The effect of gravity on the response of ventilation to abrupt change in FICO2

Recently, Arieli and Farhi (1987) formulated a model for a previous suggestion made by their group that an increased ventilation as gravity increases is due to reduced perfusion of the respiratory center which causes an elevation of tissue PCO2. Extending the model to the dynamic response, we predict a slower ventilatory response to CO2 breathing as gravity increases. To test this prediction, ventilatory response to 5% CO2 was studied in 11 seated subjects at +1 and +2 Gz in a human centrifuge. Five of these subjects were studied at +3 Gz as well. In addition, ventilatory response to 5% CO2, using breath-by-breath analysis, was measured in three subjects in supine and upright postures. The ventilatory response (mainly through tidal volume) was faster as gravity increased from +1 to +2 and to +3 Gz, and from supine to the upright position. These findings disagree with the model prediction. Therefore, an alternative explanation is suggested based on the response of CO2 sensitive stretch receptors in the lung. Increased gravity causes increased ventilation, reduction of cardiac output and increased VA/Q mismatch; all enlarge the part of the lung with low CO2 where responsiveness of the CO2 sensitive stretch receptors is large.