Exploring cardiorespiratory control mechanisms through gas exchange dynamics.

The rate of increase in oxygen uptake (VO2) at the onset of a step change in work rate can be studied to provide information about the physiological mechanisms that control this process. Several systems must interact to produce the total response. These can be grouped into oxygen transport and oxygen utilization mechanisms. In this paper, the hypothesis that one or the other of these mechanisms limits the adaptation of VO2 to a change in work rate will be examined. In addition to the traditional approach with step changes in work rate, the responses to other work rate forcing functions will be reported. These include ramp, impulse, and pseudorandom binary sequence work rate changes. The evidence that is accumulating from studies involving transitions from different baseline levels of exercise, as well as studies of the effects of hypoxia and beta-adrenergic receptor blockade, has led to the conclusion that oxygen transport mechanisms limit the rate of increase in VO2. However, the dynamic response of VO2 in the presence of adequate oxygen is not much different from that of oxygen limited conditions.