Acute respiratory distress syndrome overview

# Overview
Acute respiratory distress syndrome (ARDS), originally known as adult respiratory distress syndrome (to contrast with neonatal respiratory distress syndrome) is a serious and potentially life-threatening inflammatory lung condition that develops rapidly (usually within 24 to 48 hours) in the setting of sepsis, toxic exposures, adverse drug reactions, trauma, or other critical illnesses. ARDS is characterized by inflammation of the lung parenchyma resulting in increased permeability of the alveolar-capillary membrane, non-cardiogenic pulmonary edema, impaired gas exchange, and decreased lung compliance.
The vast majority of patients with ARDS are managed in an intensive care unit (ICU), where many will require mechanical ventilation at some point during the course of their illness and recovery. ARDS may be categorized as mild, moderate, or severe based on the degree to which oxygenation is impaired; however, all levels of severity carry a high mortality rate if appropriate measures to improve oxygenation and minimize the risk of further lung injury are not taken.

# Historical Perspective
Although the pathologic features of ARDS were first documented in the 19th century, the modern definition of ARDS did not arise until the 1960s. In 2012, the Berlin Definition of ARDS became the standard diagnostic criteria and definition of the syndrome.

# Classification
ARDS may be classified according to 2012 Berlin Definition into three subtypes: mild, moderate, and severe. These levels of severity are based on the degree to which oxygenation relative to the amount of supplemental oxygen is being delivered to the patient via positive pressure ventilation.

# Pathophysiology
ARDS is a syndrome of inflammation and increased permeability with the lung parenchyma that leads to loss of type I pneumocytes, impaired gas exchange, inappropriate cell proliferation within alveoli, and, in survivors, fibrosis.

# Causes
ARDS may be caused by either direct or indirect insults to the lung. Common causes of ARDS include sepsis, aspiration pneumonitis, and transfusion-related acute lung injury (TRALI).

# Differentiating ARDS from Other Diseases
ARDS must be differentiated from other diseases that cause hypoxemia and pulmonary infiltrates, such as pneumonia, pulmonary contusion, pulmonary edema, and pulmonary hemorrhage. Prior to the development of the Berlin Definition in 2012, a greater emphasis was placed on excluding other potential illnesses prior to making a diagnosis of ARDS. While it is important to recognize and treat and underlying cause of the patient's impaired ventilation and hypoxemia, this search for potential etiologies should not delay any focused efforts to improve oxygenation and ventilation.

# Epidemiology and Demographics
The incidence of ARDS in the United States is estimated at approximately 75 cases per 100,000 individuals, which amounts to roughly 150,000 new cases annually. There is substantial variance in the rates of ARDS between different countries and geographic regions due to factors such as mean life expectancy, prevalence of different risk factors and comorbidities, and access to healthcare.

# Risk Factors
The most potent risk factor in the development of ARDS is chronic alcoholism. Other risk factors include advanced age, cigarette smoke exposure, and chronic liver disease.

# Screening
There are no screening tools for ARDS. The best way to make an early diagnosis of ARDS is to apply the diagnostic criteria to any patient with bilateral pulmonary infiltrates on chest x ray, and new/worsening hypoxemia with an increasing supplemental oxygen requirement in whom a potential cause or risk factor for ARDS exists.

# Natural History, Complications, and Prognosis
If left untreated, 70% of patients with ARDS may progress to mortality. Common complications to ARDS include weakness, impaired lung function, and brain death. Prognosis for patients with ARDS is generally poor and varies based on the severity of illness, the precipitating insult, and medical comorbidities.

# Diagnosis

## Diagnostic Criteria
Established by the ARDS Definition Task Force in 2012, the Berlin Definition is the most current set of diagnostic criteria for ARDS.

## History and Symptoms
The history of a patient with ARDS varies according to the underlying cause. The symptoms of ARDS are fairly nonspecific and typically include rapid breathing, shortness of breath, and rapid heartbeat.

## Physical Examination
There are no physical exam findings specific to or pathognomonic of ARDS. The most notable physical exam findings tend to be those of the underlying illness or injury, as well as those of respiratory distress, critical illness, shock, and end organ damage.

## Laboratory Findings
Laboratory findings consistent with the diagnosis of ARDS include an arterial partial pressure of oxygen (PaO2) that is inappropriately low relative to the fraction of inspired oxygen (FIO2) that is being breathed by the patient. This is referred to as the PaO2/FIO2 ratio (sometimes abbreviated as P/F ratio) and is calculated by dividing the PaO2 (in mmHg) by the FIO2 (as a decimal rather than a percentage).

## Imaging Findings

### Electrocardiogram
There are no EKG findings associated with ARDS.

### X Ray
X ray is the imaging modality of choice for ARDS. By definition, findings of ARDS on x ray include bilateral airspace opacities.

### CT
There are no findings on computed tomography (CT) that are diagnostic of ARDS. CT scan may be used to better define the extent of lung injury or identify potential underlying causes of ARDS.

### Echocardiogram
Echocardiography is not especially useful in the diagnosis of ARDS, except under circumstances where cardiogenic pulmonary edema has not yet been excluded.

## Other Diagnostic Studies
There are no additional diagnostic studies that are especially useful in the diagnosis and management of ARDS.

# Treatment

## Medical Therapy
The majority of medical therapies for ARDS are aimed at treating its underlying cause (e.g., antimicrobials for infection).

### Mechanical Ventilation Therapy
Most patients with ARDS will require endotracheal intubation and mechanical ventilation at some point during the course of their illness and recovery. A mechanical ventilation strategy using lower tidal volumes of 6 mL/kg predicted body weight and higher levels of positive end-expiratory pressure (PEEP) has been shown to be most effective at improving oxygenation and minimizing volutrauma (injury to stiff lungs resulting from overdistention).

## Surgery
Surgical intervention is not recommended for the management of ARDS, except when a procedure might be recommended to treat the underlying cause.

## Prevention
There are no primary preventive measures available for ARDS. The only prevention strategy for ARDS is early recognition and treatment of medical conditions that have the potential to cause ARDS.
Secondary prevention strategies for ARDS include aggressive treatment of the underlying cause and the early application of noninvasive methods of oxygenation to slow or prevent the worsening of ARDS and potentially reduce the need for intubation and mechanical ventilation.