ARDS is a Specific Subtype of Acute Respiratory Failure
Acute respiratory failure (ARF) is a broad clinical syndrome of acute and progressive hypoxemia from various cardiorespiratory or systemic diseases, while ARDS is a specific, severe form of ARF characterized by inflammatory injury to the alveolar-capillary barrier with bilateral lung infiltrates and noncardiogenic pulmonary edema. 1
Defining Acute Respiratory Failure
ARF encompasses any acute hypoxemic condition requiring supplemental oxygen or ventilatory support, caused by diverse etiologies including pneumonia, pulmonary embolism, cardiogenic pulmonary edema, exacerbations of chronic lung disease, or neuromuscular weakness 1
ARF does not require specific radiographic findings or severity thresholds—it simply denotes inadequate gas exchange requiring intervention 1
Defining ARDS as a Distinct Entity
ARDS requires all four Berlin Definition criteria simultaneously: acute onset within 1 week of known clinical insult, bilateral opacities on chest imaging not fully explained by effusions/nodules/collapse, respiratory failure not fully explained by cardiac failure or fluid overload, and PaO₂/FiO₂ ≤300 mmHg with minimum PEEP of 5 cmH₂O 2
The pathological hallmark distinguishing ARDS is diffuse alveolar damage, characterized by inflammatory cell accumulation, vascular endothelial and alveolar epithelial injury, proteinaceous alveolar edema, hyaline membrane formation, and decreased lung compliance 3, 4
ARDS severity is stratified by degree of hypoxemia: mild (200-300 mmHg), moderate (100-200 mmHg), and severe (≤100 mmHg) PaO₂/FiO₂ ratios 2
Key Distinguishing Features
Radiographic Differences
- ARF may present with unilateral infiltrates, focal consolidation, or clear lung fields (as in neuromuscular respiratory failure), whereas ARDS mandates bilateral opacities on chest radiograph or CT 2
Mechanism of Hypoxemia
- ARF hypoxemia stems from diverse mechanisms including ventilation-perfusion mismatch, hypoventilation, diffusion impairment, or shunt, while ARDS specifically results from inflammatory disruption of the alveolar-capillary barrier with protein-rich edema flooding airspaces 4, 5
Cardiac vs. Non-Cardiac Etiology
ARDS explicitly excludes cardiogenic pulmonary edema as the primary cause of respiratory failure—echocardiography should demonstrate absence of fluid overload signs when no clear ARDS risk factor exists 2
Cardiogenic pulmonary edema causing ARF typically shows signs of volume overload, elevated filling pressures, and responds to diuresis, whereas ARDS does not 4
Clinical Implications of the Distinction
Management Differences
All ARDS patients require lung-protective ventilation with low tidal volumes (6 mL/kg predicted body weight) and plateau pressure limitation, which is only weakly recommended for general ARF 1, 6
Prone positioning for ≥12 hours daily is indicated for moderate-to-severe ARDS (PaO₂/FiO₂ <150 mmHg) but not routinely used in other forms of ARF 2, 6
Fluid management in ARDS favors restrictive strategies in patients without shock, whereas other ARF etiologies may require different fluid approaches depending on the underlying cause 1
Diagnostic Workup Differences
ARF evaluation focuses on identifying the specific underlying cause (pneumonia, heart failure, pulmonary embolism, etc.) through targeted testing 1
ARDS diagnosis requires systematic exclusion of mimics including diffuse interstitial lung diseases (acute interstitial pneumonia, organizing pneumonia, acute eosinophilic pneumonia), diffuse pulmonary infections (Pneumocystis, viral pneumonitis, miliary tuberculosis), and drug/chemical-induced lung injury (vaping injury, chemotherapy pneumonitis, amiodarone toxicity) 7
Critical Pitfalls to Avoid
Assuming all bilateral infiltrates with hypoxemia represent ARDS leads to misdiagnosis—only a minority of patients meeting Berlin criteria actually have diffuse alveolar damage on autopsy, and many have treatable ARDS-mimics requiring specific therapies like antimicrobials, immunosuppressants, or drug withdrawal 7, 2
Failing to obtain detailed exposure history (substance use including vaping, occupational exposures, medication review, travel history) delays recognition of treatable causes masquerading as ARDS 7
Not all ARF requires the intensive interventions used in ARDS—applying lung-protective ventilation, prone positioning, and restrictive fluid strategies to non-ARDS ARF may not provide benefit and could cause harm in specific contexts 1
Bronchoscopy with bronchoalveolar lavage has 41% diagnostic yield in treatment failures, identifying Legionella, resistant pathogens, tuberculosis, fungi, and Pneumocystis that alter management 7