Near-infrared spectroscopy (NIRS) has emerged as a promising tool to evaluate vascular reactivity in vivo. Whether this approach can be used to assess age-related impairments in microvascular function has not been tested. Tissue oxygen saturation (StO2) post-occlusion recovery kinetics were measured in two distinct age groups (<35 year and >65 years) using NIRS placed over the flexor digitorum profundus. Key end-points included: (1) the desaturation rate during cuff occlusion; (2) the lowest StO2 value obtained during ischemia (StO2min); (3) StO2 reperfusion rate; (4) the highest StO2 value reached after cuff release (StO2max); and (5) the reactive hyperemia area under the curve (AUC). First, using a conventional 5 minute cuff occlusion protocol, the elderly participants achieved a much slower rate of oxygen recovery (1.5 ± 0.2 vs. 2.5 ± 0.2%·s−1), lower StO2max (85.2 ± 2.9 vs. 92.3 ± 1.5%), and lower reactive hyperemia AUC (2651.8 ± 307.0 vs. 4940.0 ± 375.8%·s−1). However, due to a lower skeletal muscle resting metabolic rate, StO2min was also significantly attenuated in the elderly participants compared to the young controls (55.7 ± 3.5 vs. 41.0 ± 3.4%), resulting in a much lower ischemic stimulus. To account for this important group difference, we then matched the level of tissue ischemia in a subset of young healthy participants by reducing the cuff occlusion protocol to 3 minutes. Remarkably, when we controlled for tissue ischemia, we observed no differences in any of the hyperemic endpoints between the young and elderly participants. These data highlight the important role NIRS can serve in vascular biology, but also establishes the need for assessing tissue ischemia during cuff-occlusion protocols.
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