Bench-to-bedside review: distal airways in acute respiratory distress syndrome

Abstract

Distal airways are less than 2 mm in diameter, comprising a relatively large cross-sectional area that allows for slower, laminar airflow. The airways include both membranous bronchioles and gas exchange ducts, and have been referred to in the past as the 'quiet zone', in part because these structures were felt to contribute little to lung mechanics and in part because they were difficult to study directly. More recent data suggest that distal airway dysfunction plays a significant role in acute respiratory distress syndrome. In addition, injurious mechanical ventilation strategies may contribute to distal airway dysfunction. The presence of elevated airway resistance, intrinsic positive end-expiratory pressure or a lower inflection point on a pressure-volume curve of the respiratory system may indicate the presence of impaired distal airway function. There are no proven specific treatments for distal airway dysfunction, and protective ventilation strategies to minimize distal airway injury may be the best therapeutic approach at this time.

Figure 1

The diameters of individual airways become smaller toward the lung periphery but the number of airways increases dramatically. As a result, the total airway cross-sectional area increases markedly toward the alveoli and the gas velocities are lower.

Figure 2

Lung with the acute respiratory distress syndrome can develop a lower inflection point. Pressure–volume curves of (a) a normal lung and (b) an acutely injured lung. The lower inflection point (LIP) is indicated. Adapted with permission from Luecke and colleagues [72].

Figure 3

Peak and plateau pressures in mechanically ventilated patients. When the airway is occluded at end-inspiration, the airway pressure declines rapidly from a peak (P_peak) to a lower initial pressure (P_init), followed by a gradual decrease to a plateau pressure (P_plat).

Figure 4

Expiratory flow and detecting intrinsic positive end expiratory pressure. Intrinsic positive end-expiratory pressure (PEEP) is present if the expiratory flow does not reach zero before initiation of the next breath. PEEP can be suspected by the appearance of a truncated expiratory loop. Reproduced with permission from Dhand [73].