Fig. 2.— Multiple pressure/volume (P/V) curves of the respiratory system recorded from different levels of positive end-expiratory pressure (PEEP) (– – –: PEEP 5; ═: PEEP10; —: PEEP15) and from zero end-expiratory pressure (ZEEP; – · – ·), and related to the elastic equilibrium volume of the respiratory system at ZEEP (EELVZEEP). The curves are aligned on the same P/V diagram after correcting the starting points of each PEEP P/V curve for the volumes measured during passive expired spirograms performed at each PEEP level. The upward shift of PEEP P/V curves indicates alveolar recruitment. The recruited volume with 15 cmH2O PEEP (double-headed arrow), compared to ZEEP, is quantified by the volume difference between the curves, for the same level of airway pressure (20 cmH2O in this example). In other terms, when PEEP is applied, for the same level of airway pressure, lung volume is greater than without PEEP, suggesting the reopening of some alveolar units previously collapsed at ZEEP. Note that recruitment continues far above the value of pressure at the lower inflection point (LIP) and above to the upper inflection point. The different P/V curves tend to join at higher lung volumes, suggesting that the maximal lung volume is approached (modified from 25).