Figure 1.

This illustration shows the pressure and flow waves of pressure support. The delivered tidal volume is calculated by the area under the flow curve wave. The inspiratory airflow continuously decreases because, as air moves inside the lungs, the pressure in the alveoli builds up resulting in a reduced pressure difference between the machine and the alveoli (ΔP), and hence a decelerating flow wave. Cycling-off starts when airflow reaches a preset threshold, which is a preset percentage of maximal air flow (usually 25%). At this point, the ventilator stops to deliver inspiratory flow, the expiratory valve opens to allow passive exhalation, and pressure goes down to the set positive end-expiratory pressure. Increasing the cycling-off preset percentage of maximal airflow results in decreasing the inspiratory cycle. The illustration demonstrates two cycles; one of them has cycling-off preset percentage of maximal airflow of 25%, and the other is 50%. The inspiratory time is shorter with 50%, and hence the delivered tidal volume