Fig. (1).

Safe thresholds for transpulmonary (PL) and alveolar (Palv) pressures. This figure illustrates why the safe threshold of PL is 27 cmH2O while that of Palv may be more than 27 cmH²O. Strain must remain less than 2 to prevent lung injury [16, 17]. Because specific lung elastance (PL x FRC / b) is around 13.5 cmH²O [16], a PL divided by 13.5 must be less than 2.0. Therefore, PL must be less than 27 cmH2O. Pressure applied to the airway is utilized separately to inflate the thoracic cage (pleural pressure) and lung (transpulmonary pressure). Therefore, in some patients the safe threshold of Palv (PL + Ppl) may be higher than 30 -35 cmH²O.