Figure 2: Schematic of the sigmoid-shaped pressure (abscissa) and volume (ordinate) of the respiratory system. Two inflection points are seen following lung inflation. The lower inflection point (LIP) reflects opening of atelectatic alveoli during lung inflation. The upper inflection point (UIP) reflects decrease in pulmonary compliance due to alveolar overstretch. Deflation occurs at a lower pressure than inflation due to hysteresis. PEEP should be applied above the LIP but not above the UIP.