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

Abstract

Heterogeneity in the distribution of both blood flow (Q̇) and O2 consumption (V̇o2) has not been assessed by near-infrared spectroscopy in exercising normal human muscle. We used near-infrared spectroscopy to measure the regional distribution of Q̇ and V̇o2 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 Q̇; and 2) fractional tissue O2 saturation (StiO2) to estimate local V̇o2-to-Q̇ ratios (V̇o2/Q̇). 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 StiO2. Mean Q̇-weighted StiO2 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 StiO2 recorded at each optode, from which local V̇o2/Q̇ could be calculated by the Fick principle. Multiplying regional V̇o2/Q̇ by Q̇ yielded the corresponding local V̇o2. While six optodes along only in one muscle may not fully capture the extent of heterogeneity, relative dispersion of both Q̇ and V̇o2 was ∼0.4 under all conditions, while that for V̇o2/Q̇ was minimal (only ∼0.1), indicating in fit young subjects 1) a strong capacity to regulate Q̇ according to regional metabolic need; and 2) a likely minimal impact of heterogeneity on muscle O2 availability.