Ventilation and CO2 response during +Gz acceleration.

During foot-to-head acceleration (+Gz) ventilation increases despite a drop in alveolar PCO2. In order to investigate the underlying mechanisms, we measured ventilation (VE), VO2, VCO2 and PACO2, cardiac output (Q) and mixed venous CO2 concentration (CVCO2) using non-invasive techniques in 5 subjects breathing either air or a gas mixture containing 5% CO2 at +1, +2 and +3 Gz in a human centrifuge. Arterial PCO2 was calculated from Fick's equation, using CVCO2, Q and VCO2. VE increased from 8.7 to 18.0 L/min during air breathing and from 19.6 to 36.9 L/min during CO2 breathing at +1 and +3 Gz, respectively. The corresponding values for PACO2 are 37.9 vs 26.9 Torr and 47.8 vs 46.4 Torr. Q dropped from 5.9 to 4.8 L/min during air breathing and remained the same during CO2 breathing (6.7 vs 6.5 L/min). As the decrease of PaCO2 almost paralleled that of PACO2, the arterio-alveolar CO2 difference increased only slightly. The CO2 response curve shifts gradually to the left with an increase in +Gz, a fact that does not support the hypothesis that foot-to-head acceleration increases CO2 sensitivity.