Pulmonary O2 uptake and leg blood flow kinetics during moderate exercise are slowed by hyperventilation-induced hypocapnic alkalosis

The effect of hyperventilation-induced hypocapnic alkalosis (Hypo) on the adjustment of pulmonary O2 uptake (VO2p) and leg femoral conduit artery ("bulk") blood flow (LBF) during moderate-intensity exercise (Mod) was examined in eight young male adults. Subjects completed four to six repetitions of alternate-leg knee-extension exercise during normal breathing [Con; end-tidal partial pressure of CO2 (PetCO2) approximately 40 mmHg] and sustained hyperventilation (Hypo; PetCO2 approximately 20 mmHg). Increases in work rate were made instantaneously from baseline (3 W) to Mod (80% estimated lactate threshold). VO2p was measured breath by breath by mass spectrometry and volume turbine, and LBF (calculated from mean femoral artery blood velocity and femoral artery diameter) was measured simultaneously by Doppler ultrasound. Concentration changes of deoxy (Delta[HHb])-, oxy (Delta[O2Hb])-, and total hemoglobin-myoglobin (Delta[HbTot]) of the vastus lateralis muscle were measured continuously by near-infrared spectroscopy (NIRS). The kinetics of VO2p, LBF, and Delta[HHb] were modeled using a monoexponential equation by nonlinear regression. The time constants for the phase 2 VO2p (Hypo, 49+/-26 s; Con, 28+/-8 s) and LBF (Hypo, 46+/-16 s; Con, 23+/-6 s) were greater (P<0.05) in Hypo compared with Con. However, the mean response time for the overall Delta[HHb] response was not different between conditions (Hypo, 23+/-5 s; Con, 24+/-3 s), whereas the Delta[HHb] amplitude was greater (P<0.05) in Hypo (8.05+/-7.47 a.u.) compared with Con (6.69+/-6.31 a.u.). Combined, these results suggest that hyperventilation-induced hypocapnic alkalosis is associated with slower convective (i.e., slowed femoral artery and microvascular blood flow) and diffusive (i.e., greater fractional O2 extraction for a given DeltaVO2p) O2 delivery, which may contribute to the hyperventilation-induced slowing of VO2p (and muscle O2 utilization) kinetics.