During mechanical ventilatory support, there are four phases during each ventilatory cycle: a) the trigger phase (breath initiation), b) the flow delivery phase, c) the cycle phase (breath termination), and e) the expiratory phase. Mechanically delivered breaths can be described by what determines the trigger, flow delivery, and cycle parameters for that breath.
Triggers are of three types: machine timer, pressure change, and flow change. With a machine timed trigger, the clinician sets a rate and mechanical breaths are initiated by a machine timer. With a pressure trigger, a patient effort pulls airway/circuit pressure negative and mechanical breaths are initiated when pressure exceeds the set negative pressure threshold (pressure sensitivity). With a flow trigger, a patient effort draws flow from the circuit (often from a continuous bias flow) and mechanical breaths are initiated when flow into the patient exceeds the set flow threshold (flow sensitivity). In the above figure the pressure triggering threshold is indicated by a negative pressure level below baseline; the flow triggering threshold is indicated by an inspiratory flow above baseline bias flow. The threshold is also the triggering sensitivity.