Performance of adjustable pressure-limiting (APL) valves in two different modern anaesthesia machines

The ability to gently ventilate a patient's lungs using a self-inflating bag requires a properly working adjustable pressure-limiting (APL) valve. We compared the performance of the APL valves of the GE Aisys CS² and the Draeger Fabius anaesthetic machines during closure and opening from 1-20 and from 20-1 cmH₂ O, using standardised experimental baby and adolescent patient lung models. Airway pressures and inspiratory tidal volumes were measured using an ASL-5000 test lung and a GE Aisys CS² near-patient spirometry sensors. In both lung models, the GE Aisys CS² APL valves demonstrated non-linear behaviours for airway pressures and for inspiratory tidal volumes, with a sharp increase at set APL pressure levels of 8-10 cmH₂ O. With further closure of the GE Aisys CS² APL valves up to 20 cmH₂ O, inspiratory tidal volumes decreased to ~50% of the highest values measured. Airway pressures in the Draeger Fabius APL valves demonstrated a near linear increase and decrease. Airway pressure values measured in the Draeger Fabius were never higher than those set by the APL valves, whereas in the GE Aisys CS² , they considerably exceeded set pressures (by up to 27 cmH₂ O). We conclude that the performance of the GE Aisys CS² APL valve does not allow safe bag-assisted ventilation of a patient's lungs.