A ventilator system for delivering drive pressure ventilation to a patient, the ventilator system comprising: 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 generate output indicative of at least one of flow, volume or pressure; a display for displaying a drive pressure, wherein the drive pressure comprises a pressure for inflating the lungs of the patient against a compliance of the lungs; and a controller configured to collect and analyze the output to determine a condition, in response to the condition, the controller is configured to temporarily switch the ventilator system from a spontaneous breath subtype into a proportional assist breath subtype for at least one breath, wherein the spontaneous breath subtype does not include the proportional assist breath subtype, wherein the controller is configured to estimate a respiratory system compliance of the patient during the PA breath subtype based on the output collected during the PA breath subtype; after the at least one breath, the controller is configured to switch the ventilator system from the PA breath subtype back to the spontaneous breath subtype, characterized in that, after a return to the spontaneous breath subtype, the controller is configured to calculate the drive pressure of the patient based on the respiratory system compliance and the output after the return. 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 generate output indicative of at least one of flow, volume or pressure; a display for displaying a drive pressure, wherein the drive pressure comprises a pressure for inflating the lungs of the patient against a compliance of the lungs; and a controller configured to collect and analyze the output to determine a condition, in response to the condition, the controller is configured to temporarily switch the ventilator system from a spontaneous breath subtype into a proportional assist breath subtype for at least one breath, wherein the spontaneous breath subtype does not include the proportional assist breath subtype, wherein the controller is configured to estimate a respiratory system compliance of the patient during the PA breath subtype based on the output collected during the PA breath subtype; after the at least one breath, the controller is configured to switch the ventilator system from the PA breath subtype back to the spontaneous breath subtype, characterized in that, after a return to the spontaneous breath subtype, the controller is configured to calculate the drive pressure of the patient based on the respiratory system compliance and the output after the return.