Effect of Respiratory-Alkalosis On VO2, Mean Blood Velocity and Muscle Deoxygenation Kinetics During Moderate-Intensity Exercise

Abstract

The slower adaptation of O2 uptake (VO2) with hyperventilation-induced respiratory alkalosis (R-ALK) during moderate-intensity exercise (MOD) may be related to reduced muscle perfusion during R-ALK, because of CO2-mediated vascular changes. PURPOSE: To examine the effect of R-ALK on VO2, femoral mean blood velocity (MBV) and muscle deoxygenation kinetics at the onset of MOD. METHODS: Six young male subjects (25 ± 5 yrs; mean ± SD) performed alternate-leg knee-extension exercise to a step increase in work rate from baseline (3W) to MOD (80% estimated lactate threshold). Each subject completed 4–5 repetitions under each of 2 exercise conditions: i) during normal breathing (CON; PETCO2 ∼40 mmHg); and ii) during sustained hyperventilation (R-ALK; PETCO2 ∼20 mmHg). VO2 was measured breath-by-breath by mass spectrometry and volume turbine, and MBV was measured simultaneously at the femoral artery by Doppler ultrasound. Changes in deoxy- (HHb), oxy- (O2Hb) and total hemoglobin (HbTOT) of the vastus lateralis were measured continuously by near-infrared spectroscopy (NIRS; Hamamatsu, NIRO 300). The adaptation of VO2, MBV and HHb were modeled using a mono-exponential equation by non-linear regression. RESULTS: The phase 2 VO2 time constant (τVO2) was not different (P = 0.06) between R-ALK (55 ± 26 s) and CON (30 ± 10 s), although τVO2 was greater in all 6 subjects during R-ALK. The adaptation of MBV (τMBV) was not different amongst conditions (R-ALK: 30 ± 12 s; CON: 25 ± 7 s), however 4 of 6 subjects showed greater τMBV in R-ALK. R-ALK did not affect (P>0.05) the adaptation of HHb (τHHb), which was similar between R-ALK (10 ± 4 s) and CON (10 ± 5 s). Baseline HbTOT tended to be lower (P=0.08) in R-ALK (−0.6 ± 5 μM) than CON (3 ± 4 μM), and end-exercise HbTOT was lower (P<0.05) in R-ALK (5 ± 10 $uMM) compared to CON (10 ± 8 μM). CONCLUSION: Collectively, the trend towards slower MBV adaptation, lower blood volume, with unchanged HHb kinetics in R-ALK at the onset of MOD, suggests that the slower VO2 kinetics in R-ALK may be related, in part, to slower and/or lower blood flow adaptation during R-ALK.