A respiratory device comprising: an airway pressure sensor; an airway flow sensor; and an electronic processor programmed to control a mechanical ventilator to provide respiratory support using a pressure controlled ventilation mode and to perform a respiratory system measurement process including: controlling the mechanical ventilator to provide a pressure controlled breath at a preset pressure over an extended inspiratory breath interval that is extended by an extension time interval beyond an end of inspiration indicated by a zero crossing of the airway flow, which marks the end of physiological inspiration; controlling an exhalation valve at least during the extension time interval to maintain airway pressure estimated by the airway pressure sensor at the preset pressure; during the extended inspiratory breath interval, acquiring airway pressure estimates from the airway pressure sensor and airway flow estimates from the airway flow sensor; and determining lung compliance or elastance from the airway pressure estimates and the airway flow estimates acquired during the extended inspiratory breath interval characterized in that controlling an exhalation valve comprises: controlling the exhalation valve over the extended inspiratory breath interval to prevent the airway pressure estimated by the airway pressure sensor from rising above the preset pressure.. an airway pressure sensor; an airway flow sensor; and an electronic processor programmed to control a mechanical ventilator to provide respiratory support using a pressure controlled ventilation mode and to perform a respiratory system measurement process including: controlling the mechanical ventilator to provide a pressure controlled breath at a preset pressure over an extended inspiratory breath interval that is extended by an extension time interval beyond an end of inspiration indicated by a zero crossing of the airway flow, which marks the end of physiological inspiration; controlling an exhalation valve at least during the extension time interval to maintain airway pressure estimated by the airway pressure sensor at the preset pressure; during the extended inspiratory breath interval, acquiring airway pressure estimates from the airway pressure sensor and airway flow estimates from the airway flow sensor; and determining lung compliance or elastance from the airway pressure estimates and the airway flow estimates acquired during the extended inspiratory breath interval controlling the mechanical ventilator to provide a pressure controlled breath at a preset pressure over an extended inspiratory breath interval that is extended by an extension time interval beyond an end of inspiration indicated by a zero crossing of the airway flow, which marks the end of physiological inspiration; controlling an exhalation valve at least during the extension time interval to maintain airway pressure estimated by the airway pressure sensor at the preset pressure; during the extended inspiratory breath interval, acquiring airway pressure estimates from the airway pressure sensor and airway flow estimates from the airway flow sensor; and determining lung compliance or elastance from the airway pressure estimates and the airway flow estimates acquired during the extended inspiratory breath interval characterized in that controlling an exhalation valve comprises: controlling the exhalation valve over the extended inspiratory breath interval to prevent the airway pressure estimated by the airway pressure sensor from rising above the preset pressure..