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)over dot) and O-2 consumption ((V)over dotO(2)) 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)over dot and (V)over dotO(2) 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 O-2 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)over dot; and 2) fractional tissue O-2 saturation (Sti(O2)) to estimate local (V)over dotO(2)-to-(Q)over dot ratios ((V)over dotO(2)/(Q)over dot). Varying both exercise intensity and inspired O-2 fraction provided a (directly measured) femoral venous O-2 saturation range from about 10 to 70%, and a correspondingly wide range in Sti(O2). Mean (Q)over dot-weighted Sti(O2) over the six optodes related linearly to femoral venous O-2 saturation in each subject. We used this relationship to compute local muscle venous blood O-2 saturation from Sti(O2) recorded at each optode, from which local (V)over dotO(2)/(Q)over dot could be calculated by the Fick principle. Multiplying regional (V)over dotO(2)/(Q)over dot by (Q)over dot yielded the corresponding local(V)over dotO(2). While six optodes along only in one muscle may not fully capture the extent of heterogeneity, relative dispersion of both (Q)over dot and (V)over dotO(2) was similar to 0.4 under all conditions, while that for (V)over dotO(2)/(Q)over dot was minimal (only similar to 0.1), indicating in fit young subjects 1) a strong capacity to regulate (Q)over dot according to regional metabolic need; and 2) a likely minimal impact of heterogeneity on muscle O-2 availability.