Pulmonary · PANCE / PANRE

Acute Respiratory Distress Syndrome (ARDS)

Acute non-cardiogenic pulmonary edema with diffuse alveolar damage and severe hypoxemia.

Also known as: ARDS, acute respiratory distress syndrome, noncardiogenic pulmonary edema, Berlin criteria

Overview

Acute, diffuse inflammatory lung injury causing increased pulmonary vascular permeability and alveolar edema, leading to hypoxemic respiratory failure not fully explained by cardiac failure or volume overload. Defined by the Berlin criteria (2012) and updated by the global definition (2023).

Epidemiology

Affects ~10% of ICU patients and ~25% of mechanically ventilated patients (LUNG SAFE study). Mortality 35-46% depending on severity. Common in sepsis, pneumonia (including SARS-CoV-2), and trauma populations.

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Risk factors

Direct lung injury: pneumonia (bacterial, viral including COVID-19, fungal), aspiration, pulmonary contusion, near-drowning, inhalation injury
Indirect lung injury: sepsis (most common overall), severe trauma, massive transfusion (TRALI), pancreatitis, burns, drug overdose, post-cardiopulmonary bypass
Older age, chronic alcohol use, hypoalbuminemia, smoking, obesity

Pathophysiology

Inflammatory cascade injures pulmonary capillary endothelium and alveolar epithelium → protein-rich edema floods alveoli, surfactant dysfunction, hyaline membrane formation → diffuse alveolar damage. Stages: exudative (0-7 days), proliferative (7-21 days), fibrotic (after 3 weeks). V/Q mismatch and intrapulmonary shunt drive severe hypoxemia refractory to supplemental O2 alone.

Clinical presentation

Symptoms

Rapidly progressive dyspnea (within 1 week of inciting insult)
Tachypnea, productive or non-productive cough
Symptoms of underlying cause: fever, hypotension, trauma history

Signs / physical exam

Tachypnea, hypoxia (often refractory to supplemental O2)
Diffuse crackles bilaterally
Use of accessory muscles, retractions
Findings of underlying etiology — sepsis features, trauma findings

Differential diagnosis

Cardiogenic pulmonary edema — Elevated PCWP, BNP elevation, cardiomegaly, response to diuresis; echo shows LV dysfunction
Diffuse alveolar hemorrhage — Hemoptysis, dropping hematocrit, serial BAL progressively bloodier; vasculitis, anticoagulation, cocaine
Acute interstitial pneumonia (Hamman-Rich) — Idiopathic ARDS-like picture; bilateral GGOs; diagnosis of exclusion
Acute eosinophilic pneumonia — Days of fevers, infiltrates; BAL eosinophilia >25%; rapid response to steroids
Cryptogenic organizing pneumonia (COP) — Subacute course, peripheral consolidations; biopsy with organizing pneumonia; steroid-responsive
Hypersensitivity pneumonitis (acute) — Exposure history (birds, mold); resolves with antigen avoidance and steroids

Diagnostic workup

Diagnostic criteria

Berlin Criteria (2012): (1) acute onset ≤1 week of insult or new symptoms, (2) bilateral opacities on chest imaging, (3) respiratory failure not fully explained by cardiac failure/fluid overload, (4) impaired oxygenation with PEEP ≥5 cm H2O — mild PaO2/FiO2 200-300, moderate 100-200, severe ≤100. 2023 global definition extends to high-flow nasal O2 ≥30 L/min and resource-limited settings.

Labs

ABG — severe hypoxemia, often with respiratory alkalosis (early) progressing to acidosis
CBC, BMP, LFTs, lactate, lipase, blood and urine cultures
BNP and echocardiography to exclude cardiogenic edema
Procalcitonin if bacterial source uncertain
Respiratory viral PCR (including SARS-CoV-2, influenza)
Bronchoalveolar lavage if etiology unclear or to evaluate for atypical infection, eosinophilic pneumonia, or DAH

Imaging

Chest radiograph: bilateral opacities not fully explained by effusion, atelectasis, or nodules
CT chest (often shows heterogeneous dependent consolidation with anterior aeration — 'baby lung')
Bedside echocardiography (transthoracic): exclude LV dysfunction; identify RV strain

Diagnostic algorithm

Severity (Berlin)	PaO2/FiO2 with PEEP ≥5	Mortality
Mild	200-300	~27%
Moderate	100-200	~32%
Severe	≤100	~45%

Berlin definition severity stratification of ARDS and associated mortality (Ranieri 2012).

Treatment

First-line

Treat underlying cause (source control, antibiotics for sepsis, supportive care)
Lung-protective mechanical ventilation (ARDSnet protocol, ARMA trial):
Low tidal volume 6 mL/kg ideal body weight (start 8 then titrate down)
Plateau pressure ≤30 cm H2O
PEEP titrated to maintain oxygenation (lower or higher PEEP-FiO2 tables)
Permissive hypercapnia (pH ≥7.20 acceptable)
Conservative fluid strategy after initial resuscitation (FACTT trial)
Prone positioning ≥16 hours/day for PaO2/FiO2 <150 (PROSEVA trial) — proven mortality benefit
Neuromuscular blockade (cisatracurium) — selective use in early severe ARDS (ACURASYS supported; ROSE was negative; reserve for severe dyssynchrony)
Adjuncts for refractory hypoxemia: inhaled pulmonary vasodilators (nitric oxide, epoprostenol) — improve oxygenation but no mortality benefit

Second-line / adjunct

Veno-venous ECMO for severe ARDS with refractory hypoxemia/hypercapnia despite optimization (EOLIA, post-hoc Bayesian analysis suggests benefit) — refer to experienced center
Corticosteroids: dexamethasone 20 mg/day × 5 d then 10 mg × 5 d in moderate-severe ARDS (DEXA-ARDS); 6 mg/day in COVID-19 ARDS (RECOVERY trial)
Avoid: routine high-dose steroids late (>14 days), beta-2 agonists, statins, surfactant — none have improved outcomes in adult ARDS
Early mobilization, daily SBT and sedation interruption to limit ICU complications

Complications

Refractory hypoxemia, multiorgan failure
Ventilator-induced lung injury (volutrauma, barotrauma, atelectrauma, biotrauma)
ICU-acquired weakness, post-intensive-care syndrome (PICS): cognitive, psychiatric, physical sequelae
Pulmonary fibrosis (fibrotic ARDS) with chronic pulmonary dysfunction
Ventilator-associated pneumonia, line infections

PANCE pearls

ARDSnet low-tidal-volume (6 mL/kg IBW) ventilation is the only proven mortality-reducing intervention besides prone positioning and dexamethasone (COVID-ARDS).
Prone positioning ≥16 h/day reduces mortality in moderate-severe ARDS (PROSEVA) — implement early.
PaO2/FiO2 ratios are measured on PEEP ≥5 — without PEEP, severity grading does not apply.
Steroids are useful in COVID-19 ARDS (RECOVERY) and may benefit other moderate-severe ARDS (DEXA-ARDS); avoid in influenza ARDS unless other indication.
ECMO is rescue therapy for refractory hypoxemia — early referral to ECMO centers improves outcomes.

References

ARMA / ARDSnet — Ventilation with Lower Tidal Volumes for ARDS (ARDS Network, NEJM 2000)
PROSEVA — Prone Positioning in Severe Acute Respiratory Distress Syndrome (Guérin et al., NEJM 2013)
Berlin Definition — Acute Respiratory Distress Syndrome: The Berlin Definition (Ranieri et al., JAMA 2012)
RECOVERY — Dexamethasone in Hospitalized Patients with COVID-19 (Horby et al., NEJM 2021)

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