Outline the possible effects on oxygenation of the prone position and the potential mechanisms underlying these effects.
The effects of prone positioning on oxygenation are best studied in ARDS patients. Short lived improvements in oxygenation are common (eg. 70%) and sometimes dramatic. Some patients have no effect, and others have a long lasting effect (persisting well after rolling supine again). Potential mechanisms for improving oxygenation during proning include: an increase in end-expiratory lung volume (with better response to applied PEEP and tidal recruitment), better ventilation–perfusion matching (with more homogeneous distribution of ventilation, and less shunting), and regional changes in ventilation associated with alterations in chest-wall mechanics (allowing more of applied pressure to inflate the lungs). Prolonged benefits may be seen if inflation of recruitable lung has resulted in more lung units being held open when returned to the baseline ventilatory settings (Vt and PEEP).
Mechanisms for improved oxygenation during prone ventilation:
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