1. Pressure-Volume Curves The pressure-volume curve represents the continual relationship between changes in pressure and changes in volume of the lung. The slope of the line represents the compliance of the lung. In Figure 5, note the three lines which comprise the inspiratory limb (lower curve). The leftmost line represents noncompliant, atelectatic lung. The point at which the slope changes is known as the lower infection point, also known as Pflex. In the open lung approach (discussed later), PEEP is set to a pressure just above the lower inflection point. Physiologically, it is hypothesized that this is the point at which all atelectatic segments of lung are recruited, and that disallowing ventilator pressures from dropping below this at any point (by setting PEEP above this level) minimizes atelectasis. The point which distinguishes the second change in slope of the line (becoming flat again) is known as the upper inflection point (UIP). Pressures above this point represent overdistended, noncompliant alveoli, and thus this point represents the pressure at which the compliance of the lung decreases dramatically. In this protocol, the PV curve will be measured for each patient and the UIP utilized as the ceiling pressure at any point in the protocol. In this way, we will recruit compliant areas of the lung without the risks of overdistension.