Respiratory adaptations to dead space loading during maximal incremental exercise

We examined the effects of dead space (VD) loading on breathing pattern during maximal incremental exercise in eight normal subjects. Addition of external VD was associated with a significant increase in tidal volume (VT) and decrease in respiratory frequency (f) at moderate and high levels of ventilation (V̇I); at a V̇I of 120 l/min, VT and f with added VD were 3.31 ± 0.33 liters and 36.7 ± 6.7 breaths/min, respectively, compared with 2.90 ± 0.29 liters and 41.8 ± 7.3 breaths/min without added VD. Because breathing pattern does not change with CO₂ inhalation during heavy exercise (Gallagher et al. J. Appl. Physiol. 63: 238-244, 1987), the breathing pattern response to added VD is probably a consequence of alteration in the PCO₂ time profile, possibly sensed by the carotid body and/or airway-pulmonary chemoreceptors. The increase in VT during heavy exercise with VD loading indicates that the tachypneic breathing pattern of heavy exercise is not due to mechanical limitation of maximum ventilatory capacity, at high levels of VT.