Flow triggering occurs when the ventilator detects a drop in flow through the patient circuit during exhalation.

 

Using flow as the trigger variable is a bit more complex. Typically, the ventilator provides a continous low flow of gas through the patient circuit. The ventilator measures the flow coming out of the flow control valve and the flow through the exhalation valve. If the patient is motionless, these two flows are equal (assuming no leaks in the patient circuit). When the patient makes an inspiratory effort, the flow through the exhalation valve falls below the flow from the output valve as flow is diverted into the patient's lungs.

 

To enable flow triggering, the operator must set a flow trigger level, As an example, a ventilator has a baseline flow of 6 L/min. This allows 6 L/min of gas to pass through the patient circuit during the last part of exhalation. The sensors measure a flow of 6 L/min leaving the ventilator and 6 L /min returning to the ventilator. If the flow trigger is set at 2 L/min, the ventilator will begin an assisted breath when it detects a flow of 4 L/min (i.e., a drop of 2 L/min from the baseline) returning to the ventilator.

 

When set properly, flow triggering has been shown to require less work of breathing than pressure triggering.