Addition of inspiratory resistance increases the amplitude of the slow component of O2 uptake kinetics

The contribution of respiratory muscle work to the development of the O-2 consumption (V) over dot O-2) slow component is a point of controversy because it has been shown that the increased ventilation in hypoxia is not associated with a concomitant increase in (V) over dot O-2 slow component. The first purpose of this study was thus to test the hypothesis of a direct relationship between respiratory muscle work and (V) over dot O-2 slow component by manipulating inspiratory resistance. Because the conditions for a (V) over dot O-2 slow component specific to respiratory muscle can be reached during intense exercise, the second purpose was to determine whether respiratory muscles behave like limb muscles during heavy exercise. Ten trained subjects performed two 8-min constant-load heavy cycling exercises with and without a threshold valve in random order. (V) over dot O-2 was measured breath by breath by using a fast gas exchange analyzer, and the (V) over dot O-2 response was modeled after removal of the cardiodynamic phase by using two monoexponential functions. As anticipated, when total work was slightly increased with loaded inspiratory resistance, slight increases in base (V) over dot O-2, the primary phase amplitude, and peak (V) over dot O-2 were noted ( 14.2%, P < 0.01; 3.5%, P > 0.05; and 8.3%, P < 0.01, respectively). The bootstrap method revealed small coefficients of variation for the model parameter, including the slow-component amplitude and delay ( 15 and 19%, respectively), indicating an accurate determination for this critical parameter. The amplitude of the (V) over dot O-2 slow component displayed a 27% increase from 8.1 +/- 3.6 to 10.3 +/- 3.4 ml . min(-1) . kg(-1) ( P < 0.01) with the addition of inspiratory resistance. Taken together, this increase and the lack of any differences in minute volume and ventilatory parameters between the two experimental conditions suggest the occurrence of a (V) over dot O-2 slow component specific to the respiratory muscles in loaded condition.