A method for assessing heterogeneity of blood flow and metabolism in exercising normal human muscle by near-infrared spectroscopy

Heterogeneity in the distribution of both blood flow (Q) and O₂consumption (V_O2) has not been assessed by near-infrared spectroscopy in exercising normal human muscle. We used near-infrared spectroscopy to measure the regional distribution of Q and V_O2 in six trained cyclists at rest and during constant-load exercise (unloaded pedaling, 20%, 50%, and 80% of peak Watts) in both normoxia and hypoxia (inspired O₂ fraction = 0.12). Over six optodes over the upper, middle, and lower vastus lateralis, we recorded 1) indocyanine green dye inflow after intravenous injection to measure Q; and 2) fractional tissue O₂ saturation (Sti_O2) to estimate local V_O2-to-Q ratios (V_O2/Q). Varying both exercise intensity and inspired O₂ fraction provided a (directly measured) femoral venous O₂ saturation range from about 10 to 70%, and a correspondingly wide range in Sti_O2. Mean Q-weighted Sti_O2 over the six optodes related linearly to femoral venous O₂ saturation in each subject. We used this relationship to compute local muscle venous blood O₂ saturation from Sti_O2recorded at each optode, from which local V_O2/Q could be calculated by the Fick principle. Multiplying regional V_O2/Q by Q yielded the corresponding local V_O2. While six optodes along only in one muscle may not fully capture the extent of heterogeneity, relative dispersion of both Q and V_O2 was ∼0.4 under all conditions, while that for V_O2/Q was minimal (only ∼0.1), indicating in fit young subjects 1) a strong capacity to regulate Q according to regional metabolic need; and 2) a likely minimal impact of heterogeneity on muscle O₂availability.