Expansion
Dependent regions receive more ventilation and more perfusion in the spontaneously breathing patient
Expansion
Spontaneous breathing: the dependent part of the diaphragm is pushed higher by the abdominal contents, so it sits on a more favourable part of its length-tension curve and contracts more effectively. Ventilation therefore goes preferentially to dependent lung, which is also where perfusion is greatest, so matching is good.
Positive pressure ventilation and paralysis: gas follows the path of least resistance to the non-dependent lung, while perfusion remains dependent. Matching therefore worsens.
Clinical applications:
- Unilateral lung disease: nurse with the good lung down to place the better lung in the well-perfused dependent position and improve oxygenation. The exception is a lung abscess or major haemoptysis, where the affected lung is placed down to protect the healthy side from spillage
- Prone positioning in ARDS improves matching by recruiting dorsal lung while perfusion stays dorsal
- Obesity and pregnancy exaggerate the supine changes, which is why sitting a patient up improves oxygenation