Repeatability of vascular responsiveness measures derived from near‐infrared spectroscopy

Near‐infrared spectroscopy (NIRS)‐derived measures of tissue oxygen saturation (StO(2)) have been recently shown to significantly correlate with the widely used method for noninvasively assessing vascular endothelial function, flow‐mediated dilation (FMD). The purpose of this study was to examine the intraday and interday reliability of the reperfusion slope of StO(2) (slope 2 StO(2)) and compare it to FMD. Ultrasound‐derived FMD was quantified following 5 min of distal cuff occlusion of the popliteal artery in nine healthy young men (26 ± 3 years). An FMD test was performed each of 4 days, with a fifth involving three tests. FMD was calculated as the greatest percent change in diameter from baseline (%FMD). StO(2) was measured using NIRS throughout each test, with slope 2 StO(2) being calculated as the upslope of 10‐sec following cuff release. Reliability was determined using repeatability, intraclass correlation coefficients (ICC), and coefficient of variation (CV). Repeatability of slope 2 StO(2) was better than %FMD for both intraday (0.43 and 5.65, respectively) and interday (0.48 and 4.82, respectively) comparisons; approximately 30% of mean values for slope 2 StO(2) could be attributed to measurement error, whereas 100% of mean FMD could be for both intraday and interday comparisons. Similarly, ICC and CV values indicated stronger reliability of slope 2 StO(2) compared to %FMD for both intraday (ICC 0.92 and 0.36, respectively; CV 9 ± 4% and 44 ± 24%, respectively) and interday (ICC 0.94 and 0.25, respectively; CV 14 ± 5% and 40 ± 22%, respectively) comparisons. In conclusion, NIRS‐derived slope 2 StO(2) can be used as a reliable measure of vascular reactivity.