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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Acute Respiratory Distress Syndrome (ARDS) is a high-yield USMLE Step 2 CK respiratory topic requiring diagnosis plus next-best-step management
- Severity assessment comes first: unstable patients need immediate stabilization before definitive diagnostic workup
- Use US guideline pathways, imaging, physiology, microbiology, pathology, or risk scoring depending on the condition
- Management depends on cause, severity, comorbidities, and risk of complications
- Exam stems often hinge on classic clues, contraindications, and when to escalate to imaging, procedure, or ICU care
Overview
ARDS is diffuse inflammatory injury to the alveolar-capillary membrane leading to increased permeability, protein-rich edema, reduced compliance, shunt physiology, and refractory hypoxemia. Berlin criteria grade severity by PaO2/FiO2 ratio on PEEP or CPAP >=5 cm H2O.
Epidemiology
ARDS occurs in critically ill patients and is a major cause of ICU morbidity and mortality in the United States. Pneumonia and sepsis are among the most common triggers. Mortality increases with severity, age, shock, multiorgan failure, and immunosuppression.
Clinical Features
Symptoms
Progressive or acute dyspnea depending on severity and underlying cause
Cough, chest discomfort, fatigue, or exercise limitation
Fever, weight loss, night sweats, or hemoptysis when infection or malignancy is present
Syncope, confusion, severe hypoxemia, or rapidly worsening respiratory distress
Symptoms may be absent when the condition is detected incidentally
Signs
Abnormal breath sounds, crackles, wheeze, dullness, or reduced air entry depending on pathology
Tachypnea, tachycardia, or oxygen desaturation when clinically significant
Cyanosis, hypotension, altered mental status, or respiratory exhaustion
Clubbing, lymphadenopathy, cachexia, or signs of chronic disease when present
Physical examination can be normal in early or mild disease
Investigations
First-line
ABGCalculate PaO2/FiO2 ratio to grade ARDS severity and assess ventilation and acid-base status
Chest X-rayBilateral diffuse opacities not fully explained by effusions, lobar collapse, or nodules
Basic labs and culturesCBC, CMP, lactate, blood cultures, respiratory cultures, and source-directed studies based on suspected trigger
Second-line
Laboratory testingCBC, CMP, inflammatory markers, cultures, viral testing, autoimmune tests, or disease-specific markers depending on presentation
Pulmonary function testingUseful for chronic dyspnea, obstructive/restrictive patterns, DLCO, and treatment response
Microbiology or pathologySputum studies, cytology, biopsy, or fluid analysis when infection, cancer, or inflammatory disease is suspected
Specialist
Specialist referralPulmonology, infectious disease, oncology, sleep medicine, critical care, or thoracic surgery depending on diagnosis and severity
Advanced testingBronchoscopy, PET-CT, sleep study, HRCT, echocardiography, or interventional radiology procedures when indicated
1
Treat cause and support oxygenation
- Source control and antibiotics for sepsis or pneumonia
- Prevent further aspiration, stop offending transfusion or drug exposure, and manage pancreatitis or trauma
- Use high-flow nasal cannula or noninvasive ventilation only in carefully selected early cases; intubate if worsening work of breathing or oxygenation failure
2
Lung-protective ventilation
- Tidal volume 4-8 mL/kg predicted body weight, usually 6 mL/kg
- Plateau pressure <=30 cm H2O
- Use adequate PEEP while monitoring hemodynamics
- Permissive hypercapnia is acceptable if pH remains tolerable and no contraindication exists
3
Severe ARDS strategies
- Prone positioning for moderate to severe ARDS, especially PaO2/FiO2 <150
- Short course neuromuscular blockade in selected severe ventilator dyssynchrony or refractory hypoxemia
- Conservative fluid strategy after initial shock resuscitation
- Consider ECMO referral for refractory severe hypoxemia despite optimal management
Complications
- Respiratory failure: Severe disease can progress to hypoxemia, hypercapnia, or need for ventilatory support
- Secondary infection: Damaged or obstructed lung is prone to bacterial infection
- Pulmonary hypertension: Chronic hypoxemia or parenchymal disease can increase pulmonary vascular resistance
- Delayed diagnosis: Incidental or nonspecific presentations may hide malignancy, TB, PE, or ILD
- Treatment toxicity: Antimicrobials, corticosteroids, anticoagulants, chemotherapy, or procedures can cause harm
USMLE Step 2 CK Exam Tips
- 1ARDS is noncardiogenic pulmonary edema: bilateral infiltrates plus hypoxemia not explained by heart failure
- 2PaO2/FiO2 ratio is the tested severity marker: <=300 with PEEP >=5 meets oxygenation criterion
- 3Best ventilator strategy = low tidal volume based on predicted body weight, not actual body weight
- 4Plateau pressure goal <=30 cm H2O is high yield
- 5Severe ARDS with refractory hypoxemia = prone positioning
- 6Pulmonary capillary wedge pressure is no longer required in Berlin criteria
- 7ARDS after transfusion within 6 hours = transfusion-related acute lung injury
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