Pressure support is only applied to spontaneous breaths, the trigger of the breath could be either pressure or flow (sensitivity). In pressure support ventilation, all the breaths are supported breath type and are initiated by the patient. Once the breath is triggered, the ventilator will deliver the pressure support at the limit of the set level above the CPAP/PEEP and the breath will be cycled off when the patient's inspiratory flow declines to a value determined by the clinician (for example, 25% of peak inspiratory flow). In PSV the volume and the flow are both variable and determined by the resistance, compliance, inspiratory effort, and level of pressure support; in addition, the inspiratory time is variable as well.
Pressure support ventilation is a pressure preset mode in which each breath is patient’s triggered, and machines supported. It provides a means of a positive pressure that is synchronized with the inspiratory effort of the patient.
The trigger is either pressure or flow depending on the ventilator used; the set level for the trigger (sensitivity) can be determined by the clinician.
The inspiratory pressures in pressure supported breath are set by the operator. The peak pressure is determined by the addition of the level of pressure support to the level of CPAP/PEEP (i.e., peak pressure = pressure support + CPAP/PEEP). There are no plateau pressures in pressure supported breaths as it is impossible to achieve an inspiratory pause. The speed of pressurization may be fixed by the ventilator or adjustable by setting the rise time.
The flow in pressure support must vary so that the preset level of pressure support is achieved and maintained throughout the breath. Flow cannot, therefore, be set by the operator. Likewise, the flow waveform cannot be set but tends to be decelerating in nature. Initially a high flow rate is delivered to the patient in order distend the alveoli and overcome the resistance of the endotracheal tube. Once the alveoli opening occurs and the preset pressure has been obtained the rate of flow decreases - producing a decelerating flow waveform.
The termination of the pressure support breath is based on the decline of inspiratory flow. Inspiration cycles off when inspiratory flow falls to a preset value. This value may be a percentage of peak inspiratory flow (e.g., 25%) or a fixed amount of flow (e.g., 4 liters / min). The decline of inspiratory flow suggests that the patient’s inspiratory muscles are relaxing and that the patient is approaching the end of inspiration. At this point the inspiratory phase is cycled off. The ventilator terminates the pressure support and opens its exhalation valve. The expiratory phase is free of assistance and returns to baseline pressure which may be level of CPAP/PEEP that is applied.
Pressure support ventilation is thus defined as a mode of ventilation that is patient initiated with a preset pressure, variable flow, volume, and inspiratory time, and is flow cycled.