PEEP has a protective effect in acute lung injury, and probably in the prevention of ventilator induced lung injury.
The reasons proposed for this effect are:
- Curtailment of quantitative and qualitative surfactant depletion
- Reduced shearing stresses within junctional tissues, reduced parenchymal injury and cytokine release.
PEEP only prevents derecruitment (it does not recruit lung tissue – a sustained recruitment maneuver is needed for this). In the past when large tidal volumes were used to ventilate patients, most lung units were probably opened due to repetitive high pressure expansion. Every time a patient is disconnected from a ventilator, the lungs derecruit, and will not inflate after reinstatement of the previous ventilator settings. A recruitment maneuver is required.
In the absence of PEEP, many alveoli expand and collapse during the respiratory cycle. Others, “sticky” alveoli do not participate in gas exchange. Using PEEP, the alveoli that open in inspiration remain recruited. However some “sticky” alveoli remain collapsed. Application of sustained pressure to the lung pops open the residual alveoli, and PEEP maintains recruitment.
A successful recruitment will result in improved oxygenation, reduced end-tidal CO2 and improved compliance. On occasion, it may be necessary to increase the CPAP further to guarantee recruitment, up to 20 cmH2O above the pressure limit.