Acute Respiratory Distress Syndrome

Definition

Definition of Acute Respiratory Distress Syndrome
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 or acute infection to the lung. ARDS is a severe lung syndrome (not a disease) caused by a variety of direct and indirect issues. 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 often fatal, usually requiring mechanical ventilation and admission to an intensive care unit. A less severe form is called acute lung injury (ALI).

Symptoms

Symptoms of Acute Respiratory Distress Syndrome
People usually present with shortness of breath, tachypnea leading to hypoxia and providing less oxygen to the brain occasionally causing confusion .

ARDS can occur within 24 to 48 hours of an injury (trauma, burns, aspiration, massive blood transfusion, drug/alcohol abuse) or an acute illness (infectious pneumonia, sepsis, acute pancreatitis).

ARDS is characterized by:

1. Acute onset
2. Bilateral infiltrates on chest radiograph sparing costophrenic angles
3. Pulmonary artery wedge pressure < 18 mmHg (obtained by pulmonary artery catheterization), if this information is available; if unavailable, then lack of clinical evidence of left ventricular failure suffices
4. if PaO2:FiO2 < 300 mmHg (40 kPa) acute lung injury (ALI) is considered to be present
5. if PaO2:FiO2 < 200 mmHg (26.7 kPa) acute respiratory distress syndrome (ARDS) is considered to be present

Causes

Causes of Acute Respiratory Distress Syndrome
Three clinical settings account for 75% of ARDS cases: 1. Sepsis syndrome - most important cause 2. Severe multiple trauma 3. Aspiration of saliva/gastric contents and it could also be a complication of pneumonia if left untreated known as aspiration pneumonia.

Some cases of ARDS are linked to large volumes of fluid used during resuscitation post trauma. Other causes include shock, near-drowning, multiple transfusions and inhalation of irritants or toxic fumes that damage the alveolar epithelium.

Diagnosis

Diagnosis of Acute Respiratory Distress Syndrome
An arterial blood gas analysis and chest X-ray allow formal diagnosis by the aforementioned criteria. Although severe hypoxemia is generally included, the appropriate threshold defining abnormal PaO2 has never been systematically studied. Note though, that a severe oxygenation defect is not synonymous with ventilatory support. Any PaO2 below 100 (generally saturation less than 100%) on a supplemental oxygen fraction of 50% meets criteria for ARDS. This can easily be achieved by high flow oxygen supplementation without ventilatory support.

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.

Treatment

Treatment of Acute Respiratory Distress Syndrome
Acute respiratory distress syndrome is usually treated with mechanical ventilation in the Intensive Care Unit. Ventilation is usually delivered through oro-tracheal intubation, or tracheostomy whenever prolonged ventilation (=2 weeks) is deemed inevitable.

The possibilities of non-invasive ventilation are limited to the very early period of the disease or, better, to prevention in individuals at risk for the development of the disease (atypical pneumonias, pulmonary contusion, major surgery patients).

Treatment of the underlying cause is imperative, as it tends to maintain the ARDS picture. Appropriate antibiotic therapy must be administered as soon as microbiological culture results are available. Empirical therapy may be appropriate if local microbiological surveillance is efficient. More than 60% ARDS patients experience a (nosocomial) pulmonary infection either before or after the onset of lung injury. The origin of infection, when surgically treatable, must be operated on. When sepsis is diagnosed, appropriate local protocols should be enacted.

Commonly used supportive therapy includes particular techniques of mechanical ventilation and pharmacological agents whose effectiveness with respect to the outcome has not yet been proven. It is now debated whether mechanical ventilation is to be considered mere supportive therapy or actual treatment, since it may substantially affect survival.

Complications

Complications of Acute Respiratory Distress Syndrome
Since ARDS is an extremely serious condition which requires invasive forms of therapy it is not without risk. Complications to be considered are:

1. Pulmonary: barotrauma (volutrauma), pulmonary embolism (PE), pulmonary fibrosis, ventilator-associated pneumonia (VAP).
2. Gastrointestinal: hemorrhage (ulcer), dysmotility, pneumoperitoneum, bacterial translocation.
3. Cardiac: arrhythmias, myocardial dysfunction.
4. Renal: acute renal failure (ARF), positive fluid balance.
5. Mechanical: vascular injury, pneumothorax (by placing pulmonary artery catheter), tracheal injury/stenosis (result of intubation and/or irritation by endotracheal tube.
6. Nutritional: malnutrition (catabolic state), electrolyte deficiency.