Pulmonary · Topic 2 of 28

Acute Respiratory Distress Syndrome (ARDS)

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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.

01 · Severity by P/FMild ≤ 300 · moderate ≤ 200 · severe ≤ 100

02 · Every ARDS patient6 mL/kg ideal weight, plateau ≤ 30

03 · Severe ARDSProne over 12 hours a day

Why it happens

A leaky alveolar–capillary membrane floods the lung at normal pressures

Flooded or collapsed alveoli create shunt

Shunt hypoxemia resists high FiO₂

Collapse sits mostly in dependent lung

Inflammatory injury moves from an exudative phase to a fibroproliferative phase.

What causes it

Sepsis and pneumonia cause most cases

Direct lung injury

PneumoniaGastric aspirationPulmonary contusionNear-drowningInhalation injury

Indirect injury

SepsisTrauma with shockPancreatitisTRALIMassive transfusion

Diagnosis: Berlin definition

All four, then grade by P/F on PEEP ≥ 5

Onset within 1 week of a known insult

Bilateral opacities not explained by effusion, collapse or nodules

Not fully explained by heart failure or fluid overload

On PEEP or CPAP ≥ 5 cmH₂O

Mild: P/F 201–300 mmHg

Moderate: P/F 101–200 mmHg

Severe: P/F ≤ 100 mmHg

FiO₂ as a decimal: PaO₂ 80 on FiO₂ 0.5 = P/F 160

Berlin remains the board standard. The 2023 global definition keeps these criteria and cut-offs for intubated patients, and adds patients on high-flow oxygen ≥ 30 L/min, SpO₂/FiO₂ ≤ 315 (when SpO₂ ≤ 97%) in place of an ABG, and lung ultrasound for bilateral opacities.

Red flags

Plateau pressure over 30 cmH₂O: cut tidal volume lower Vt

Tidal volume set from actual weight in an obese patient use ideal weight

P/F ≤ 100 in the first 48 h: prone, consider paralysis prone

Prolonged recruitment (PEEP ≥ 35 for ≥ 60 s) raised mortality avoid

Noninvasive support with P/F ≤ 150 can delay intubation plan intubation

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

Treatment

Protect the lung — low tidal volume cuts mortality

Ventilator moves, in order

Tidal volume 6 mL/kg ideal body weightrange 4–8; plateau ≤ 30 cmH₂O

→

Higher PEEP in moderate–severe ARDSe.g. ARDSNet higher PEEP/FiO₂ table

→

Prone over 12 h/day if severeimproves survival

→

Conservative fluids once out of shockfewer ventilator and ICU days

Low Vtevery patient

6 mL/kg ideal body weight, from height and sex

Plateau over 30: cut 0.5–1 mL/kg, floor 4 mL/kg

Plateaucheck every 4 h

End-inspiratory hold every 4 h and after each change

Target 30 cmH₂O or less

OxygenSpO₂ 88–95%

Lowest FiO₂ that reaches SpO₂ 88–95%, ideally ≤ 0.6

Normal saturations are not the goal

CO₂permissive hypercapnia

Raise rate up to 35/min; accept pH down to about 7.15

Don't enlarge the breaths to clear CO₂

Pronesevere ARDS

Prone over 12 hours a day — improves survival

Also used in moderate–severe ARDS

Paralysisearly severe

Cisatracurium

Consider within 48 h of onset with P/F ≤ 100

Always with adequate sedation

Steroidssuggested

Corticosteroids Dexamethasone (COVID-19 ARDS)

ATS 2024 suggests corticosteroids in adult ARDS

Conditional, moderate certainty

VV-ECMOselected severe

Consider if P/F under 80, or pH under 7.25 with PaCO₂ ≥ 60, under 7 days in

High-volume center, after proning and paralysis fail

Fluids and outcomes

Conservative fluids: watch perfusionhypotension, oliguria or cool extremities mean back off

Mortality about 30–35% at 28 daysmostly from sepsis and multiorgan failure

Survivorsnear-normal lung function by 6–12 months, but weakness and cognitive impairment may persist

ATS words the plateau limit as under 30 cmH₂O; ARDSNet targets ≤ 30. PROSEVA: prone sessions of at least 16 h with P/F under 150 on FiO₂ ≥ 0.6. Paralysis is a conditional ATS 2024 suggestion; Merck does not recommend it routinely after a larger trial showed no benefit. Merck still calls steroid data in non-COVID ARDS inconclusive.

ARDS or cardiogenic pulmonary edema?

	ARDS	Cardiogenic edema
Mechanism	Leaky alveolar–capillary membrane (low pressure)	High capillary hydrostatic pressure
Examination	No S3; signs of the trigger (sepsis, pneumonia)	S3, JVD, peripheral edema
Chest x-ray	Bilateral, more peripheral opacities	Central opacities, cardiomegaly, wide vascular pedicle
Oxygen response	Shunt: hypoxemia resists high FiO₂	Improves fast with diuretics and CPAP/BiPAP
Definition clause	Not fully explained by heart failure or fluid overload	Explained by LV failure or volume

LV dysfunction on echo is nonspecific in sepsis.

Pearl

Protect the lung, not the blood gas: small breaths, plateau ≤ 30, accept some CO₂ and SpO₂ 88–95%.

Sources · Acute Hypoxemic Respiratory Failure, Including ARDS (Merck Manual Professional) · ARDS: the Berlin definition (JAMA 2012) · ATS/ESICM/SCCM guideline: mechanical ventilation in adult ARDS (2017) · ATS guideline update: management of adult ARDS (2024) · A new global definition of ARDS (AJRCCM 2024) · Overview of Mechanical Ventilation (Merck Manual Professional) · NHLBI ARDS Network mechanical ventilation protocol summary · Prone positioning in severe ARDS: PROSEVA (NEJM 2013) · Follow your institution's protocols.