is considered an early and integral manifestation of vascular disease, which predicts cardiovascular events (10,14,18,20,25), the FMD approach has become a popular and frequently applied technique in clinical and physiological research.FMD results depend critically upon the methodology used to assess and analyze vascular images. For example, several important issues relating to stimulus magnitude were recently outlined (30), and the time course of the response is also a critical consideration that impacts upon data interpretation (2). One important methodological issue that has not been critically examined relates to the baseline diameter selected for calculation of the FMD.There are two commonly applied approaches used to assess baseline diameter, which differ according to whether measurements are collected before or during ...

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