Characteristics of the ventilatory exercise stimulus.

Simple mathematical models were used to quantitatively examine a number of hypotheses concerning the nature of the exercise stimulus. The modelling demonstrated the following for an exercise intensity of 5 times the resting metabolic rate. (1) During the steady state, a deviation in the coupling between VE and metabolic rate by +/- 25% of the value necessary for isocapnia, results in a deviation of PaCO2 of +/- 2 torr from isocapnia. (2) In the transient phase, a mismatch between VE and Q (and thus CO2 flow) of 50% results in a change of PaCO2 of only 1 torr. (3) When resting PaCO2 is changed by 10 torr and it is assumed that the coupling between VE and VCO2 does not change, PaCO2 deviates from isocapnia by less than 2 torr. It is concluded that (1) to experimentally test hypotheses of the exercise stimulus requires resolution of small changes in PaCO2; (2) good regulation of PaCO2 does not necessarily imply precise coupling between VE and VCO2; (3) the ventilatory exercise stimulus need not be a precise function of metabolic rate; (4) in the steady state, the normal CO2 controller will be very effective in minimizing changes in PaCO2 due to a mismatch between ventilation and metabolic rate.