With time triggering, the breath begins when the ventilator has measured an elapsed amount of time. In other words, the rate of breathing is controlled by the ventilator; therefore, this mode sometimes is called controlled ventilation. The breath is a mandatory breath because ventilator starts it.

 

The operator sets up time triggering with the rate (or frequency) control, which may be a knob or a touch pad. As an example, if the rate is set at 12 breaths/min, a breath occurs every 5 seconds (60 sec/min divided by 12 breaths/min = 5 sec). The internal timing device on the ventilator triggers inspiration after 5 seconds elapses.

 

There are several time intervals of interest during expiration. One is the expiratory flow time . Gas travels from the patient to the atmosphere during this time. Another interval is the expiratory pause time , during which expiratory flow has ceased but inspiratory flow has not yet started. An expiratory hold is sometimes imposed intentionally to measure autoPEEP. The sum of expiratory flow time and the expiratory pause time is the expiratory time.