Ventilatory work and oxygen consumption during exercise and hyperventilation

The work of breathing (W(B)), and thus the energy requirement of the respiratory muscles, is increased any time minute ventilation (V̇E) is elevated, by either exercise or voluntary hyperventilation. Respiratory muscle O₂ consumption (V̇RM(O₂)) in humans has generally been estimated by having subjects breathe at a level comparable to that during exercise while the change in O₂ consumption (V̇O₂) is measured. The difference between V̇O₂ at rest and during hyperventilation is attributed to the respiratory muscles and is assumed to be similar to V̇RM(O₂) during exercise at the same V̇E. However, it has been suggested that W(B) differs between exercise and hyperventilation and that W(B) during exercise is lower than during hyperventilation at the same V̇E. In this study we measured W(B) during exercise and hyperventilation and from these measurements estimated V̇RM(O₂). W(B), V̇E, and V̇O₂ were measured in five male subjects during rest and during exercise or hyperventilation at levels of V̇E ranging from 30 to 130 l/min. V̇E/W(B) relationship was determined for both hyperventilation and exercise. Multiple regression analysis showed that the shape of the two curves was different (P < 0.0001), with W(B) at high levels of V̇E being ≤25% higher in hyperventilation than in exercise. In a second study in which frequency, tidal volume, and duty cycle were controlled as well as V̇E, there was no difference in W(B) between exercise and hyperventilation. V̇O₂ was significantly correlated with W(B), and the estimated V̇RM(O₂) did not increase as a fraction of total V̇O₂ as exercise intensity rose. From these results we suggest that when carefully controlled for both pattern and V̇E, hyperventilation can be used to mimic exercise and to estimate the metabolic cost of breathing. However, if only V̇E is controlled, it is necessary to measure W(B) to estimate the energy used by the respiratory muscles.