Effect of altered body CO2 stores on pulmonary gas exchange dynamics during incremental exercise in humans

The lactate threshold is a widely used and, at times, controversial constru ct in exercise physiology and pathophysiology. Its non-invasive estimation during incremental exercise depends upon CO2 output increasing as a functio n of O-2 uptake, i.e. 'V-slope', as a result of bicarbonate buffering durin g the lactic acidosis. However, we hypothesised that the V-slope deflection could also occur as a consequence of metabolic CO2 being diverted proporti onally more into the CO2 stores in the early phase of exercise. Eight healt hy males performed two incremental exercise tests on a cycle ergometer, wit h and without controlled prior hyperventilation; the hyperventilation cause d end-tidal P-CO2 to decline by similar to 10 mmHg, with the clearance of a CO2 volume averaging 2547 mi. This corresponded to an 'effective' CO2 capa citance of some 3.12 mi mmHg(-1) kg(-1). Gas exchange was determined breath -by-breath, and blood was sampled from the dorsum of the heated hand. Our r esults demonstrate that the early dynamics of CO2 wash-in to the previously depleted body stores can result in a 'pseudo-threshold', i.e. significantl y before the onset of the actual lactic acidosis. Precautions should theref ore be taken to avoid hyperventilation prior to non-invasive estimation of the lactate threshold.