Effect of Blood pH on VO2 Kinetics During High-Intensity Exercise

Abstract

1590 Several investigators have reported faster Phase II (i.e., rapid component) VO2 kinetics for exercise that was preceded by exercise above the lactate threshold. The initial exercise bout decreased blood pH for the start of the subsequent bout, and investigators theorized that this enhanced circulation and increased oxygen delivery to working muscle for the subsequent bout. However, the initial exercise bout may have also caused a metabolic disturbance that contributed to the faster VO2 kinetics. PURPOSE: To investigate the effects of blood pH on VO2 kinetics by altering pH without prior exercise. METHODS: Nine active subjects (29 ± 9 y; 80.1 ± 12.0 kg) completed two 6-min cycling bouts (214 ± 42 W) at 25 W above their ventilatory threshold on separate days. One h prior to exercise, each subject ingested 0.3 g·kg−1 of sodium bicarbonate with ∼1 L of water or water alone. VO2 was measured breath-by-breath and interpolated to 1-s values; 5-s averages were used to examine the VO2 kinetics using a three-component mono-exponential model. RESULTS: Prior to exercise, blood pH was 7.421 ± 0.023 and 7.505 ± 0.031 for the control and bicarbonate trials, respectively. The Phase II VO2 time constant (τ2) was slower for the bicarbonate trial (P = 0.020), but the overall mean response time (MRT) was unaffected. Moreover, blood pH did not affect the amplitude (A3) for Phase III (i.e., slow component) or the difference in VO2 between minutes 3 and 6 (VO2 diff63). The increased blood pH during the bicarbonate trial is thought to have caused a leftward shift of the oxygenhemoglobin disassociation curve and to have diminished vasodilation in working muscle. This would have decreased oxygen delivery to working muscle and slowed VO2 kinetics. CONCLUSION: Our results suggested that oxygen delivery is a limiting factor during exercise above the lactate threshold for only the rapid, but not slow, component of VO2 kinetics.Table