A feedback controller for ventilatory therapy

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Abstract
A computerized system that uses feedback of end-tidal CO2 fraction (FET CO 2) to adjust minute volume of a ventilator has been developed and tested. The effectiveness and robustness of the controller were evaluated in five anesthetized dogs. The controlled responded to step-changes in the set-point for FET CO 2 by adjusting minute volume so that the FET CO 2 settled to the new set-point in less than 60 sec with less than 20% overshoot. The system exhibited suitable dynamic response to step-changes in set-point with loop gains as large as two times and as small as one-half the optimal value. The breath-to-breath variation in FET CO 2 values during prolonged periods of closed-loop controlled ventilation was smaller than the variation during periods of constant minute volume ventilation in three of five experiments. The controller generally maintained FET CO 2 within ±0.1 vol% of the set-point. A disturbance to the controlled system was produced by releasing an occlusion of a branch of the pulmonary artery. The controller always responded to this disturbance in a stable manner, returning the FET CO 2 to its desired value within 30 sec. Accurate control of arterial partial pressure of CO2(PaCO 2) will require modifications enabling the system to determine the relationship between FET CO 2 and PaCO 2.