Positive end-expiratory pressure is an anti-de-recruitment force!
What does PEEP do?
PEEP should be used to keep the patient's lung from collapsing / derecruiting
PEEP is the antagonist to de-recruitment forces from decreased compliance of the chest wall from lack or dysfunction of surfactant, alveolar collapse and superimposed pressure
PEEP maintains lung volume and reduces risk of V/Q mismatch
After a Recruitment Manoeuvre (RM) PEEP allows to maintain a certain EELV potentially increasing respiratory system compliance (Crs) and decreasing driving pressure - both associated with better outcomes
PEEP reduces Atelecttrauma and ventilation heterogeneity
PEEP reduces cardiac output by decreasing RV preload
PEEP increases PVR (pulmonary vascular resistance)
High PEEP increases alveolar dead space (West Zone 1 equivalent) and can cause regional overinflation
Paediatric Intensivists generally use a low-moderate amount of PEEP (5 - 10cm H2O). A degree of apprehension regarding higher levels of PEEP seems to exist.
We advocate for moderate to high levels of PEEP as long as they are haemodynamically tolerated, especially in patients with pARDS. This is in keeping with data from 2018 where patients with PARDS managed with lower PEEP relative to FiO2 than recommended by the ARDSNet model had higher mortality.
The PROSpect trial that is currently recruiting might be able to help answer this question. It uses the same FiO2/PEEP protocol as the ARDSnet trial.
How to set PEEP at the bedside?
Stress is equivalent to trans-pleural pressure (PL).
Strain is equivalent to TV/FRC (without PEEP).
5 CRUCIAL CONCEPTS
- Crs is proportional to the baby lung size in diseased lungs
- Crs tends to increase with recruitment and then starts to decrease again, once overinflation is reached
- Changes in Crs are therefore a key aspect of PEEP titration
- At the same TV any change in Crs will be reflected in the driving pressure. If pressure controlled ventilation is used, the same pressure settings will yield different TVs if the compliance changes
- Assess whether you patient's lungs are recruitable or not. Absence of changes in compliance during a short RM (sustained inflation OR stepwise via ventilator) is a sign for overinflation and/or inability to recruit any lung tissue at this stage.
- Keep increasing PEEP until Crs stops increasing
- If Crs decreases when PEEP is increased consider the lungs to be hyper inflated and decrease PEEP
- Assess changes in oxygenation (paO2) and ventilation (pCO2) while increasing PEEP
- Consider an increase in paO2 with the same FiO2 while increasing PEEP a sign of recruitment
- An increased or steady CO2 elimination can be interpreted as a sign of recruitment
- Decrease in CO2 clearance should be interpreted as a sign of overinflation and the PEEP should be reduced
- During all of the above make sure to maintain a maximum Pplat or PIP of 28 cm H2O to limit strain unless the chest wall compliance is reduced - in that case consider up to 32 cmH2O or measure oesophageal pressure to appreciate trans-pulmonary pressures.
- Aim at a driving pressure of < 15 cm H2O as is is associated with better outcomes
- Adhering to the above will generate TVs that are in accordance with the patient's underlying disease and therefor minimise strain
Using oesophageal pressure to titrate PEEP
In patients with ARDS the chest wall compliance (Ccw) can be significantly reduced. This leads to an increase in pleural pressure (Ppl) which - if greater than the alveolar pressure - can lead to alveolar collapse potentially causing continuous de- and re-inflation of alveoli which is known to be a significant factor of iatrogenic harm to lung tissue. The same is true for abdominal hypertension causing an increase in Ppl.
In these situations Ppl can be approximated using an oesophageal balloon measuring oesophageal pressure (Pes) and then used to avoid alveolar collapse by setting PEEP accordingly.
We are currently awaiting the results of the EPVent 2 trial comparing ventilation guided by trans-pulmonary pressure versus a conventional approach.