Airway Resistance: Lung Volumes

The lung pressure-volume curve is representative of the lung and chest wall together (if measured without an esophageal balloon) – the “respiratory system.” As pressure rises on the x-axis, volume rises on the y-axis as a function of the respiratory system compliance (see figure below). The curve is measured at zero PEEP, forming a sigmoid shape because of lung recruitment at low pressure-volume and hyperinflation at high pressure-volume. In other words, at low pressure-volume, there is a lower inflection point (LIP) created by alveolar opening pressure. Below the LIP, there is little volume change on the y-axis; however, after the LIP pressure investment, volume rises. Initially, the LIP was thought to be the end of alveolar recruitment and PEEP was set 2 cm H₂O above this level for ventilated patients in the ICU, particularly ARDS patients. Just the opposite of LIP, the upper inflection point (UIP) reveals the moment when increased airway pressure no longer raises volume on the y-axis. The UIP was felt to mark the moment of alveolar over-distension; hence, safe ventilation was determined in simple terms to be between the LIP and UIP.

Conspicuously, there is a pressure difference between inflation and deflation in the pressure volume curve; this is known as “hysteresis” and occurs primarily because inflation pressure must overcome the attraction between water molecules at the water-air interface. Surfactant mitigates inter-molecular attraction of water such that as pressure falls on deflation and a larger volume is achieved.