Proportional Assist Ventilation
Your patient is dyssynchronous on the ventilator. No matter what adjustments you make, they still are dyssynchronous, appear air hungry, and uncomfortable. It’s at this point that the clinician should think about PAV. What is PAV you ask? PAV is like power steering. Proportional Assist Ventilation (PAV) is a spontaneous breathing mode created to improve ventilator synchrony by creating more natural breathing. PAV is a form of synchronized partial ventilator assistance where support is adaptive to keep the work of breathing constant.
How it works
The ventilator generates the instantaneous rate and tidal volume of gas flow from the ventilator to the patient by using the equation of motion of the respiratory system. The equation of motion states the pressure applied by the respiratory muscles to the respiratory system is used to overcome elastic and resistive opposing forces. PAV reduces the inspiratory effort needed to overcome respiratory elastance and resistance by applying pressure in proportion to the patient’s effort and need. In pressure support mode the support is fixed driving pressure (i.e. inspiratory pressure). On the other hand, with PAV there is no set target, flow, volume, or pressure which allows the patient to determine their own rate, volume, and timing of each breath controlling their own ventilatory pattern. PAV provides flow and volume assistance. The greater the patient effort, the more pressure the ventilator delivers to decrease the work of breathing thus decreasing the need for sedation. This pattern of support is essentially the opposite of PRVC, where more work by the patient results in LESS support from the vent. See Figure 1 for comparison of support across different modes.
Figure 1: Comparison of patient effort (Pmusc) vs vent support (Pvent) across different modes. PAV = proportional assist ventilation, NAVA = neurally adjusted ventilatory assist, PC = pressure control, PS = pressure support, VC = volume control, PRVC = pressure-regulated volume control, VS = volume support. Concept adapted from prior work by Eduardo Mireles-Cabodevila, MD.
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!.
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.
- Allows patients neurophysiology to direct ventilation creating more natural breathing
- The ventilator and the patient share the work by setting a % support
- Estimates work of breathing and calculates respiratory mechanics determining how much support the patient needs
- Kacmarek, R. (2011, February 01). Proportional assist ventilation AND Neurally ADJUSTED Ventilatory Assist. http://rc.rcjournal.com/content/56/2/140