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 (V̇o_2p) 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 (PET_CO2) ~40 mmHg] and sustained hyperventilation (Hypo; PET_CO2 ~20 mmHg). Increases in work rate were made instantaneously from baseline (3 W) to Mod (80% estimated lactate threshold). V̇o_2p 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 (Δ[HHb])-, oxy (Δ[O₂Hb])-, and total hemoglobin-myoglobin (Δ[Hb_Tot) of the vastus lateralis muscle were measured continuously by near-infrared spectroscopy (NIRS). The kinetics of V̇o_2p, LBF, and Δ[HHb] were modeled using a monoexponential equation by nonlinear regression. The time constants for the phase 2 V̇o_2p (Hypo, 49 ± 26 s; Con, 28 ± 8 s) and LBF (Hypo, 46 ± 16 s: Con, 23 ± b s) were greater (P < 0.05) in Hypo compared with Con. However, the mean response time for the overall Δ[HHb] response was not different between conditions (Hypo, 23 ± 5 s; Con, 24 ± 3 s), whereas the Δ[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 ΔV̇_2p) O₂ delivery, which may contribute to the hyperventilation-induced slowing of V̇o_2p (and muscle O₂ utilization) kinetics. [ABSTRACT FROM AUTHOR]