A feedback controller for the maintenance of FRC during tidal liquid ventilation: theory, implementation, and testing

The necessity of controlling functional residual capacity (FRC) during tida l liquid ventilation has been recognized since the first description of thi s respiratory support technique by Kylstra et al in 1962. We developed a mi crocomputer feedback system that adjusts the inspired tidal volume (V-T,V-I ) of a liquid ventilator based on the end-expiratory quasi-static alveolar pressure (P-A,P-EE), in order to maintain a stable FRC. The system consists of three subunits: (I) a tracheal pressure catheter to estimate breath by breath FRC changes, derived from P-A,P-EE changes, and (2) a roller pump in terfaced with (3) a personal computer in which a closed-loop control is imp lemented. The regulator sets the actual P-A,P-EE against the corresponding desired value. Any discrepancy is offset by changes in V-T,V-I and the requ ired change in pump velocity is communicated to the roller pump. The size o f any change in pump velocity is determined to both the observed and target or desired P-A,P-EE (i.e., the error) and the (calibration) pressure-volum e curve. To evaluate the efficacy of the controller, a set of laboratory bench tests were conducted under steady state and transient conditions. Closed-loop co ntrol was effective in keeping FRC and P-A,P-EE near the desired level, wit h an acceptable oscillatory behaviour. The feedback controller successfully compensated for transient disturbances of PFC liquid balance. The steady s tate stability was confirmed during a five hour period of liquid ventilatio n in five preterm lambs.