Expansion
It reverses the normal negative intrathoracic pressure, reducing venous return
Expansion
Cardiovascular
- Raised intrathoracic pressure reduces venous return, so cardiac output falls, especially in hypovolaemia
- Right ventricular afterload rises, as alveolar vessels are compressed
- Left ventricular afterload falls, which can benefit a failing left ventricle; this is why non-invasive ventilation helps acute pulmonary oedema
Respiratory
- Ventilation is preferentially distributed to non-dependent lung, the opposite of spontaneous breathing, worsening V/Q matching
- PEEP recruits collapsed alveoli, raises functional residual capacity and improves oxygenation, but excessive PEEP overdistends and creates West zone 1 dead space
- Barotrauma and volutrauma risk from high pressures and volumes
Other
- Reduced renal and hepatic blood flow, with sodium and water retention
- Raised intracranial pressure through impaired cerebral venous drainage
These effects are the rationale for lung-protective ventilation, using low tidal volumes of about 6 ml per kilogram of predicted body weight and limiting plateau pressure.