Intrinsic PEEP and the Expiratory Hold Manoeuvre

Created on Tue, 06/16/2015 - 18:13
Last updated on Tue, 06/16/2015 - 18:31
The expiratory flow manoeuvre allows time for pressure to equalise between the different parts of the respiratory circuit, revealing the intrinsic PEEP in the system.

Performing the expiratory hold manoeuvre

The expiratory hold manoeuvre

If there is gas trapped in the alveoli, and expiration is not sufficient in clearing that gas out, a certain amount of positive pressure will still be found in the lungs following expiration.

This positive pressure is termed "intrinsic PEEP".

An expiratory breath hold stops all flow in the airways; so you can eliminate the expiratory airway resistance (the flow dependent component of intrinsic PEEP).

Thus you are able to measure the "static PEEP", the PEEP due to the elastic recoil of the lungs putting pressure on the gas trapped inside them.

Briefly, about intrinsic PEEP

airway pressure vs alveolar pressure

When you exhale, airflow is driven by the difference between these two pressures.

The difference is created by the elastic recoil of the lungs and the chest wall.

The flow of air out of the lungs is also resisted by the EXPIRATORY AIRWAY RESISTANCE.

 Typically, it takes 1.5 seconds to exhale a tidal breath. 

Things which increase intrinsic PEEP are things which

  • Impair elastic recoil
    • Emphysema
  • Increase expiratory resistance
    • Bronchospasm
    • Airway collapse at the equal-pressure point (where intrathoracic pressure equals intrabronchial pressure)

Gas trapping and dynamic hyperinflation

gas trapping

The key issue is that  THERE IS NOT ENOUGH TIME FOR EXPIRATION.

 The solution to this problem is to increase the I:E ratio.

The patient needs more time to exhale the volume.

     

Limitations of the expiratory hold manoeuvre

Unfortunately, at the end of expiration many of the small airways have closed, and the highly pressurised alveoli at the end of them are not equilibrating their pressure with the rest of the respiratory circuit. This means that the expiratory hold manoeuvre underestimates the true extent of AutoPEEP.

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References

Most of this information comes from only two textbooks. With "Basic Assessment and Support in Intensive Care" by Gomersall et al (as well as whatever I picked up during the BASIC course) as a foundation, I built using the humongous and canonical "Principles and Practice of Mechanical Ventilation" by Tobins et al – the 1442 page 2nd edition.