Gas leakage from a ventilator circuit and patient interface is estimated by a method that relies on breath-by-breath or time-to-time variability in the pattern of pressure delivery by the ventilator, comprising generating signals that correspond to pressure (PCIRCUIT) and flow rate (Flow) in the ventilator circuit, integrating flow (Integrated Flow), measuring the change in Integrated Flow (ΔV) over an INTERVAL extending from a selected point in the expiratory phase of a ventilator cycle to a selected point during the expiratory phase of a nearby ventilator cycle, repeating the measurement of ΔV for at least two said INTERVALs, processing PCIRCUIT to produce an index or indices that reflect(s) the pattern of pressure delivery by the ventilator during each of said INTERVALs, applying statistical methods to determine the relation between the differences in ΔV among 1NTERVALs and the corresponding differences in said index or indices of PCIRCUIT, and establishing from said relation the leak rate at specified PCIRCUIT values.
