Open loop-gain of the CO2 ventilatory control system.

We devised a method of measuring the open-loop gain for CO2-ventilatory feedback control system. The procedure is to gradually increase then decrease the PICO2 while measuring PICO2 and PACO2. We theorized that the gain is delta PICO2/delta PACO2-1. We measured this gain in 11 young healthy subjects and found the value to be 3.24, quite different from what had been reported so far by a conventional method. The conventional method calculates the gain as the ratio of the slope of CO2-response curve(s) over that of metabolic hyperbola. Analysis revealed that the conventional method requires an accurate knowledge of s-value at the very operating point. The assumption that the CO2 response curve is linear down to the operating point is probably the source of error. The open-loop gain for CO2 ventilatory control is in the order of 3, from which we may extrapolate that the slope of CO2-response curve at the operating point is much flatter than the normal value of more than 21/min/mmHg.