A method and apparatus for operating a ventilator to control the fraction ofpressure inspired oxygen (FiO2) to a patient that includes: providing aventilator controller that includes a software algorithm, a pulse oximeter anda (FiO2) flow rate controller； measuring the pulse oximetry of the patient andcomputing an average pulse oximetry value over a time period； selecting afirst, second and third pulse oximetry levels as set points for the ventilatorcontroller； selecting an update time interval； decreasing the (FiO2) flow rateby a first incremental amount when the average pulse oximetry value is greaterthan the first level； increasing the (FiO2) flow rate by a second incrementalamount when the average pulse oximetry value is less than the second level；increasing the (FiO2) flow rate to the maximum and initiating an alarmcondition when the average pulse oximetry value is less than the third level.
