Heterogeneity of blood flow and metabolism in exercising muscle by near-infrared spectroscopy (NIRS) in COPD patients

Abstract

Introduction: Heterogeneity in the distribution of both blood flow and oxygen consumption (VO 2 ) within exercising skeletal muscle may contribute to exercise limitation in COPD, but its extent is unknown. Methods: We used NIRS to measure regional distribution of blood flow and VO 2 in six COPD patients (FEV 1 :46±12% predicted) across different exercise intensities (unloaded, 20%, 50% and 80% of peak watts) in air and 100% oxygen. From six optodes placed over the upper, middle and lower vastus lateralis, we recorded: a) indocyanine green dye inflow after intravenous injection to measure local blood flow (Q), and b) fractional tissue O 2 saturation (StO 2 ). Results: Varying both exercise intensity and FIO 2 provided a femoral venous O 2 saturation (SfvO 2 ) range from 15 to 60% and a wide range in StO 2 (from 50% to 75%). Mean StO 2 over the 6 optodes related linearly to SfvO 2 in each subject (r=0.70). We used this relationship to compute local muscle venous blood O 2 saturation from StO 2 recorded at each optode, from which local VO 2 /Q ratios could be calculated by the Fick principle. Multiplying regional VO 2 /Q by Q yielded the corresponding local VO 2 . Relative dispersion (RD) of both Q and VO 2 was about 0.5 under all conditions, while that for VO 2 /Q was less than 0.1. Exercise intensity and FIO 2 had no measurable effect on RD. These results are almost identical to those in healthy young subjects (1) Conclusion: In COPD there appears to be a strong capacity to regulate blood flow according to regional metabolic need and therefore a likely minimal impact of heterogeneity on muscle O 2 availability.