Can we measure Plateau pressure during pressure support? And what does it indicate?

Alice Grassi, M.D.
Department of Anesthesia and Pain Management, University Health Network, Toronto School of Medicine,
University of Milan Bicocca, Monza, Italy

from https://coemv.ca/can-we-measure-plateau-pressure-during-pressure-support-and-what-does-it-indicate/

Pressure Support Ventilation is one of the most commonly used modes of assisted ventilation during the weaning phase. In patients who are recovering from the acute phase of both obstructive and restrictive lung diseases, monitoring respiratory effort and the total pressure distending the respiratory system is of invaluable help in order to maintain a “protective” ventilation setting but it is not straightforward in the presence of spontaneous breathing efforts. In fact, the maximum inspiratory pressure (Peak Pressure, Ppeak) recorded by the mechanical ventilator during a tidal breath corresponds to the sum of PEEP and Pressure Support and does not take into account the negative pressure exerted by patient’s muscles. In the presence of the same set Pressure Support Level, the tidal volume received by the patient varies based on the amount of inspiratory effort that he generates and on the lung compliance.

Most of the mechanical ventilators in use allow us to perform inspiratory holds during spontaneous modes of ventilation, including Pressure Support ((1), Supplementary materials). However, some ventilators such as the PB 840 and Medtronic 980 do not allow this maneuver to be done. The Plateau Pressure (Pplat) displayed during Pressure Support Ventilation allows:

To measure Driving Pressure (DP=Pplat-PEEP) and to compute Respiratory System Compliance (Tidal Volume/DP), as during fully controlled modes of ventilation; in a small prospective study, DP during PSV was shown to correlate with outcome as during controlled ventilation (2).
You can also use the difference between Ppeak and Pplat to estimate the amount of muscular pressure that the patient was exerting at the moment of the inspiratory hold (it has been called the Pressure Muscle Index (PMI): it correlates to the muscular pressure measured by an esophageal catheter at the moment of the hold (reference (3) and FIG 2). This happens because if the patient muscles relax during the hold, and the negative pressure that they were generating “appears” as positive pressure on the airway pressure trace.

Some specific issues have to be taken into account while performing an inspiratory hold during Pressure Support Ventilation:

While during the controlled mode of ventilation, Pplat is invariably lower Ppeak (FIG 1), this is not always true during Pressure Support Ventilation (4), when Pplat can be higher than Ppeak. In fact, as mentioned, the negative pressure generated by the patient’s muscles at the moment of the hold is recorded as a positive pressure on the airway tracing during the hold and the total tidal volume generated depends on this total pressure (FIG 2).
Pushing the inspiratory hold button during a spontaneous breath leads to the generation of a plateau that can not always be considered reliable and sometimes has to be discarded. The most common reason for this will be that the patient’s muscle are not relaxing and he/she tries to exhale while the inspiratory hold is at place. It is important to perform the maneuver properly and to be able to recognize which holds can not be deemed reliable, in order not to get misleading information (i.e. measuring a DP higher than the actual one). Some suggestions can be taken into account to properly measure Pplat during PSV:
1. Pplat has to be flat, with a steep increase or decrease or no change from “normal” airway pressure tracing.
2. Maintain the hold for 2-3seconds.
3. Flow has to be 0 during PPlat.
4. The presence of small incisures on pressure curves (inspiratory efforts while the valve is closed) does not make the plateau unreliable, as long as Pplat trace is flat before and after the incisures and can be measured in the flat part.
5. Pplat has to be considered unreliable if it shows a curve shape; if it decreases or increases along time; if the increase from airway peak pressure to plateau pressure is not steep; if the flow does not go to 0 during the inspiratory hold if the patient is clearly contracting expiratory muscles during the hold.

Figures 2 and 3 show examples of readable Pplats, while Figures 4 and 5 show not-readable Pplats.

Keeping all of this in mind, we suggest to TRY this maneuver at the bedside. It does not need any additional monitor and may add information of invaluable help in setting protective spontaneous ventilation.

Figure 1 shows inspiratory holds during controlled modes of ventilation. In both Pressure Controlled (Panel A) and Volume Controlled (Panel B) Pplat is lower than Ppeak.

Figure 2: Inspiratory hold during Pressure Support Ventilation. Pplat may be (as in this case) higher than Ppeak. Pes,ei Pes at end inspiration. Pes,min Minimum Pes.

Figure 3: This is a normal breath followed by a breath with an inspiratory hold (flow is 0 during the hold). Tidal volume is the same in both breaths, as indicated by the equal inspiratory flow. The inspiratory hold reveals that the actual pressure distending the system to generate this tidal volume is not the only the Ppeak (green horizontal line, PEEP + Pressure Support Level) but it is at least 5 cmH2O higher (Pplat, red horizontal line).

In Figure 4, the plateau cannot be considered readable/interpretable because the pressure does not increase steeply from Ppeak to Pplat, possibly indicating muscular activity.