Proportional assist ventilation has been around in various shapes and forms since the late 1990s. The most advanced current iteration – PAV+ – is unique to Puritan Bennett ventilators. It is a closed loop mode of ventilation. That means that the ventilator dynamically changes the level of assistance that the patient receives in response to patient effort.
PAV+ is neither volume controlled nor pressure controlled but is patient (and operator) controlled. The operator adjusts the percentage support that the ventilator delivers to the patient. The patient breathes – triggering the ventilator – and the ventilator amplifies the patient’s breath. Consequently the more work that the patient does to generate muscular effort the more work the ventilator performs to match the patient’s workload.

It has been known for some time that the diaphragm becomes both atrophic and dysfunctional in acute critical illness, in particular due to disuse during control of mechanical ventilation. In most assisted modes, all the patient needs to do is trigger the ventilator. Patient workload may be inversely proportional to ventilator workload. Frequently the patient’s diaphragm and ventilator are out of synchrony.

PAV+ is patient triggered and flow cycled so it should be seen as a form of pressure support ventilation. PAV+ contrasts with standard pressure support in that the degree of support changes from breath to breath and indeed within breath depending on patient effort. Pressure support delivers a fixed airway pressure for every single breath irrespective of patient effort. Consequently if we map patient effort to ventilator workload there is only one point where the two will intersect. Conversely in proportional assist ventilation the workload of the ventilator and the workload of the patient increase and decrease linearly.

PAV+ works by utilizing very high quality flow and pressure sensors. The ventilator determines when the patient initiates the breath and when the breath is completed. Having instructed the ventilator what proportion of work of breathing that the ventilator should perform, one observes, using a work of breathing bar, if the patient is doing satisfactory work or whether they need to increase or decrease their workload. The work of breathing (WOB) is determined by the ventilator by measuring compliance, resistance and intrinsic peep dynamically every 9 to 12 breaths. As such a Green Zone between 0.3 and 0.7 joules per litre is indicative of ideal work of breathing for the patient; I call this the “sweet spot.” As long as the patient’s WOB resides within the sweet spot of the toolbar the bedside clinician can be satisfied that the patient is both comfortable and safe.

As the tidal volume relates to the patient’s neural activity that results in diaphragmatic power one should not be unduly concerned about high or low tidal volumes in this mode.

If one wishes to put a patient on proportional assist ventilation it is imperative that one determines if the patient is breathing spontaneously and taking an adequate minute ventilation prior to using this mode. The reason for this is that there is no backup rate in PAV+. Usually one starts with 70% support: that means 70% of the work of breathing is performed by the ventilator on 30% by the patient. After a couple of minutes, once one has observed the work of breathing bar, one can make adjustments either to increase the workload of the ventilator or to reduce it by keeping the patient within that Green Zone sweet spot. Generally failure of the patient to settle on this mode is manifest by a respiratory rate of more than 35. Once the patient has been on 20% support for an hour or more and is awake, obeying commands, protecting their airway, and not being suctioned frequently then the patient can be extubated.

Studies that have looked at PAV+ versus pressure support have indicated that weaning is more rapid with PAV+.