A hybrid algorithm for terrain simplification

A fluidically-operated respirator comprises an apneic event circuit (10) and a demand gas circuit (20). The apneic event circuit (10) comprises a variable capacitance device (132) and an exhaust means (130) which rapidly discharge fluid from the circuit (10) when an inspiration occurs. If an apneic event occurs, the circuit (10) activates one or more signals (136) as a predetermined volume of fluid is built up in the circuit (10). The demand gas circuit (20) of the respirator supplies respirating gas to a patient at the beginning of an inspiration and for a time period which is a fraction of the duration of the inspiration.