THE RELATIONSHIP BETWEEN PULMONARY O2 UPTAKE KINETICS AND MUSCLE DEOXYGENATION.

Abstract

The temporal relationship between the kinetics of phase 2 pulmonary O2 uptake (VO2 p) and deoxygenation of the vastus lateralis muscle was examined during constant-load leg cycling exercise. Young adults (6 male, 6 female; 23 ± 3 yr; mean ± S.D.) performed repeated transitions on 3 separate days from 20 W to exercise intensities corresponding to moderate (80% ventilatory threshold; MOD) and heavy (δ50%; HVY) exercise. Breath-by-breath VO2 p was measured by mass spectrometer and volume turbine. Oxy-(HbO2), deoxy-(HHb) and total hemoglobin/myoglobin (Hbtot) were determined by near-infrared spectroscopy (NIRS; Hamamatsu NIRO-300). VO2 p data were filtered, interpolated to 1 s, and averaged to 5 s bins. HHb-NIRS data were filtered and averaged to 5 s bins. VO2 p and HHb data were fit with a single-component exponential model; NIRS data were further analyzed by determining the time delay from exercise onset to the start of an increase in HHb. The delay prior to an increase in HHb was 13 ± 2 s (MOD) and 7 ± 1 s (HVY). This delay may reflect either a delay in activation of muscle O2 consumption and/or an early close matching of muscle O2 consumption and local muscle blood flow. The phase 2 time constants (τ) for VO2 p, 30τ 8 s (MOD) and 35 ± 5 s (HVY) were slower (p < 0.01) than the ± HHb, 10 ± 3 s (MOD) and 12 ± 5 s (HVY) and in several subjects, the initial rise in HHb during MOD showed an early overshoot relative to the exponential fit. These data demonstrate that the kinetics of local muscle deoxygenation assessed by NIRS are faster than those of phase 2 VO2 p during both MOD and HVY intensity exercise and suggest that a mismatch exists between O2 delivery and utilization during the exercise on-transient. Supported in part by NSERC, Canada