VENTILATORY CONTROL DURING EXERCISE WITH INCREASED RESPIRATORY DEAD SPACE IN GOATS

Our objectives were to determine 1) the effects of increased respiratory dead space (VD) on the ventilatory response to exercise and 2) whether changes in the ventilatory response are due to changes in the chemoreceptor feedback (rest to exercise) vs. changes in the feedforward exercise stimulus. Steady-state ventilation (V̇I) and arterial blood gas responses to mild or moderate hyperoxic exercise in goats were compared with and without increased VD. Responses were compared using a simple mathematical model with the following assumptions: 1) steady state, 2) linear CO2 chemoreceptor feedback, 3) linear feedforward exercise stimulus proportional to CO2 production (V̇CO2) and characterized by an exercise gain (Gex), and 4) additive exercise stimulus and CO2 feedback producing the system gain (Gsys = ΔV̇I/ΔV̇CO2). Model predictions at constant Gex [assuming VD-to-tidal volume (VT) ratio independent of V̇CO2] are that increased VD/VT will 1) increase arterial PCO2 (Pa(CO2)) and V̇I at rest and 2) increase Gsys via changes in chemoreceptor feedback due to a small increase in the Pa(CO2) vs. V̇CO2 slope. Experimental results indicate that increased VD increased VD/VT, Pa(CO2), and V̇I at rest and increased Gsys during exercise. However, measurable changes in the Pa(CO2) vs. V̇CO2 slope occurred only at high VD/VT or running speeds. Gex was estimated at each VD for each goat by using the model in conjunction with experimental measurements. With 0.2 liter VD, Gex increased 40% (P < 0.01); with 0.6 liter VD, Gex increased 110% between 0 and 2.4 km/h and 5% grade (P < 0.01) but not between 2.4 and 4.8 km/h. Thus, Gex is increased by VD through a limited range. In goats, increases in Gsys with increased VD result from increases in both Gex and CO2 chemoreceptor feedback. These results are consistent with other experimental treatments that increase the exercise ventilatory response, maintaining constant relative Pa(CO2) regulation, and suggest that a common mechanism linked to resting ventilatory drive modulates Gex.