Suitability of the muscle O(2) resaturation parameters most used for assessing reactive hyperemia: a near-infrared spectroscopy study

BACKGROUND: There is a spectrum of possibilities for analyzing muscle O(2) resaturation parameters for measurement of reactive hyperemia in microvasculature. However, there is no consensus with respect to the responsiveness of these O(2) resaturation parameters for assessing reactive hyperemia. OBJECTIVES: This study investigates the responsiveness of the most utilized muscle O(2) resaturation parameters to assess reactive hyperemia in the microvasculature of a clinical group known to exhibit impairments of tissue O(2) saturation (StO(2)). METHODS: Twenty-three healthy young adults, twenty-nine healthy older adults, and thirty-five older adults at risk of cardiovascular disease (CVD) were recruited. Near-infrared spectroscopy (NIRS) was used to assess StO(2) after a 5-min arterial occlusion challenge and the following parameters were analyzed: StO(2slope_10s), StO(2slope_30s), and StO(2slope_until_baseline) (upslope of StO(2) over 10s and 30s and until StO(2) reaches the baseline value); time to StO(2baseline) and time to StO(2max) (time taken for StO(2) to reach baseline and peak values, respectively); ∆StO(2reperfusion) (the difference between minimum and maximum StO(2) values); total area under the curve (StO(2AUCt)); and AUC above the baseline value (StO(2AUC_above_base)). RESULTS: Only StO(2slope_10s) was significantly slower in older adults at risk for CVD compared to healthy young individuals (p < 0.001) and to healthy older adults (p < 0.001). Conversely, time to StO(2max) was significantly longer in healthy young individuals than in older adult at CVD risk. CONCLUSIONS: Our findings suggest that StO(2slope_10s) may be a measure of reactive hyperemia, which provides clinical insight into microvascular function assessment.