An Algorithm for Automatic Control of O2 and CO2 in Artificial Ventilation

This paper describes a control algorithm for a system built to deliver and control, for artificially ventilated patients, O2 and CO2 concentrations of either the inspiratory, expiratory, or alveolar gases without using an external supply of CO2. The system used is composed of a slightly modified respirator and a rebreathing bag controlled by a microcomputer. If αand ßdenote the relative opening of mixing valves controlling, respectively, the intake of pure oxygen and of expiratory gases from the bag, the inspiratory gas sent to the patient is therefore composed of a fraction a of pure oxygen, a fraction (1 -α)ßtaken from the rebreathing bag, and the balance (1-α)(1-ß) being room air. This multivariable (two-input and two-output signals) and nonlinear system has been identified with the correlation method using pseudorandom binary input signals. For each cycle of the respirator, α and ßare computed by an algorithm based on the state feedback optimal control by minimization of a quadratic criterion. The model of the system has been described by a first-order transfer matrix. Tests show excellent stability with no steady-state error. The settling time to within ±5 percent is of the order of 60 s for FIO2 and 90 s for FICO2.