Safe Pressure Support ventilation

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Patient Self Inflicted Lung Injury (PSILI) is a relatively new concept within mechanical ventilation. It implies that a patient can cause harm to their own lungs, creating barotrauma due to excessive transpulmonary pressure as a result of increased patient respiratory effort.

In Pressure Support mechanical ventilation it's not always easy to identify patients at risk of PSILI. This page will attempt to show you how.

Intensivist Dr. Bertoni proposed a method to estimate respiratory effort: he proposed that by performing an expiratory hold and determining the nadir of the pressure curve, you could determine the force that the patient is applying to the ventilator in order to receive an inspiration, i.e. the predicted muscular pressure or Pmus. Despite seemingly adequate respiratory support by the ventilator, some patients still show signs of increased respiratory effort. The predicted force applied specifically to the lungs, i.e. the predicted transpulmonary pressure or Ptp, is derived from this as well.

How to perform the measurement

  • Press record on your ventilator
  • Perform an expiratory hold
  • Retroactively, note the peak pressure, total PEEP and Pnadir (the lowest point of the inspiratory effort of the patient)

How to calculate Pmus and Ptp

Or… use VentICalc to make this a whole lot easier!

Refresher: P0.1

The airway occlusion pressure can also be described by the P0.1 value, i.e. the pressure drop in the first 0.1 seconds (i.e. 100 ms) of the inspiration. This pressure is always negative (as it is an inspiratory effort), but is sometimes depicted as positive on the ventilator.

I’m using positive P0.1 values here:

  • A normal P0.1 should be between 1 and 2
  • P0.1 <1 (i.e. a slow decline in pressure) implies low respiratory effort and could indicate overassist (NB. this is expert opinion)
  • P0.1 >4 (i.e. a steep decline in pressure) implies high respiratory effort and you should be concerned for for PSILI (patient self-inflicted lung injury)

How I interpret these findings

The study performed by dr. Bertoni is a small study which should be regarded as a proof of principle. Further studies are needed, and thus, my views on this subject are first and foremost with great caution, and secondly, subject to change. That said, this is how I interpret the results.

P0.1 and Pmus do match:

  1. Both are low → oversupport
    ▸ Decrease pressure support
    ▸ Decrease sedation
  2. Both are high → undersupport
    ▸ Increase pressure support. Repeat measurements.
    ▸ If this doesn’t help, increase sedation
    ▸ If this doesn’t help, use a controlled ventilation mode

P0.1 and Pmus do not match:

  1. P0.1 is high and Pmus is low:
    ▸ Hickups?
    ▸ Discomfort
  2. P0.1 is low and Pmus is high:
    ▸ Is the patient young and strong? This may not be as bad as we think.
    ▸ Opioid breathing pattern

Don't forget to check out VentICalc to aid with respiratory mechanics for safe mechanical ventilation!

Written by

I'm an intensivist and clinical pharmacologist, spreading the love for and knowledge of acute and critical care medicine on YouTube

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