Acute respiratory distress syndrome overview

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Acute respiratory distress syndrome (ARDS), also known as respiratory distress syndrome (RDS) or adult respiratory distress syndrome (in contrast with IRDS) is a serious reaction to various forms of injuries to the lung. This is the most important disorder resulting in increased permeability pulmonary edema.

ARDS is a severe lung disease caused by a variety of direct and indirect insults. It is characterized by inflammation of the lung parenchyma leading to impaired gas exchange with concomitant systemic release of inflammatory mediators causing inflammation, hypoxemia and frequently resulting in multiple organ failure. This condition is life threatening and often lethal, usually requiring mechanical ventilation and admission to an intensive care unit. A less severe form is called acute lung injury (ALI).

ARDS formerly most commonly signified adult respiratory distress syndrome to differentiate it from infant respiratory distress syndrome in premature infants. However, as this type of pulmonary edema also occurs in children, ARDS has gradually shifted to mean acute rather than adult. The differences with the typical infant syndrome remain.

Below is a table showing The Berlin definition of Acute Respiratory Distress Syndrome:

	Acute Respiratory Distress Syndrome
Timing	❑ Within 1 week of a known clinical insult or new or worsening respiratory symptoms
Chest imaging i.e., CXR or CT	❑ Bilateral opacities—not fully explained by effusions, lobar/lung collapse, or nodules
Origin of edema	❑ Respiratory failure not fully explained by cardiac failure or fluid overload ❑ Need objective assessment (e.g., echocardiography) to exclude hydrostatic edema if no risk factor present
Oxygenation (Corrected for altitude)
Mild	❑ 200 mm Hg < PaO2/FiO2 ≤ 300 mmHg with PEEP or CPAP > 5 cm H2O
Moderate	❑ 100 mm Hg < PaO2/FIO2 ≤ 200 mm Hg with PEEP ≥ 5 cm H2O
Severe	❑ PaO2/FiO2 ≤ 100 mm Hg with PEEP ≥ 5 cm H2O

Diagnosis

ARDS usually occurs within 24 to 48 hours of the initial injury or illness. The patient usually presents with shortness of breath, tachypnea, and symptoms related to the underlying cause, i.e. shock.

An arterial blood gas analysis and chest X-ray allow formal diagnosis by inference using the aforementioned criteria. Although severe hypoxemia is generally included, the appropriate threshold defining abnormal PaO₂ has never been systematically studied.

Any cardiogenic cause of pulmonary edema should be excluded. This can be done by placing a pulmonary artery catheter for measuring the pulmonary artery wedge pressure. However, this is not necessary and is now rarely done as abundant evidence has emerged demonstrating that the use of pulmonary artery catheters does not lead to improved patient outcomes in critical illness including ARDS.

While CT scanning leads to more accurate images of the pulmonary parenchyma in ARDS, its has little utility in the clinical management of patients with ARDS, and remains largely a research tool. Plain Chest X-rays are sufficient to document bilateral alveolar infiltrates in the majority of cases

References

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