ARDS is a type of acute respiratory failure seen after direct (eg, aspiration, infection) or indirect (eg, severe trauma) lung injury. Initially, when this clinical syndrome was thought to be related to the underlying shock state and its resuscitation, names such as "shock lung" and "traumatic wet lung" were applied to the subsequent acute respiratory insufficiency. It is now recognized that many types of insults result in damage to the alveolar-capillary membrane, resulting in leakage of proteinaceous fluid into the alveolar space. This injury, with its resulting interstitial and alveolar edema, produces a clinical picture ranging from mild to severe pulmonary dysfunction that can be fatal.

ARDS was formerly defined as:

• Acute onset of respiratory failure.

• Bilateral infiltrates on chest radiography.

• Pao₂/fraction of inspired oxygen (Fɪo₂) ratio of 200 or less.

• Pulmonary capillary wedge pressure less than 18 mm Hg.

However, this definition was reevaluated in 2012 secondary to concerns regarding its accuracy. First, the diagnosis of bilateral infiltrates on chest radiograph had wide interobserver variability and thus was inconsistent. Next, the Pao₂/Fɪo₂ ratio varied greatly based on ventilator status and needed to be refined. Third, patients with heart failure with elevated left atrial pressures can have ARDS, so high pulmonary capillary wedge pressures are not enough to rule out the diagnosis of ARDS. Finally, the definition was found to have low specificity when compared to autopsy results of patients with diffuse alveolar damage.

For these reasons, the Berlin definition was established, which characterizes ARDS by the following:

Oxygenation

• Mild—Pao₂/Fɪo₂ less than 200 mm Hg or less than or equal to 300 mm Hg with positive end- expiratory pressure (PEEP) or continuous positive airway pressure (CPAP) greater than or equal to 5 cm H₂O

• Moderate—Pao₂/Fɪo₂ less than or equal to 100 mm Hg or less than or equal to 200 mm Hg with PEEP greater than or equal to 5 cm H₂O

• Severe—Pao₂/Fɪo₂ less than or equal to 100 mm Hg with PEEP greater than or equal to 5 cm H₂O

• Timing—onset within 1 week of a known clinical insult or the patient has worsening respiratory symptoms

• Chest radiograph—demonstrates opacities bilaterally that are not due to pleural effusions, atelectasis, or nodules

• Origin of edema—respiratory insufficiency that is not solely caused by cardiogenic causes, that is, left heart failure or volume overload

• Acute lung injury—a term for a less severe form of ARDS was removed from the definition and is no longer used.

References

• Fanelli V, Vlachou A, Ghannadian S, et al. Acute respiratory distress syndrome: new definition, current and future therapeutic options. J Thorac Dis. 2013;5:326-334.
• Faugno AJ, Epstein SK. Acute respiratory failure. In: Kellerman RD, Heidelbaugh JJ, Lee EM, eds. Conn's Current Therapy. 1st ed. Elsevier; 2025:913-918.
• Hallman MR. Critical care medicine. In: Cullen BF, Stock MC, Ortega R, et al, eds. Barash, Cullen, and Stoelting's Clinical Anesthesia. 9th ed. Wolters Kluwer; 2024:1539-1565.