Pressure support ventilation (PSV) is a special form of assisted ventilation. For PSV, the patient must have a consistent, reliable spontaneous respiratory pattern. The ventilator provides a constant pressure during inspiration once it senses that the patient has inspiratory effort. The operator sets the inspiratory pressure, PEEP, and sensitivity level. The patient establishes the rate, inspiratory flow, and T I . PSV is always an assist mode (patient triggered). The flow curve resembles a descending ramp, and the patient can vary the inspiratory flow on demand. A PS breath is patient triggered, pressure limited, and flow cycled. The machine senses a decrease in flow and determines that inspiration is ending. The decrease in flow corresponds to the decrease in the pressure gradient between the mouth and lungs as the lungs fill.
 
PSV is used for three basic functions:
To overcome WOB for spontaneously breathing patients, including patients on CPAP or SIMV, through the ventilator valve system, the ventilator circuit, and the endotracheal tube.
 
To reduce WOB further in CPAP or SIMV by setting the pressure level higher than that required to overcome system resistance.
 
To provide full ventilatory support in the assist mode, in which each patient breath is a PS breath. The patient must have a dependable, intact respiratory center and a fair stable lung condition for this use because VT can vary. This is sometimes referred to as maximum pressure (PSmax)
 
PSV can be used with an artificial airway or a mask to provide non-invasive ventilation.
 
Inspiratory flow in PSV ends when the ventilator determines that flow has dropped to a certain level (flow cycling). In some ventilators this is a fixed value. Newer ventilators have an adjustable flow cycle criterion, which can range from 1 % to 80% of the measured peak inspiratory flow, depending on the ventilator. Manufacturers have given this feature names such as inspiratory cycle percent, inspiratory flow termination, and expiratory flow sensitivity .