Flow-by triggering in mechanical ventilation

Flow-by triggering involves the monitoring of gas flow through the circuit as it is passed by the patient. The machine is triggered once the return flow is less than delivered flow, due to the patient's inspiratory effort. The ventilator monitors return flow (left upper panel), which decreases as patient effort begins (left middle panel). The airway pressure and inspiratory flow required by flow triggering (red dashed arrows) are less than those required to trigger a demand valve (black solid arrows). Once the return flow has diminished, the base flow increases as the machine is triggered; increased gas flow is then delivered to the patient as shown in the graph in the upper right