Pulmonary · PANCE / PANRE

Acute Respiratory Distress Syndrome (ARDS)

Written & clinically reviewed by Ryan Busch, PA-C · Updated August 2026 · Report an error

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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Question 1PulmonaryMedium
A 72-year-old man is mechanically ventilated after acute respiratory distress syndrome (ARDS) from pneumonia. He is on assist-control ventilation with tidal volume (TV) 520 mL, positive end-expiratory pressure (PEEP) 8 cmH2O, FiO2 0.65. His weight is 70 kg. The plateau pressure is 38 cmH2O. What ventilator adjustment is most appropriate?
  • AIncrease FiO2 to 1.0
  • BIncrease PEEP to 12 cmH2O
  • CDecrease tidal volume to 420 mL
  • DSwitch to pressure-support ventilation
Reveal answer & full explanation
Correct answer: C — Decrease tidal volume to 420 mL
  • AIncrease FiO2 to 1.0
  • BIncrease PEEP to 12 cmH2O
  • CDecrease tidal volume to 420 mL✓
  • DSwitch to pressure-support ventilation

Why Decrease tidal volume to 420 mL is correct

  • ARDSNet (ARDS lung-protective ventilation) protocol targets tidal volume 6 mL/kg ideal body weight and plateau pressure below 30 cmH2O
  • At 70 kg ideal body weight, the target TV is 420 mL (6 mL/kg × 70 kg)
  • The current TV of 520 mL (7.4 mL/kg) and plateau pressure of 38 cmH2O indicate volutrauma and barotrauma risk
  • Reducing TV is the priority intervention shown to reduce mortality in ARDS

Why the others are wrong

  • A) Increase FiO2 to 1.0 — the plateau pressure problem (barotrauma/volutrauma) is the priority; increasing FiO2 alone does not address the ventilator-induced lung injury risk
  • B) Increase PEEP to 12 cmH2O — PEEP adjustments are guided by oxygenation tables after TV is corrected; increasing PEEP before lowering TV does not address the elevated plateau pressure
  • D) Switch to pressure-support ventilation — pressure-support ventilation is used for weaning, not for managing volutrauma in acute ARDS
Question 2PulmonaryMedium
A 65-year-old male with sepsis develops bilateral diffuse pulmonary infiltrates and severe hypoxemia. PaO2/FiO2 ratio is 145 on positive end-expiratory pressure (PEEP) of 8. Echocardiogram is normal. Which of the following Berlin definition severity criteria does this meet?
  • AAcute lung injury
  • BMild ARDS
  • CModerate ARDS
  • DSevere ARDS
Reveal answer & full explanation
Correct answer: C — Moderate ARDS
  • AAcute lung injury
  • BMild ARDS
  • CModerate ARDS✓
  • DSevere ARDS

Why Moderate ARDS is correct

  • Berlin definition of acute respiratory distress syndrome (ARDS) severity by PaO2/FiO2 ratio: mild 201–300, moderate 101–200, severe ≤100.
  • This patient's PaO2/FiO2 ratio of 145 falls in the 101–200 range, classifying this as moderate ARDS.

Why the others are wrong

  • Acute lung injury — a legacy term from the older AECC definition (PaO2/FiO2 ≤300) that the Berlin definition retired; it is not a Berlin severity category.
  • Mild ARDS — requires a ratio of 201–300; this patient's ratio of 145 is below that range.
  • Severe ARDS — requires a ratio of 100 or less; this patient's ratio of 145 does not meet that threshold.

Additional high-yield points

  • Berlin definition diagnostic criteria: acute onset within 1 week, bilateral infiltrates not fully explained by effusions/atelectasis/nodules, not fully explained by cardiac failure, and positive end-expiratory pressure (PEEP) ≥5 cmH2O.
  • Management principles: lung-protective ventilation (tidal volume 6 mL/kg predicted body weight (PBW), plateau pressure <30 cmH2O), prone positioning for severe ARDS (supported by the PROSEVA trial), neuromuscular blockade for very severe cases, conservative fluid strategy, and extracorporeal membrane oxygenation (ECMO) for refractory cases.
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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 ≥5Mortality
Mild200-300~27%
Moderate100-200~32%
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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Related Pulmonary diagnoses

AsthmaChronic Obstructive Pulmonary Disease (COPD)Community-Acquired Pneumonia (CAP)Hospital-Acquired and Ventilator-Associated Pneumonia (HAP/VAP)Aspiration Pneumonia and PneumonitisTuberculosis (Active and Latent)Pulmonary Embolism (PE)Pulmonary HypertensionPneumothorax (Spontaneous and Tension)Pleural EffusionLung Cancer (Small Cell and Non-Small Cell)BronchiectasisCystic FibrosisSarcoidosis

Differentials & related conditions

Hypersensitivity Pneumonitis

Educational use only. This outline is a study aid for PA students and is not medical advice or a substitute for clinical judgment. FirstPassPA is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCCPA or PAEA. PANCE® and PANRE® are registered trademarks of the National Commission on Certification of Physician Assistants; End of Rotation™ is a program of the Physician Assistant Education Association.