The elastance and a resistance of a subject being ventilated are determined. The determination of elastance and resistance of the breathing of the subject is made without adjusting the ventilation of the subject to facilitate the determination. That is, the determination of elastance and resistance of the subject is made without manipulating one or more parameters of the ventilation in a manner not dictated by a treatment algorithm that is designed to ventilate the subject effectively and/or comfortably.
