Fig. 1

The respiratory system quasi-static pressure–volume curve and recruitment-to-inflation ratio. The respiratory system quasi-static pressure–volume curve was recorded using an automatic ventilator tool for low-flow inflation from 0 to 40 cm H₂O and low-flow deflation from 40 to 0 cm H₂O, with a constant pressure variation of 2 cm H₂O/s. The evaluation parameters in the pressure–volume curve were as follows: volume difference between the inspiratory and expiratory limbs at 20 cm H₂O (distance at 20 cm H₂O); normalized distance at 20 cm H₂O, which is the distance at 20 cm H₂O weighted by the maximal volume (V_max); maximal volume difference between the inspiratory and expiratory limbs at the same pressure (maximal distance); and normalized maximal distance, which is the maximal distance weighted by the V_max