A mechanical ventilator (10) is controlled by an electronic processor (14) to provide respiratory support to a patient (12) using a pressure controlled ventilation mode while being monitored by an airway pressure sensor (30) and an airway flow sensor (32). The electronic processor also controls the ventilator to perform a respiratory system measurement process (44) including: controlling the ventilator to provide a pressure controlled breath at a preset pressure (Ppreset) over an extended inspiratory breath interval that is extended by an extension time interval (TIE) beyond end of physiological inspiration； controlling an exhalation valve (40) at least during the extension time interval to maintain airway pressure at the preset pressure (Ppreset). Lung compliance or elastance is determined from airway pressure measurements and airway flow measurements acquired during the extended inspiratory breath interval.
