Monitoring lung mechanics during mechanical ventilation has become an important parameter for managing pulmonary disease and acute changes in respiratory status in patients. Additionally, this monitoring is used to adjust ventilator settings for prevention of alveolar overdistention (or barotrauma) and to attempt to improve clinical outcomes. The two most common airway-pressure parameters used are peak inspiratory pressure (PIP) and plateau pressure (Pplat).

PIP is measured at the end of inspiratory inflation (see Fig. 116.1) and is a function of the inflation volume, the flow resistance of the airways, and the compliance of the lungs and chest wall. Since PIP is greatly influenced by resistance in the upper airways and ventilator equipment, it represents a poor marker of alveolar pressures. The most accurate measurement of alveolar pressure is plateau pressure. This value reflects airway pressure during a 1- to 2-second inspiratory
pause and therefore is less influenced by ventilator equipment. The Pplat approximates small airway and alveolar pressure more closely than PIP. It is believed that control of the Pplat is important as excessive stretch of alveoli has been implicated as the cause of ventilator-induced lung injury. The recent Acute Respiratory Distress Syndrome (ARDS) Network multicenter prospective randomized trial showed that maintaining plateau pressures at ≤30 cm H₂O, with resultant permissive hypercapnia, resulted in a statistically significant decrease in the number of ventilator days and improved mortality.