The invention provides a method for automatically determining the resistance of the respiratory system of a spontaneously breathing intubated patient, comprising measuring flow (V') and airway pressure (P aw ) as a function of time, determining the airway pressure at the end of the occlusion period as end of occlusion airway pressure P aw (t c )=P occl , analysing the time dependence of the flow starting at the end of the occlusion period to determine a transition time t d at which the slope of the flow changes from a first high value to a second lower value, and/or analysing the time dependence of the airway pressure starting at the end of the occlusion period to determine a transition time t d at which the slope at the airway pressure changes from a first high value to a second lower value, or by detecting an increase of the airway pressure over a predetermined threshold a value to determine the transition time t d , determining the flow at the transition time as transition time flow V'(t d )=V' peak and determining the airway pressure at the transition time as transition time pressure P aw (t d ), calculating the resistance on the basis of the ratio of an estimated driving pressure at the transition time P drv (t d ) and the transition time flow V' peak , wherein the value of the transition time driving pressure P drv (t d ) is calculated as sum of the transition time airway pressure P aw (t d ) and the muscular effort P mus (t d ) at the transition time, which muscular effort P mus (t d ) is extrapolated based on a predetermined time dependence of P mus (t) during the occlusion period and based on value of the end of occlusion airway pressure P occl. .
