The auto-PEEP that EHFO generates may well improve resting lung volume and hence improve oxygenation for any given fractional inspired oxygen (Fio 2). It is unlikely that this is the complete explanation of why improvements in oxygenation in many patients are both very rapid and well maintained. A possible solution may be in the positioning of the cuirass, lying as it does over the lower half of the chest and the upper half of the abdomen. The inspiratory phase of EHFO may be capable of initiating the opening of collapsed lung units in dependent areas of high ventilation-perfusion mismatch. This could then allow conventional ventilation to expand the lung units further, but from a non-collapsed (i.e. more compliant) starting point. In this way, EHFO may be looked upon as a 'director' of ventilation, allowing conventional ventilation to recruit lung units which would otherwise be impossible to expand within safe inflation pressure limits.
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