Sodium bicarbonate ingestion alters the slow but not the fast phase of VO2 kinetics

Purpose: The influence of metabolic alkalosis (ALK) on pulmonary O-2 uptake (pVO(2)) 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 pVO(2) 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% pVO(2max) 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 pVO(2) 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 pVO(2) response was not different between conditions (CON: 29 +/- 6 vs ALK: 32 +/- 7 s; P = 0.21). However, the onset of the pVO(2) 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 pVO(2) (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 pVO(2) kinetics but alters the pVO(2) slow-component response, possibly as a result of the effects of NaHCO3 ingestion on muscle pH.