A ventilator system for delivering drive pressure ventilation to a patient comprises:a pressure generating system that generates a flow of breathing gas；a ventilation tubing system including a patient interface for connecting the pressure generating system to the patient；one or more non-invasive sensors operatively coupled to at least one of the pressure generating system or the ventilation tubing system, wherein the one or more non-invasive sensors are configured to generate output indicative of at least one of flow, tidal volume or pressure；a controller that is configured to receive a selection of a base breath subtype, wherein the base breath subtype comprises a spontaneous breath subtype other than a proportional assist (PA) breath subtype and comprises at least one target setting；the controller is configured to determine a percent support setting for a PA breath subtype based on the at least one target setting；the controller is configured to determine whether a condition has occurred, the condition comprising the expiration of a predetermined amount of time, the delivery of a predetermined number of breaths or a change in non-invasively monitored pressure, flow or tidal volume；in response to the condition, the controller temporarily switches the ventilator system from the spontaneous breath subtype into the proportional assist (PA) breath subtype thereby to deliver at least one PA breath using the determined percent support setting；wherein the controller estimates a respiratory system compliance of the patient based on the output collected during the at least one PA breath；after the at least one PA breath, the controller switches the ventilator system from the PA breath subtype back to the spontaneous breath subtype；after a return to the spontaneous breath subtype, the controller calculates a drive pressure of the patient based on the respiratory system compliance and the output after the return；the controller is conf
