Abstract: The electrical impedance between two selected points on a person's chest will change in response to spontaneous respiration. The rising edge of the impedance signal is detected to indicate the onset of spontaneous inspiration. The supply of gas by the respirator is synchronized with a selected point during the spontaneous inspiration time period. Apnea conditions are detected to trigger a backup system for supplying gas to the patient periodically despite the lack of spontaneous respiration. If so desired, the triggering and turning off of the respirator need not be synchronized to be simultaneous with the onset of inspiration and exhilation; instead, these may be accomplished at selected times after the initiation of inspiration or exhilation. The triggering and turning off of the respirator can also be accomplished in a manner so that the effects of a wandering baseline are minimized.