Ventilatory and occlusion-pressure responses to incremental-load exercise

Mouth occlusion pressure (P0.1), minute ventilation (V), and mean inspiratory and expiratory flows were studied in eight normal subjects at rest and during exercise on a cycle ergometer, the load of which was increased in steps of 10 W every minute. All four variables rose curvilinearly as the load was increased from 0 to 200 W. The ratio of P0.1 to mean inspiratory flow, like the ratio P0.1/V, increased with work load in the range 40-200 W, indicating that P0.1 increased considerably faster than mean inspiratory flow and V at rates higher than about 0.7 L X sec-1 and 15 L X min-1, respectively. Evidence is presented that the progressive divergence of the P0.1 and ventilatory responses was a result of raised respiratory impedance consequent to increasing respiratory frequency and resistance, and that, concurrently, the respiratory drive as assessed by P0.1 was enhanced because of an active load-compensating response. In this way, the respiratory drive increased with work load in a self-adjusting fashion, compensating for the impedance-dependent alterations in ventilatory responses. We also conclude that in moderate and heavy exercise P0.1 is a more representative index of the respiratory drive than are V and mean inspiratory flow.