Expansion
It makes ventilation more uniform while perfusion remains relatively unchanged
Expansion
Several mechanisms combine:
- More uniform pleural pressure gradient, so ventilation is distributed more evenly
- Recruitment of dorsal lung, which is normally the most dependent, most compressed and most collapsed region, and also the largest by mass
- Perfusion remains predominantly dorsal, so improved dorsal ventilation directly improves matching
- The heart rests on the sternum rather than compressing the left lower lobe
- Reduced compression by the abdominal contents on the diaphragm
- More homogeneous distribution of stress and strain, reducing ventilator-induced injury
In severe ARDS, proning for at least 16 hours a day improves mortality, not merely oxygenation, which is attributed largely to the reduction in ventilator-induced lung injury.
Practical hazards are accidental extubation, line displacement, pressure sores and facial oedema, which is why it requires a trained team.