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:

  1. 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).
  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:

  1. 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).
  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.
In Figure 5, the pressure keeps increasing during the hold, therefore the Pplat value should be discarded.
This video is an example of performing an inspiratory hold during pressure support on the Servoi ventilator to measure plateau pressure.

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


References

  1. Bellani G, Grassi A, Sosio S, Foti G. Plateau and driving pressure in the presence of spontaneous breathing. Intensive Care Med. 2019;45:97-98.
  2. Bellani G, Grassi A, Sosio S, Gatti S, Kavanagh BP, Pesenti A, et al. Driving Pressure Is Associated with Outcome during Assisted Ventilation in Acute Respiratory Distress Syndrome. Anesthesiology. 2019;131(3):594-604.
  3. Foti G, Cereda M, Banfi G, Pelosi P, Fumagalli R, Pesenti A. End-inspiratory airway occlusion: a method to assess the pressure developed by inspiratory muscles in patients with acute lung injury undergoing pressure support. Am J Respir Crit Care Med. 1997;156(4 Pt 1):1210–6.
  4. Sajjad H, Schmidt GA, Brower RG, Eberlein M. Can the Plateau Be Higher Than the Peak Pressure? Ann Am Thorac Soc. 2018;15(6):754–9.
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12 Comments

  1. Bex on January 13, 2020 at 11:39 pm

    Hello! Thanks for this! I think the axis labels in figure 1 have got switched…

    • Thomas Piraino on January 14, 2020 at 9:34 am

      Thank you, we have corrected the figure.

  2. Victor Perez on June 19, 2020 at 2:07 pm

    Hello. I would like to ask what the airway occlusion pressure is in the video named AOP with the Servo i, 11 cmH2O or 16 cmH2O?

    Thanks.

  3. Angela Rutter on October 6, 2020 at 7:23 pm

    How can I use this information? What is a good PMI, what is a bad PMI? I find this fascinating but want to know how to use this information to better determine the patient’s ability to extubate. Thank you

  4. Luis Ruiz del Fresno on September 27, 2021 at 4:57 am

    Congratulations on all the content on the page. It is of direct clinical utility for us.

    I’d like to know your thought about a discrepancy between tidal volume and plateau pressure in some patient-triggered breaths. Regarding plateau pressure in pressure support, we have observed the same in pressure control when the patient triggers the inspiration. In PC-sIMV + PS we can compare three types of breaths: spontaneous, ventilator-initiated-controlled breaths, and patient-initiated-controlled breaths. Breaths triggered by the patient use to have a bigger tidal volume than those totally controlled, and plateau pressure use to be bigger as well, but sometimes these patient-triggered breaths do not have a higher plateau pressure despite a bigger tidal volume. We speculate that when these patients make an inspiratory effort, they favourably change the chest wall configuration, getting the room for a bigger pulmonary volume, and in that way they accommodate a bigger tidal volume without increasing plateau pressure. If correct, it could be beneficial not to paralyse them but to allow some inspiratory activity.

  5. David Hersey on October 13, 2021 at 11:20 pm

    Hello,

    For mandatory pressure-control breaths can you measure the plateau pressure with an inspiratory hold?

    Won’t the inspiratory hold just show the airway pressure as the summation of the PControl and PEEP?

    I would think the flow waveform, in pressure control, could ID a resistance issue.

    Sorry for so many questions. An ICU Nurse here trying to work this out.

    All the best,

    David

    • Thomas Piraino on October 14, 2021 at 8:55 am

      It is best practice to perform an inspiratory hold in AC-PC to measure plateau. It sometimes can be the summation of PC + PEEP, but only if the decelerating flow waveform is reaching (or close to reaching) zero during each breath. In some cases the flow will not be near zero and therefore the pressure will not reflect plateau. If resistance is high during pressure control the flow will decelerate more slowly, and high compliance will also cause the flow to decelerate slowly.

  6. David Herse on October 14, 2021 at 8:49 pm

    hello Thomas,

    Almost there.

    Basically, like figure 10 in this link:

    http://rc.rcjournal.com/content/65/6/739/tab-figures-data

    The decelerating flow waveform starts looking flattened out?

    David

    • Thomas Piraino on October 14, 2021 at 9:42 pm

      Yes. High resistance will have lower peak flow and prolonged deceleration, high compliance will retain the higher peak flow, but still have prolonged deceleration. The time it takes the lungs to fill effects the waveform deceleration rate. The time it takes to fill is based on the “time constant” which is RxC.

      • David Hersey on October 21, 2021 at 1:25 pm

        Thanks you,

        David

      • Luis Ruiz del Fresno on October 22, 2021 at 5:13 pm

        You are right, just to reinforce what you said in your 8:55 am Oct 14 post about compliance ‘high compliance will also cause the flow to decelerate slowly’, because in the 9:42 pm post it is written the opposite ‘poor compliance … still have prolonged deceleration’

        • Thomas Piraino on October 22, 2021 at 5:21 pm

          Correct, I just updated my comment thank you.

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