A manual patient ventilation system includes a breathing bag manually compressed by a clinician to ventilate a patient via a patient interface device, a gas supply system supplying fresh gas to the breathing bag at a fresh gas flow rate, a pressure sensor measuring a patient airway pressure (PAW) throughout a compression and release cycle of the breathing bag by the clinician, a fresh gas controller comprising a processor, and a manual ventilation control module executable on the processor by the fresh gas controller. The manual ventilation control module includes instructions executable to receive the Paw measurements and determine a fresh gas flow rate based thereon. The gas supply system is then controlled to supply the fresh gas flow rate into the breathing circuit.