Cardiorespiratory responses to sudden release of circulatory occlusion during exercise

Cardiovascular and respiratory parameters were measured during and after release of pressure in thigh cuffs which occluded circulation to the legs of four human subjects exercising on a bicycle ergometer. The subjects exercised at 200 kg/min while thigh cuffs were inflated for 4 min and then released. Responses from 6 to 8 identical experiments were ensemble averaged so the precise timing of delays could be obtained. Five to ten seconds following cuff release, end-tidal CO2 increased, marking arrival of the trapped blood at the lungs. Ten to eighteen seconds after this increase in end-tidal CO2, ventilation, respiratory rate and tidal volume increased. This delay in ventilation must have resulted in an increase in arterial PCO2 and suggests that arterial chemoreceptors mediated the responses, and that no venous chemoreceptors or CO2-flux or disequilibrium receptors in the lung need be postulated.