Proportional Assist Ventilation

When using PAV the patient must be spontaneously breathing and have an intact neuromuscular drive.  Note, however PAV tends not to work well in the setting of severe dynamic hyperinflation, expiratory flow limitation, and/or respiratory muscle weakness.  Initially, it takes 15 breaths for the ventilator to adapt to the patient’s mechanics.  PAV estimates work of breathing every 5 milliseconds and thus changes the pressure support breath to meet the patient’s demands. PAV calculates respiratory mechanics (resistance and elastance) every 4-10 breaths by applying a short pause at the end of inspiration.  This measurement determines the pressure needed for each breath. In PAV mode the clinician sets a % support to determine how much the ventilator will do and how much the patient will do. Set the % support setting in a place that offloads enough of the total work onto the ventilator allowing the patient to continue to breathe spontaneously. 

If the clinician sets a % support of 40%, this means the ventilator gives 40% and the patient is responsible for the other 60%.   The clinician will notice a work of breathing meter that will show just how much the patient is doing as opposed to the ventilator. It will tell the clinician if the patient is working too hard, not hard enough, or not at all.  The clinician should adjust the % support to maintain the patient’s work of breathing within the green zone. Note, however it is important to enter the patient’s ETT size and the patient’s ideal body weight beforehand.  The other setting the clinician needs to set is the pressure limit. Fio2 and peep are set as you would normally. When it comes to weaning, wean the % support setting low enough to achieve a low delta p before extubation. Once the level is 20% or less, you are providing nearly no inspiratory support, and it’s time to think about pulling the tube!.

Improves Synchrony

PAV improves flow synchrony by matching the patient’s inspiratory demands.  As patient demand increases the ventilator increases pressure and flow.  The ventilator generates enough flow to unload the resistive burden and volume to unload the elastic burden. Improved Cycling synchrony is due to the ventilator auto-adjusting at the end of inspiration based on patient demand.  However, triggering is not directly improved as the patient still needs to generate enough flow to trigger the ventilator.