Differences in overall 'gain' of CO2-feedback system between dead space and CO2 ventilations in man

The differences in overall gain of the CO2-ventilation feedback control system obtained by external dead space ventilation (GCO2,ds) and by CO2 inhalation (GCO2,inh) were elucidated in nine healthy male subjects. They breathed into a closed circuit and were subjected to the CO2 steady-state test. This procedure was conducted with 250, 500 and finally 750 ml added dead space (ds), consecutively. About 1 h after these ds runs, a further steady-state CO2 inhalation test (inh) was carried out, maintaining PETCO2 levels equivalent to those in the 250, 500 and 750 ml ds runs. In the ds and the inh runs, PETO2 was maintained at the air breathing level. GCO2 was calculated from the ratio of the slope of the CO2-response curve (S) to that of the metabolic hyperbola (SL). The absolute value of GCO2,ds was larger than that of GCO2,inh. This was accounted for by both a higher slope of the CO2-response curve and a lower slope of the metabolic hyperbola in dead space ventilation than those in CO2 inhalation. From both GCO2,ds and GCO2,inh, the magnitude of the expected change in PETCO2 (delta PETCO2,exp) was calculated for dead space and CO2 breathing, respectively. The delta PETCO2,exp thus calculated was in good agreement with the delta PETCO2 increment experimentally observed (delta PETCO2,act).