Fig. 8

Tidal airway P/V curves of tidal volumes of 297 ml (red arrows) at ZEEP and 8.4 cmH₂O of PEEP in the model with two test lungs (nominal lung compliance 38 ml/cmH₂O). Red circles: end-expiratory airway P/V points, arrows: end-inspiratory airway P/V points. Transpulmonary P/V curve (blue arrow) of tidal volume of 297 ml. To determine the elastance of the lung, a volume change must be induced and the increase in transpulmonary pressure caused by the change in volume must be measured. A change in volume can be achieved by tidal inflation or by changing the end-expiratory airway pressure, PEEP inflation. In this case, changing PEEP by 8.4 cmH₂O resulted in an increase in end-expiratory lung volume (281 ml) of the same amount as the tidal volume used (297 ml). In the experiment in the figure, the end-inspiratory transpulmonary pressure calculated conventionally as ΔPAW-ΔPPL is 14.1–5.3 = 8.8 cmH₂O and calculated based on a PEEP step maneuver as (ΔPEEP/ΔEELV) × VT is (8/281) × 297 = 8.9 cmH₂O