Sodium Bicarbonate Ingestion Alters the Slow but Not the Fast Phase of VO2 Kinetics

Purpose: The influence of metabolic alkalosis (ALK) on pulmonary O2 uptake (pV·O2) kinetics during high-intensity cycle exercise is controversial. The purpose of this study was to examine the influence of ALK induced by sodium bicarbonate (NaHCO3) ingestion on pV·O2 kinetics, using a sufficient number of repeat-step transitions to provide high confidence in the results obtained. Methods: Seven healthy males completed step tests to a work rate requiring 80% pV·O2max on six separate occasions: three times after ingestion of 0.3 g·kg-1 body mass NaHCO3 in 1 L of fluid, and three times after ingestion of a placebo (CON). Blood samples were taken to assess changes in acid-base balance, and pV·O2 was measured breath-by-breath. Results: NaHCO3 ingestion significantly increased blood pH and [bicarbonate] both before and during exercise relative to the control condition (P < 0.001). The time constant of the phase II pV·O2 response was not different between conditions (CON: 29 ± 6 vs ALK: 32 ± 7 s; P = 0.21). However, the onset of the pV·O2 slow component was delayed by NaHCO3 ingestion (CON: 120 ± 19 vs ALK: 147 ± 34 s; P < 0.01), resulting in a significantly reduced end-exercise pV·O2 (CON: 2.88 ± 0.19 vs ALK: 2.79 ± 0.23 L·min-1; P < 0.05). Conclusions: Metabolic alkalosis has no effect on phase II pV·O2 kinetics but alters the pV·O2 slow-component response, possibly as a result of the effects of NaHCO3 ingestion on muscle pH.