Respiratory responses following lifting the legs in normal man.

This study reinvestigates the problems of ventilatory drive linked to the rate of CO2 flow to the pulmonary circulation, and the respiratory response to the changes in cardiac output, which are both known to be capable of matching ventilation to perfusion so that the arterial blood remains isocapnic. Taking advantage of the known influence of posture on venous return, 21 healthy humans were subjected to the experimental maneuver which consisted of lifting the legs from the lower to uppermost position. This maneuver increased venous return to the right heart by bringing stagnant blood from the legs carrying more CO2. In all, 34 tests were performed. The results showed a rapid increase in minute ventilation of 12.3 percent. There were also increases in end-tidal CO2, CO2 production and cardiac output. All of the changes were fully reversible and reproducible. The results support the concept that a higher gain CO2 receptor system is operating during increased CO2 delivery to the lungs via the circulation. The possibility of reflex hyperpnea elicited from pressoreceptors localized in pulmonary circulation is also considered.