Abstract: The arterial CO.sub.2 monitor and closed loop controller for use with a ventilator monitors a patient's breath and determines PaCO.sub.2 based upon a determination of a deadspace ratio, which is the ratio of the alveolar deadspace to alveolar tidal volume. The method generally comprises the steps of continuously monitoring measurable parameters of a patient's breath; obtaining an input value of PaCO.sub.2 from a blood sample of the patient and using the patients breath parameters and the input value to calculate the deadspace ratio; and continuously determining PaCO.sub.2 based on the assumption that the deadspace ratio subsequently remains constant. Decision rules obtained from other measurable data are preferably also used to identify the onset of changes in the deadspace ratio, and a new deadspace ratio is then determined from the patient's breath parameters and further input value of PaCO.sub.2 from the patient's blood sample.