Heterogeneity in the distribution of both blood flow and O2 consumption (VO2) has not been assessed by NIRS in exercising normal human muscle. We used NIRS to measure the regional distribution of blood flow and VO2 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 (FIO2=0.12). Over six optodes over the upper, middle and lower vastus lateralis, we recorded: a) indocyanine green dye inflow after intravenous injection to measure blood flow (Q), and b) fractional tissue O2 saturation (StO2) to estimate local VO2/Q ratios. Varying both exercise intensity and FIO2 provided a (directly measured) femoral venous O2 saturation (SfvO2) range from about 10 to 70%, and a correspondingly wide range in StO2. Mean Q-weighted StO2 over the 6 optodes related linearly to SfvO2 in each subject. We used this relationship to compute local muscle venous blood O2 saturation from StO2 recorded at each optode, from which local VO2/Q ratios could be calculated by the Fick principle. Multiplying regional VO2/Q by Q yielded the corresponding local VO2. While six opt odes along only in one muscle may not fully capture the extent of heterogeneity, relative dispersion of both Q and VO2 was about 0.4 under all conditions, while that for VO2/Q was minimal (only about 0.1), indicating in fit subjects: a) a strong capacity to regulate blood flow according to regional metabolic need and b) a likely minimal impact of heterogeneity on muscle O2 availability.
- Near Infrared Spectroscopy
- Oxygen uptake
- Muscle blood flow
- Healthy subjects
- Copyright © 2014, Journal of Applied Physiology