Factors which alter the relationship between ventilation and carbon dioxide production during exercise in normal subjects

The slope of the linear relationship between ventilation (V-E) and carbon dioxide production (VCO2) has been thought to indicate that VCO2 is one of the major stimuli to V-E. A group of 15 normal subjects undertook different incremental treadmill exercise protocols to explore the relationship between V-E and VCO2, An incremental protocol using 1 instead of 3-min stages of exercise resulted in an increase in the V-E to VCO2 ratio [26.84 (SEM 1.23) vs 31.08 (SEM 1.36) (P < 0.008) for the first stage, 25.24 (SEM 0.86) vs 27.83 (SEM 0.91) (P < 0.005) for the second stage and 23.90 (SEM 0.86) vs 26.34 (SEM 0.81) (P = 0.001) for the third stage]. Voluntary hyperventilation to double the control level of V-E during exercise resulted in an increase in the V-E to VCO2 slope [from 21.3 (SEM 0.71) for the control run to 35.1 (SEM 1.2) for the hyperventilation run (P < 0.001)]. Prolonged hyperventilation (5 min) during exercise at stage 2 of the Bruce protocol resulted in a continuted elevation of VCO2 and the V-E/VCO2 slope. A steady state of V-E and metabolic gas exchange can only be said to have been present after at least 3 min of exercise, Voluntary hyperventilation increased the slope of the relationship between V-E and VCO2, End-tidal carbon dioxide fell, but remained within the normal range. These results would suggest that a non-carbon dioxide factor may have been responsible for the increase we found in V-E during exercise, and that factors other than increased dead space ventilation can cause an increased ventilation to VCO2 slope, such as that seen in some pathophysiological conditions, such as chronic heart failure.