How ARDS is tested on Step 1

Expect a septic, pancreatitis, or trauma patient who develops worsening hypoxemia with bilateral infiltrates that heart failure does not explain, and questions about the histology, mechanism, or ventilator strategy.

workspace_premium Noncardiogenic edema

ARDS is pulmonary edema from leaky capillaries, not high pressure. The 2012 Berlin definition requires respiratory failure not fully explained by cardiac failure or fluid overload (echocardiography helps exclude a cardiac cause). It replaced the older criterion of a wedge pressure ≤ 18 mm Hg, which is still the classic way to contrast it with cardiogenic edema.

Pathogenesis

A direct (pneumonia, aspiration) or indirect (sepsis, pancreatitis) insult activates alveolar macrophages and neutrophils.

InsultSepsis,pneumonia,traumaCytokinesIL-1, IL-8,TNF-αNeutrophilsProteases,free radicalsLeakymembraneExudate,hyalinemembranesShuntRefractoryhypoxemia
ARDS pathogenesis. Damage to type II pneumocytes reduces surfactant, causing alveolar collapse.
Diffuse alveolar damage over timeDay 0Day 3Day 7Week 2Week 3+Exudativehyaline membranesProliferativetype II repairFibroticsome patients
Diffuse alveolar damage. Survivors may recover near-normal lung function, but some develop fibrosis.

Diagnosis

The Berlin definition requires all four elements.

CriterionRequirement
TimingWithin 1 week of a known insult or new or worsening symptoms
ImagingBilateral opacities not explained by effusions, collapse, or nodules
Origin of edemaNot fully explained by cardiac failure or fluid overload
OxygenationPaO₂/FiO₂ ≤ 300 mm Hg with PEEP or CPAP ≥ 5 cm H₂O

Example: PaO₂ 80 mm Hg on FiO₂ 0.8 gives a ratio of 100 — severe ARDS.

ARDSIncreased permeabilityProtein-rich exudateNot explained by heart failurePatchy, peripheral infiltratesRefractory hypoxemia (shunt)Cardiogenic edemaIncreased hydrostatic pressureProtein-poor transudateHigh left atrial pressure (PCWP)Central infiltrates, effusions,cardiomegalyResponds to diureticsvs
ARDS vs cardiogenic edema. Elevated BNP and an enlarged heart point toward cardiogenic edema.

Management

Supportive care buys time for the lung to heal; ventilator strategy prevents further injury.

InterventionRationaleEvidence
Low tidal volume (6 mL/kg ideal weight)Prevents volutrauma of remaining healthy lung↓ Mortality
Plateau pressure ≤ 30 cm H₂OLimits barotraumaPart of lung-protective strategy
PEEPRecruits collapsed alveoli, improves oxygenationHigher PEEP for moderate–severe ARDS
Prone positioningImproves V/Q matching in dorsal lung↓ Mortality in severe ARDS
Conservative fluidsLess lung waterMore ventilator-free days
Permissive hypercapniaAccept ↑ PaCO₂ to avoid high volumesTolerated if pH ≥ 7.20

Corticosteroids are now conditionally recommended for ARDS (2024 ATS guideline); surfactant has not reduced mortality in adults.

Clinical application

Pancreatitis and sepsis are classic indirect causes.

functions Worked example — hypoxemia after pancreatitis

Two days after admission for severe gallstone pancreatitis, a patient needs FiO₂ 0.7 to maintain PaO₂ 63 mm Hg. Chest x-ray shows bilateral fluffy infiltrates; echocardiogram shows normal LV function. What is the diagnosis and the key ventilator setting?

  1. Bilateral infiltrates within a week of a systemic inflammatory insult suggest ARDS.
  2. Normal LV function argues against cardiogenic edema.
  3. PaO₂/FiO₂ = 63 / 0.7 = 90 — severe ARDS.
  4. Pancreatic enzymes and cytokines injure the alveolar–capillary membrane.

Answer. Severe ARDS from pancreatitis. Use lung-protective ventilation with 6 mL/kg ideal body weight tidal volumes, plateau pressure ≤ 30, adequate PEEP, and prone positioning.