A method of operating a ventilator or pressure support system that automatically determines a timed back up rate for the patient based on the patients spontaneous breathing patterns. The aggressiveness of the determined rate increases as the patient takes less spontaneous breaths. Also, a method wherein one or more mechanisms are used to determine whether a machine breath should be issued. Each mechanism consists of a basic timing element coupled with one or more logical permits. Both the timing element and each of the one or more logical permits must be satisfied to allow the timing mechanism to trigger a machine breath.
