Current Concepts of ARDS: A Narrative Review

Abstract

Acute respiratory distress syndrome (ARDS) is characterized by the acute onset of pulmonary edema of non-cardiogenic origin, along with bilateral pulmonary infiltrates and reduction in respiratory system compliance. The hallmark of the syndrome is refractory hypoxemia. Despite its first description dates back in the late 1970s, a new definition has recently been proposed. However, the definition remains based on clinical characteristic. In the present review, the diagnostic workup and the pathophysiology of the syndrome will be presented. Therapeutic approaches to ARDS, including lung protective ventilation, prone positioning, neuromuscular blockade, inhaled vasodilators, corticosteroids and recruitment manoeuvres will be reviewed. We will underline how a holistic framework of respiratory and hemodynamic support should be provided to patients with ARDS, aiming to ensure adequate gas exchange by promoting lung recruitment while minimizing the risk of ventilator-induced lung injury. To do so, lung recruitability should be considered, as well as the avoidance of lung overstress by monitoring transpulmonary pressure or airway driving pressure. In the most severe cases, neuromuscular blockade, prone positioning, and extra-corporeal life support (alone or in combination) should be taken into account.

Keywords: acute respiratory distress syndrome; critically ill patients; lung-protective ventilation; positive end-expiratory pressure.

Figure 1

Diagnostic approach to identify the causal pathogen in patients with pulmonary ARDS.

Figure 2

Ideal model depicting the effects of increased permeability in terms of increased superimposed pressure, with the inhomogeneous coexistence of areas of hyperinflation, normal inflation, collapse and areas of consolidation (as indicated by arrows), along with the necessary pressure that needs to be applied to the lung in order to overcome the superimposed pressure generated by the lung mass and by the chest wall and recruit the alveolar units (i.e., to inflate the collapsed lung regions) and to maintain these regions open. ∞ represents infinite pressure, i.e., areas that can never be open despite increased positive airway pressure.

Figure 3

Example of lung CT scan of patients with high (upper panel) or low (lower panel) potential of lung recruitment. Arrows depict the morphologic change from a condition of low airway pressure (i.e., 5 cm H₂O), to one of high airway pressure (i.e., 45 cm H₂O).

Figure 4

Possible ultrasonographic findings at lung examination. 0: Normal aeration with normal sliding, with A-lines pattern; 1: Multiple B-lines but separated by at least 5 mm; 2: Multiple, coalescent, not well-separated B-lines; 3: Lung consolidation, hyperechoic area with air bronchogram. Numbers on the left side of each ultrasound image represent the depth (in cm).

Figure 5

Pressure-time curve showing different recruitment maneuvers. (A) sustained inflation sigh using continuous positive airway pressure (CPAP) of 35 cm H₂O for 40 s (as depicted by the arrow); (B) stepwise recruitment maneuver using both plateau pressure and PEEP increase, keeping a fixed driving pressure of 15 cm H₂O; after recruitment, a decremental PEEP titration is performed until an optimal level is identified (e.g., one associated with the best compliance or best oxygenation).