Abstract: A patient ventilation system and a computing device-implemented method determine oxygen consumption in a mechanically ventilated subject includes receiving a signal representing a total flow at an inspiratory outlet of a ventilator, calculating a transport delay time between inspiratory and expiratory gas sample points in a patient ventilation circuitry, transmitting a signal to an inlet selector valve to selectively open a fluid path between the inspiratory gas sample point and an oxygen sensor or between the expiratory gas sample point and the oxygen sensor, receiving data representing oxygen content and carbon dioxide content over a period of time, calculating oxygen consumption data over the period of time from the data representing the total flow, oxygen content and carbon dioxide content and from the transport delay time, and displaying the oxygen consumption data over the period of time.