Performing 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
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
- Increase expiratory resistance
- Airway collapse at the equal-pressure point (where intrathoracic pressure equals intrabronchial pressure)
Gas trapping and dynamic hyperinflation
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.