Dynamics of ventilation, circulation, and gas exchange to incremental and decremental ramp exercise

Transient responses of minute volume (VE), O2 uptake (VO2), CO2 output (VCO2), heart rate (HR), and cardiac output (Q) to a step change and ramp changes with slopes ranging from 33.3 to 14.3 W/min were studied in five healthy human subjects over the load range from 25 to 125 W. The ramp responses were fitted to a first-order model with a pure time delay (td) and a time constant (TC), while most of the step responses fitted better to a second-order model, consisting of a fast and a slow component. No significant asymmetry was observed between the on- and off-responses to step forcing. The mean response time (MRT = td+TC) of the incremental ramp response was prolonged, whereas the MRT of the decremental ramp response was shortened or unchanged, with decreasing ramp slope. The asymmetry was commonly observed in respiratory and gas exchange variables and, to a lesser extent, also in circulatory variables. Neural and humoral factors that might be responsible for this phenomenon are discussed.