An assessment of overall "gain" of the O2-feedback control system with and without external dead space breathing.

Overall gain of the O2-ventilation feedback control system (GO2) was determined in 9 male and one female healthy subjects. GO2 progressively increased with decreasing end-tidal PO2 (PETO2). This value did not exceed the overall gain of the CO2-ventilation feedback system (GCO2) even at a PETO2 level of 40 mmHg, suggesting that hypoxic stimulation did not become predominant in the present experimental condition. With addition of 250 ml of external dead space, PETO2 decrement (delta PETO2 X actual) was experimentally observed. The delta PETO2 X actual thus obtained was found to be in good agreement with the PETO2 decrement deduced from GO2 (delta PETO2 X expected). This result was similar to that found in the PETCO2 change previously seen in normoxia.