Intrinsic PEEP
OVERVIEW
Definition
- Intrinsic PEEP is also known as autoPEEP or PEEPi
- Intrinsic PEEP occurs when the expiratory time is shorter than the time needed to fully deflate the lungs, preventing the lung and chest wall from reaching an elastic equilibrium point
- This is sometimes referred to as ‘gas trapping’
- ARDS or cardiogenic pulmonary oedema tend to have low levels of PEEPi (e.g. 3-4cmH20)
- asthma or COPD patient have higher levels of PEEPi (e.g. 14cmH2O), which is why they are susceptible to dynamic hyperinflation (‘breath stacking’ from incomplete exhalations) and its complications
MEASUREMENT
Expiratory hold manoeuvre
- PEEPi is measured by performing an end expiratory pause or hold manoeuvre
- expiratory circuit occlusion for 3-5 seconds allows alveolar pressure to equilibrate with airway pressure
Other methods
- Observe the expiratory flow curve, if there is ‘gas trapping’ the curve may not reach baseline
- Disconnect the circuit from the ETT to observe for a prolonged exhalation and audible wheeze. Resolved hypotension and decreased Pplat on reconnection suggests that dynamic hyperinflation causing high PEEPi was present
Limitations
- at the end of expiration the smaller airways may close preventing the over-pressurised alveoli to which they connect from equilibrating with the rest of the respiratory circuit. Thus the expiratory hold manoeuvre will underestimate the actual PEEPi
- Similarly, failure of the expiratory flow curve to reach baseline is an insensitive test
- Plateau pressure (Pplat) will also tend to increase as PEEPi increases, but this could also occur due to decreased lung compliance so increased PEEPi cannot be assumed
CAUSES OF INCREASED PEEPi
- increased expiratory resistance
- bronchospasm e.g. asthma, COPD
- narrowed/kinked ETT
- inspissated secretions
- exhalation valves
- HME filter
- impaired elastic recoil
- emphysema
- increased minute ventilation
- inadequate expiratory time
CONSEQUENCES OF HIGH PEEPi
Dynamic hyperinflation leads to excessively high PEEPi (‘breath stacking’ resulting in ‘gas trapping’)
- increased intrathoracic volume
-> increased pressures for given tidal volume
-> risk of barotrauma - increased intrathoracic pressure
-> decreased venous return, preload and cardiac output (e.g. hypotension post-intubation in severe asthma)
-> increased inspiratory work to trigger the ventilator
MANAGEMENT
- treat reversible factors (bronchospasm, secretions, expiratory devices)
- prolong expiratory time (decrease I:E ratio, decrease RR, increase inspiratory flow)
- decrease tidal volume
- measure PEEPi (expiratory hold)
- set exogenous PEEP to ~2/3 PEEPi, this will decrease inspiratory triggering work and improve distribution of inspired gas (others prefer a PEEP of zero, or ZEEP)
- if in doubt and dynamic hyperinflation is suspected, disconnect the ETT from the circuit to allow spontaneously exhalation, then reconnect to the ventilator
References and Links
LITFL
Critical Care
Compendium
Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.
After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.
He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE. He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.
His one great achievement is being the father of three amazing children.
On Twitter, he is @precordialthump.
| INTENSIVE | RAGE | Resuscitology | SMACC
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Auto-PEEP does not necessarily equate to dynamic hyperinflation. Longer time constant lung units can have autopeep when expiratory time is too short for the pressure in those lung units to fall to the PEEP at the trachea. This does not mean that the expiratory volume in those lung units is less than the inspiratory volume in those lung units. During general anaesthesia for patients with COPD, it is common to ventilate for hours with auto-PEEP evident on the expiratory flow curve and with no evidence of hyperinflation.