Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) is an acute, diffuse inflammatory lung injury that produces severe hypoxaemia and stiff, fluid-filled lungs not explained by cardiac failure or fluid overload. It is a defining condition of critical care because most affected patients require mechanical ventilation, and its management has shaped modern intensive care practice.

Definition

Under the 2012 Berlin definition, ARDS is acute (within one week of a known insult) hypoxaemic respiratory failure with bilateral opacities on chest imaging that are not fully explained by cardiac failure or fluid overload, graded as mild, moderate, or severe by the ratio of arterial oxygen tension to inspired oxygen fraction (PaO2/FiO2) measured with a minimum level of positive end-expiratory pressure (ARDS Definition Task Force, 2012).

Scope

This entry covers the Berlin definition of ARDS, its underlying pathophysiology of diffuse alveolar injury, the gas-exchange and mechanical derangements that characterise it, and the epidemiologic burden. It distinguishes ARDS as a critical-care nursing topic from the cardiology and respiratory-medicine entries on related conditions, and it is a conceptual reference rather than a ventilation protocol or treatment guide.

Core questions

• What distinguishes ARDS from cardiogenic pulmonary oedema?
• How does the Berlin definition grade severity?
• What pathophysiologic process produces the hypoxaemia of ARDS?
• Why is ARDS considered a syndrome rather than a single disease?

Key concepts

• Diffuse alveolar damage
• Non-cardiogenic pulmonary oedema
• Increased alveolar-capillary permeability
• Refractory hypoxaemia and the PaO2/FiO2 ratio
• Reduced lung compliance
• Ventilator-induced lung injury
• Berlin definition severity grading

Mechanisms

ARDS arises from injury to the alveolar-capillary barrier, triggered directly (for example by pneumonia or aspiration) or indirectly (for example by sepsis or trauma). Inflammatory mediators recruit neutrophils and increase the permeability of the barrier, so that protein-rich fluid floods the alveoli, surfactant function is impaired, and gas exchange collapses (Ware & Matthay, 2000). The lungs become stiff and poorly compliant, and ventilation-perfusion mismatch and shunting produce hypoxaemia that responds poorly to supplemental oxygen. Because the injury is heterogeneous, mechanical ventilation can itself aggravate the damage, which is why lung-protective strategies and positioning have been central to outcomes research (Guérin, 2013).

Clinical relevance

ARDS is a core reference syndrome for critical care nursing because it frames the physiology of severe hypoxaemic respiratory failure and the rationale behind protective ventilation and positioning. This entry explains how ARDS is defined, classified, and studied; it is descriptive and is not a basis for individual ventilator settings, positioning decisions, or other treatment, which follow current clinical guidelines and institutional protocols.

Epidemiology

ARDS accounts for a substantial proportion of patients requiring mechanical ventilation in intensive care and carries high mortality that rises with severity grade. Population-based estimates show a meaningful incidence and a heavy burden of associated death and long-term morbidity, although figures vary with case definition and ascertainment (Rubenfeld, 2005).

History

The syndrome was first described by Ashbaugh and colleagues in 1967 as acute respiratory distress in adults, and the terminology and definitions evolved over subsequent decades. The 1994 American-European Consensus Conference produced an early standard definition, which was superseded in 2012 by the Berlin definition that refined the timing, imaging, and oxygenation criteria and introduced severity grading (ARDS Definition Task Force, 2012). Recognition that ventilation itself can injure the lung reshaped management toward lung-protective strategies.

Debates

How should ARDS be defined and graded?

Successive definitions have refined the timing, imaging, and oxygenation thresholds, and debate continues over how to classify milder injury and how the criteria perform across resource settings.

Seminal works

• ware-matthay-2000
• ards-definition-2012
• rubenfeld-2005

Frequently asked questions

How is ARDS different from heart failure causing fluid in the lungs?

Both produce pulmonary oedema, but ARDS results from inflammatory injury that makes the alveolar-capillary barrier leaky, whereas cardiogenic oedema results from high pressures due to a failing heart; the Berlin definition specifically requires that the opacities not be fully explained by cardiac failure or fluid overload.

Why is ARDS called a syndrome?

Because it is a common pattern of acute lung injury and hypoxaemia that can be triggered by many different insults, rather than a single disease with one cause.