Abstract: An apparatus for assisting in ventilating the lungs of a patient comprises a respiratory gas source means; a breathing mask means; a conduit means for fluidly connecting the respiratory gas source means to the mask; a means fop regulating the pressure within the conduit means; a flow meter means for detecting the flow rate of the respiratory gas; and a pressure sensor means for detecting the pressure within the conduit adjacent to the breathing mask. The apparatus further comprises a means for calculating the flow impedance parameter of the flow system downstream the pressure sensing means; a means for storing the relationship between the operation of the pressure regulating means, the flow impedance parameter, and the pressure within the conduit means; a means for predicting the flow impedance parameter after a predetermined time interval; and a means for generating a target pressure to which the pressure regulating means regulates the pressure within the conduit.