Role of airway resistance in the control of ventilation during exercise

To analyze the interdependence of respiratory drive, ventilation and airway resistance during exercise, mouth occlusion pressure (P0.1), minute ventilation (V) and mean inspiratory flow (VT/TI) were studied in eight normal subjects performing cycle-ergometer exercise at loads ranging from 0 W to 200 W under two different ambient conditions: 1) during oxygen breathing at 1.3 ATA, and 2) during air breathing at 6 ATA (PO2 = 1.3 ATA). Comparison of measurements at 6 ATA with those at 1.3 ATA indicated that a 4.2-fold increase in respired gas density (D) had little or no influence on the V and VT/TI responses whereas P0.1 at any given VT/TI was increased by a factor of 1.9. In both conditions, P0.1 increased at a faster rate than VT/TI as the work load increased. At loads higher than 40 W, the relationship between P0.1, D and VT/TI was found to approximate the equation P0.1 = K X D0.5(VT/TI)1.4, where K is a constant that varies among subjects. The results indicate that the ratio P0.1/(VT/TI), an estimate of respiratory impedance, increased with both D and VT/TI. Evidence is presented that the respiratory drive was reflexly enhanced in response to loading as airway resistance increased with D and/or VT/TI. We conclude that neural mechanisms compensating for internal flow-resistive loading play an important role in the control of ventilation during exercise, both at normal and at raised air pressures.