Respiration during recovery from exercise: effects of trapping and release of femoral blood flow.

To investigate the contribution of vascular and metabolic stimuli to the sustained hyperpnea after exercise, the respiratory effects of obstructing and then releasing the femoral blood flow were recorded in 15 normal volunteers during recovery from steady-state cycle exercise (80 W). Obstruction was achieved using cuffs around the upper thighs, inflated for the first 2 min of recovery to a pressure of 200 mmHg. Cuff inflation significantly reduced ventilation during recovery compared with control (P less than 0.001); the subsequent release of pressure was accompanied by an increase in ventilation (averaging 3.2 l/min), which began on the first breath after release. This preceded a rise in end-tidal CO2 (maximum 8.3 Torr increase), which first became significant on the fourth breath after release and led to a further rise in ventilation. The first-breath increase in ventilation after cuff release persisted, although slightly attenuated (averaging 2.5 l/min), in additional experiments with inspired O2 fraction of 1.0. The pattern of ventilatory response was also similar when the experiments were performed with 5% CO2 in air as the inspirate. The immediate rise in ventilation on cuff release, together with the persistent response on 100% O2, suggests that the vascular changes resulting from cuff release exert an influence on ventilation independent of the effects of released metabolites on the known chemoreceptors. The persistence of the response on 5% CO2 indicates that CO2-sensitive lung afferents do not have a major role in these responses.