Respiratory and cardiac responses to dynamic exercise in man

Ventilation (VE), cardiac output (Q), oxygen consumption (VO2), carbon dioxide production (VCO2), and end tidal gas tensions (PETO2 and PETCO2) were measured in four healthy men during stepwise, steady state increases in work rate on a bicycle ergometer (25, 50, 75, 100, 125, and 150 W). Both the ventilation equivalent (VE/VCO2) and the cardiac equivalent (Q/VCO2) for carbon dioxide, fell during a steady state exercise at 150 W to 2/3 and to 1/3 of the initial levels, respectively. This stepwise reduction in the carbon dioxide production with increasing work rate was compatible with a non-chemical stimulus increasing in proportion to work rate, and governing both ventilation and circulation. These observations do not support the cardio-dynamic hypothesis.