Studies into the determinants of skeletal muscle oxygen consumption: novel insight from near-infrared diffuse correlation spectroscopy.

Diffuse correlation spectroscopy (DCS) is emerging as a powerful tool to assess skeletal muscle perfusion. Combining DCS with near-infrared spectroscopy (NIRS) introduces exciting possibilities for understanding the determinants of muscle oxygen consumption; however, no investigation has directly compared NIRS-DCS to conventional measures of oxygen delivery and utilization in an exercising limb. To address this knowledge gap, nine healthy males performed rhythmic handgrip exercise with simultaneous measurements by NIRS-DCS, Doppler blood flow and venous oxygen content. The two approaches showed good concurrent validity, with directionally similar responses between: (a) Doppler-derived forearm blood flow and DCS-derived blood flow index (BFI), and (b) venous oxygen saturation and NIRS-derived tissue saturation. To explore the utility of combined NIRS-DCS across the physiological spectrum, we manipulated forearm arterial perfusion pressure by altering the arm position above or below the level of the heart. As expected, Doppler-derived skeletal muscle blood flow increased with exercise in both arm positions, but with markedly different magnitudes (below: +424.3 ± 41.4 ml/min, above: +306 ± 12.0 ml/min, P = 0.002). In contrast, DCS-derived microvascular BFI increased to a similar extent with exercise, regardless of arm position (P = 0.65). Importantly, however, the time to reach BFI steady state was markedly slower with the arm above the heart, supporting the experimental design. Notably, we observed faster tissue desaturation at the onset of exercise with the arm above the heart, resulting in similar muscle oxygen consumption profiles throughout exercise. Taken together, these data support a novel role for NIRS-DCS in understanding the determinants of skeletal muscle oxygen utilization non-invasively and throughout exercise.