Determinants of gas exchange kinetics during exercise in the dog

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Abstract
Following exercise onset, CO2 output (.ovrhdot.VCO2) and O2 uptake (.ovrhdot.VO2) increase exponentially, but with appreciably different time constants. To determine the time course sensitivity of these variables to altered ventilatory kinetics, rhythmic exercise was induced abruptly in anesthetized dogs by bilateral stimulation of the peripheral ends of the cut sciatic and femoral nerves. This increased the metabolic rate by 83 .+-. 25 (SD) %. The dogs were ventilated with a constant-volume pump, the frequency of which was changed exponentially from the start of the exercise until the ventilation that returned arterial CO2 and O2 pressure (PCO2 and PO2) in the steady state to resting levels. The time constant (.tau.) of the increase in ventilation (.ovrhdot.VE) was varied among trials. .ovrhdot.VCO2, .ovrhdot.VO2, end-tidal PCO2 and PO2 and arterial PCO2 were measured breath by breath. .tau..ovrhdot.VO2 was constant at approximately 18 s regardless of alterations in .tau..ovrhdot.VE. .tau..ovrhdot.VCO2 was strongly dependent on .tau..ovrhdot.VE, apparently due to the larger body stores for CO2; the transitions were isocapnic when .tau..ovrhdot.VE was approximately 40 s. Ventilatory dynamics markedly influenced the dynamics of CO2 exchange during exercise, but had no appreciable effect on O2 uptake dynamics.

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