Day‐to‐day Variability in Arterial Diameter and Brachial Artery Flow‐Mediated Dilation in Sedentary Young Men and Women

Brachial artery flow‐mediated dilation (FMD), an index of endothelial function, is predictive of cardiovascular disease risk. A wide range of FMD day‐to‐day variability has been reported, but whether this variability differs between men and women is unknown. We examined potential sex‐based differences in FMD and its day‐to‐day variability. Sedentary but otherwise healthy men (N=10, age: 22±3 years) and women (N=7, age: 21±2 years) completed two brachial FMD tests 24 hours apart. Women were tested during the mid‐follicular phase of their menstrual cycle (days 7‐12). Baseline arterial diameter (AD BL ) and maximum reactive hyperemia diameter (AD RH ) were determined, and relative FMD was calculated as FMD% = (AD RH – AD BL )/AD BL . Day 1 measures and between‐day differences were assessed with independent t‐tests. Day‐to‐day variability was assessed with the coefficient of variation (CV) and intraclass correlation coefficient (ICC). Compared to men, women had smaller arterial diameters (p<0.05), but a similar FMD response (p=0.828). No sex‐differences were observed in between‐day differences for any measures (p>0.05). Day‐to‐day variability was low for AD in both sexes (CV of 3‐5%), but high for FMD (men: 43%, women: 55%). Similarly, ICCs were substantial to almost perfect for AD in both sexes (ICC of 0.77‐0.92), but poor for FMD (men: 0.01, women: 0.07). These findings suggest that arterial diameter exhibits better day‐to‐day repeatability than relative FMD in both men and women. Despite women having greater FMD variability than men, between‐day differences were not statistically significant. A larger sample size may be required to detect potential sex‐differences. Funded by NSERC, OGS and GSSI.