Abstract: A method and apparatus for obtaining an accurate measure of the gas volume expired by a patient by deriving a signal that provides a positive indication of when the patient is exhaling. This signal is used to limit the gas flow monitoring to the expired flow of the patient, thus eliminating the monitoring of a gas volume that leaks through a partially closed ventilator expiration valve during patient inspiration. The gas flow through the expiration valve and the mouth pressure of the patient are used to obtain an indication of the resistance of the expiration valve. This resistance indication is then compared with a preselected threshold valve resistance to accurately determine the time of expiration.