Causes of Acute Respiratory Distress Syndrome (ARDS)
ARDS is most commonly caused by pneumonia, sepsis, aspiration, or trauma, with these direct and indirect lung injuries leading to increased vascular permeability and protein-rich pulmonary edema. 1
Direct vs. Indirect Lung Injury
ARDS can be categorized based on the mechanism of injury:
Direct Lung Injuries (ARDSp)
- Pneumonia (bacterial, viral, fungal)
- Aspiration of gastric contents
- Pulmonary contusion
- Inhalation injury
- Near-drowning
- Vaping-associated lung injury 2
- COVID-19 and other viral pneumonias 2, 1
Indirect Lung Injuries (ARDSexp)
- Sepsis (most common indirect cause)
- Trauma with shock
- Pancreatitis
- Major burns
- Non-pulmonary infections
- Multiple blood transfusions
- Drug overdose/toxicity
Iatrogenic and Secondary Factors
Several iatrogenic factors can aggravate or contribute to ARDS development:
- Injurious mechanical ventilation (ventilator-induced lung injury) 2
- Excessive fluid administration 2
- Blood product transfusions 2
- Superimposed nosocomial infections 2
These factors often act as "second hits" after the initial insult, adding complexity to the syndrome and potentially worsening outcomes 2.
Pathophysiological Mechanisms
The underlying pathophysiology of ARDS involves:
- Increased vascular permeability causing protein-rich exudative pulmonary edema 1
- Significant surfactant dysfunction contributing to alveolar collapse 1
- Severely reduced lung compliance (often <25% of normal) 1
- Extensive right-to-left intrapulmonary shunting (>25% of cardiac output) 1
- Ineffective or absent hypoxic pulmonary vasoconstriction 1
ARDS-Mimics
Some conditions can present with similar characteristics to ARDS but have distinct pathophysiology and require specific treatments:
- Diffuse interstitial acute lung diseases
- Diffuse pulmonary infections
- Drug/chemical-induced diffuse lung disease 2
Recognition of these mimics is crucial as they may respond to specific treatments like immunosuppressants, antimicrobials, or withdrawal of the offending agent 2.
Clinical Course and Complications
In some patients, ARDS may progress to fibroproliferation, contributing to unfavorable outcomes 2. The resolution timeline is variable and may take weeks, with survivors often having permanent mild to moderate impairment of lung function 1.
Key Distinctions from Cardiogenic Pulmonary Edema
ARDS must be distinguished from cardiogenic pulmonary edema, which:
- Results from increased hydrostatic pressure due to cardiac dysfunction
- Has minimal direct alveolar epithelial injury
- Contains transudative fluid with lower protein content
- Has better preserved surfactant function initially
- Shows less severely reduced lung compliance compared to ARDS 1
Clinical Implications
Understanding the specific cause of ARDS is essential for appropriate management, as one of the fundamentals of ARDS care is adequate treatment of the underlying disease 2. This approach can significantly impact mortality and morbidity outcomes, as demonstrated by the discovery of dexamethasone as an effective treatment for severe COVID-19 pneumonia 2.