Dynamics of cardiac, respiratory, and metabolic function in men in response to step work load

Stroke volume, heart rate, cardiac output, tidal volume, respiratory frequency, minute ventilation, end-tidal tensions of O2 and CO2, O2 uptake, CO2 output, and respiratory exchange ratio were measured simultaneously in healthy male volunteers before, during, and after upright bicycle exercise from 0 to 360 and 720 kpm/min. The circulatory variables were determined continuously once per 20 cardiac cycles and the respiratory variables breath by breath using separate computer-based systems in which an impedance pneumograph and an impedance cardiograph were incorporated. Stroke volume, heart rate, and cardiac output started to increase without measurable delay at the onset of exercise. Stroke volume increased by 20% from resting control value in response to the mildest exercise and essentially leveled off with a further increase in work load. Time constant for cardiac output increased with the increasing work load. Time constant for minute ventilation was much longer than that for cardiac output and independent of work intensity. A good synchronization between the ventilation and cardiac output responses at an initial period of transitions from rest to exercise and from exercise to rest seems to support the concept of cardiodynamic hyperpnea.