[Open loop gain of the CO2-ventilation feedback control system in chronic obstructive pulmonary disease]

To evaluate the stability of the CO2-ventilation feedback system, we measured its open loop gain (G) in 12 patients with chronic obstructive pulmonary disease (COPD) and 15 control subjects. Then, we compared G to the conventional slope of the CO2-ventilation response line (S) and that of the metabolic hyperbola (SL). G was determined as the ratio of S to SL by applying external dead space of 250 and 500 ml. G, S and 1/SL in the control and the COPD were +17.1 +/- 7.2 (Mean +/- SD), 1.70 +/- 0.75 L.min-1.Torr-1 and -10.4 +/- 2.0 L-1.min.Torr, and -7.2 +/- 3.3, 0.48 +/- 0.27 L.min-1.Torr-1 and -16.1 +/- 6.4 L-1.min.Torr, respectively. G was significantly correlated with S in both groups, but that was not the case in 1/SL. The magnitude of G and S in COPD was about 42% and 28% of the control, indicating that G was maintained more stable than S. These data suggest that the decreased G in the COPD resulted from insufficient compensation of ventilatory drive, whereas 1/SL increased higher than the control. We conclude that G can be used to indicate the stability of the CO2-ventilation feedback system better than S.