Pulmonary · Topic 2 of 28

Acute Respiratory Distress Syndrome (ARDS)

Pulmonary · acute respiratory failure

Acute Respiratory Distress Syndrome (ARDS)

Bilateral noncardiogenic edema with P/F ≤ 300 on PEEP ≥ 5: ventilate at 6 mL/kg ideal body weight, keep plateau ≤ 30, prone the severe.

Key numbers

Severity by P/F: 300, 200, 100P/F ≤ 300 · ≤ 200 · ≤ 100

Tidal volume 6 mL/kg ideal body weight6 mL/kg · Pplat ≤ 30

Prone > 12 h/day if severeprone > 12 h/day

01 · The numberSeverity by P/F: 300, 200, 100

Severity by PaO₂/FiO₂ on PEEP ≥ 5: mild 201–300, moderate 101–200, severe ≤ 100 mmHg (FiO₂ as a decimal, e.g. PaO₂ 80 on FiO₂ 0.5 = 160).

02 · DoTidal volume 6 mL/kg ideal body weight

Ventilate every ARDS patient with low tidal volumes — start at 6 mL/kg predicted (ideal) body weight, range 4–8 — and keep plateau pressure at or below 30 cmH₂O; this lowers mortality.

03 · DoProne > 12 h/day if severe

Prone positioning for more than 12 hours a day is strongly recommended in severe ARDS (and used in moderate–severe ARDS); it improves survival, likely by reducing ventilator-induced lung injury.

Causes & risk factors

Why it happens

Sepsis and pneumonia cause most ARDSDirect causes: pneumonia, gastric aspiration, contusion, near-drowning, inhalation injury; indirect: sepsis, trauma with shock, pancreatitis, TRALI and massive transfusion. Sepsis and pneumonia cause most cases.

Diagnosis & monitoring

Work-up

Bilateral opacities within 1 week, not cardiac must knowBerlin definition: onset within 1 week of an insult; bilateral opacities not explained by effusion, collapse or nodules; failure not fully explained by heart failure or fluid overload; on PEEP or CPAP ≥ 5 cmH₂O.

Target SpO₂ 88–95%Aim for SpO₂ 88–95% on the lowest FiO₂ that achieves it (ideally FiO₂ ≤ 0.6); normal saturations are not the goal and high FiO₂ risks oxygen toxicity.

Base tidal volume on ideal body weightSet tidal volume from predicted (ideal) body weight, calculated from height and sex — never from actual weight, which overinflates the lungs of obese patients.

Check plateau pressure every 4 hoursCheck plateau pressure with an end-inspiratory hold at least every 4 hours and after each PEEP or tidal-volume change; above 30, cut tidal volume in 0.5–1 mL/kg steps to a minimum of 4 mL/kg.

Do right now

In this order

First moves

Tidal volume 6 mL/kg ideal body weight6 mL/kg · Pplat ≤ 30

→

Higher PEEP, no prolonged recruitment maneuvers

→

Prone > 12 h/day if severeprone > 12 h/day

→

Conservative fluids once shock resolves

Why each step

Tidal volume 6 mL/kg ideal body weightVentilate every ARDS patient with low tidal volumes — start at 6 mL/kg predicted (ideal) body weight, range 4–8 — and keep plateau pressure at or below 30 cmH₂O; this lowers mortality.

Higher PEEP, no prolonged recruitment maneuversIn moderate–severe ARDS use a higher-PEEP strategy (e.g., the ARDSNet higher PEEP/FiO₂ table) to recruit alveoli and lower FiO₂; avoid prolonged recruitment maneuvers (PEEP ≥ 35 cmH₂O for ≥ 60 s), which raised mortality.

Prone > 12 h/day if severeProne positioning for more than 12 hours a day is strongly recommended in severe ARDS (and used in moderate–severe ARDS); it improves survival, likely by reducing ventilator-induced lung injury.

Conservative fluids once shock resolvesOnce the patient is out of shock, use a conservative fluid strategy: it shortens mechanical ventilation and ICU stay without changing mortality; watch for hypotension, oliguria or cool extremities.

Management

Orders

Permissive hypercapnia to pH ≥ 7.15Raise the respiratory rate (up to 35/min) to offset low tidal volumes, and accept respiratory acidosis down to a pH of about 7.15 rather than enlarge the breaths.

Paralysis for early severe ARDSConsider a neuromuscular blocker (cisatracurium was used in the trials) in early severe ARDS — under 48 hours from onset with P/F ≤ 100 — always with adequate sedation (conditional ATS 2024 recommendation).

Corticosteroids are suggested in ARDSATS 2024 suggests corticosteroids for adults with ARDS (conditional, moderate certainty); dexamethasone is standard for COVID-19 ARDS.

VV-ECMO for refractory severe ARDSConsider VV-ECMO in selected severe ARDS (P/F < 80, or pH < 7.25 with PaCO₂ ≥ 60, under 7 days in) at a high-volume ECMO center once proning and paralysis have failed.

ARDS or cardiogenic pulmonary edema?

Commonly confused

Mechanism	Leaky alveolar–capillary membrane (low pressure)
Examination	No S3; signs of the trigger (sepsis, pneumonia)
Chest x-ray	Bilateral, more peripheral opacities
Oxygen response	Shunt — hypoxemia resists high FiO₂
Definition clause	Not fully explained by heart failure or fluid overload

More detail

Reference

Show 5 reference facts

The 2023 global definition adds patients on high-flow nasal oxygen ≥ 30 L/min, accepts SpO₂/FiO₂ ≤ 315 (when SpO₂ ≤ 97%) instead of an ABG, and allows lung ultrasound for bilateral opacities.

Flooded or collapsed alveoli create shunt, so hypoxemia resists high FiO₂; inflammatory injury moves from an exudative phase to a fibroproliferative phase, with collapse mostly in dependent lung.

High-frequency oscillatory ventilation is strongly recommended against in moderate–severe ARDS; inhaled nitric oxide and surfactant replacement do not reduce morbidity or mortality.

Noninvasive support (high-flow or NIV) may be tried in milder hypoxemia with close monitoring, but use it cautiously when P/F ≤ 150 — failure delays intubation and may raise mortality (see the NIV/HFNC topic).

ARDS 28-day mortality is about 30–35%, mostly from sepsis and multiorgan failure; survivors often recover near-normal lung function by 6–12 months but may keep weakness and cognitive impairment.

Pearl

Protect the lung, not the blood gas: small breaths, plateau at or below 30, and accept some CO₂ and an SpO₂ of 88–95%.

Sources · Merck Manual Professional — Acute Hypoxemic Respiratory Failure (AHRF), Including ARDS · ARDS Definition Task Force. Acute respiratory distress syndrome: the Berlin definition. JAMA 2012;307:2526–2533 · Fan E et al. ATS/ESICM/SCCM Clinical Practice Guideline: Mechanical Ventilation in Adult Patients with ARDS. Am J Respir Crit Care Med 2017;195:1253–1263 · Qadir N et al. An Update on Management of Adult Patients with ARDS: An Official ATS Clinical Practice Guideline. Am J Respir Crit Care Med 2024;209:24–36 · Matthay MA et al. A New Global Definition of Acute Respiratory Distress Syndrome. Am J Respir Crit Care Med 2024;209:37–47 · Merck Manual Professional — Overview of Mechanical Ventilation (sedation and neuromuscular blockade) · NIH NHLBI ARDS Clinical Network — Mechanical Ventilation Protocol Summary (predicted body weight, plateau ≤ 30, SpO₂ 88–95%) · Guérin C et al. Prone positioning in severe acute respiratory distress syndrome (PROSEVA). N Engl J Med 2013;368:2159–2168 · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).