Breathing mechanics during exercise with added dead space reflect mechanisms of ventilatory control

Small increases in external dead space (VD) augment the exercise ventilatory response via a neural mechanism known as short-term modulation (STM). We hypothesized that breathing mechanics would differ during exercise, increased VD and STM. Men were studied at rest and during cycle exercise (10–50 W) without (Control) and with added VD (200–600 ml). With added VD, VT increased via increased end-inspiratory lung volume (EILV), with no change in end-expiratory lung volume (EELV), indicating recruitment of inspiratory muscles only. With exercise, VT increased via both decreased EELV and increased EILV, indicating recruitment of both expiratory and inspiratory muscles. A significant interaction between the effects of exercise and VD on mean inspiratory flow indicated that the augmented exercise ventilatory response with added VD (i.e. STM) resulted from increased drive to the inspiratory muscles. These results reveal different patterns of respiratory muscle recruitment among experimental conditions. Hence, we conclude that fundamental differences exist in the neural control of ventilatory responses during exercise, increased VD and STM.