Acute Respiratory Distress Syndrome (ARDS)

ARDS is acute hypoxemic respiratory failure from diffuse, non-cardiogenic lung injury. The 2024 Global Definition asks for three things. Onset or worsening within 1 week of a known insult, or new respiratory symptoms. Bilateral opacities on chest imaging. Respiratory failure that cardiac failure or fluid overload does not fully explain. Severity is graded only in intubated patients, by PaO₂/FiO₂ on PEEP of at least 5 cmH₂O. Non-intubated patients meet a single threshold.

• Severity, intubated patients (PaO₂/FiO₂): mild above 200 to 300; moderate above 100 to 200; severe 100 or lower. Mild may also be met on CPAP of 5 cmH₂O or more.
• SpO₂/FiO₂ equivalent (when SpO₂ is 97% or lower): mild above 235 to 315; moderate above 148 to 235; severe 148 or lower.
• Non-intubated patients: a separate category with one threshold and no severity bands: PaO₂/FiO₂ at or below 300 (or SpO₂/FiO₂ at or below 315 when SpO₂ is 97% or lower) on high-flow nasal oxygen at 30 L/min or more, or on NIV or CPAP with end-expiratory pressure of at least 5 cmH₂O.
• Resource-limited settings: the modified definition uses SpO₂/FiO₂ at or below 315 (SpO₂ 97% or lower) and does not require PEEP, oxygen flow or a blood gas.
• Mechanism: the alveolar-capillary barrier is injured, so it leaks protein-rich fluid. Alveoli collapse. Blood flowing past them picks up no oxygen, and that is the shunt. Stiff lungs pass ventilator pressure straight into the lung, which is why the ventilator can injure them further.
• Triggers: the common insults are pneumonia, aspiration, sepsis, major trauma, pancreatitis and transfusion.

The 2012 Berlin Definition used the same oxygenation bands and the same 1-week rule. You will still see it in trial and protocol criteria.

Typical Presentation

• Progressive hypoxemia: SpO₂ keeps falling even as you turn the oxygen up. Dyspnea and a high work of breathing come with it.
• Imaging: you will see bilateral opacities on chest X-ray or CT. On lung ultrasound, the same picture shows as bilateral B-lines and consolidation.
• Timing: it starts within 1 week of the insult. Early on, it often looks like pneumonia or sepsis.

Red Flags

• PaO₂/FiO₂ at or below 100 (severe range)
• Plateau pressure at or above 30 cmH₂O
• pH below 7.25 with PaCO₂ of 60 mmHg or higher despite optimal conventional ventilation
• Falling blood pressure during PEEP changes or recruitment maneuvers

ARDS is a clinical diagnosis, confirmed with the arterial blood gas and the ventilator the patient is on. Work out PaO₂/FiO₂ with the PEEP that is actually set. Not a bag. Not a mask.

• Cardiogenic pulmonary edema: bilateral opacities with a raised BNP, a dilated heart, or a response to diuresis. The Global Definition rules it out.
• Pneumonia or aspiration: these often cause ARDS. One-sided or lobar disease without diffuse bilateral opacities is not ARDS.
• Pneumothorax or auto-PEEP on a ventilator: rising airway pressures and a falling BP can look like worse ARDS. Check breath sounds and chest movement first.
• Pleural effusion or lobar collapse: the hypoxemia has a focal pattern, not a diffuse one.

1. Scene safety and airway: confirm the airway is patent and note the reason for respiratory distress.
2. Position: sit the patient upright if breathing is hard and the patient can tolerate it.
3. Oxygen: start supplemental oxygen at a high flow and titrate to SpO₂ per protocol.
4. Ventilation support: assist with a bag-valve-mask if effort is inadequate; use a PEEP valve when the protocol provides one.
5. Notify ALS and transport early: report the onset time, cause and any known ventilator settings.

Leave the settings alone on a patient already on a ventilator from home or a facility. Change them only if the protocol directs it. Prehospital CPAP is an option where your protocol allows it.

Doses vary by protocol; follow local protocols and medical direction.

• Monitoring: continuous SpO₂, EtCO₂ and ECG; arterial line where the ICU protocol requires it (Paramedic/RN/CCT).
• Intubation: use a planned approach with preoxygenation and a rehearsed backup plan. See Rapid Sequence Intubation.
• Ventilation after intubation: tidal volume 4 to 8 mL/kg predicted body weight (PBW), with plateau pressure below 30 cmH₂O per the ATS 2024 guideline. PBW comes from height and sex, not actual weight (formulas in ED & ICU Management). Target SpO₂ 88 to 95% (ARDS Network protocol).
• Sedation and analgesia: analgesia first. Propofol 5 to 50 mcg/kg/min IV infusion titrated to the sedation goal, per the ICU protocol. The label starts at 5 mcg/kg/min and titrates in 5 to 10 mcg/kg/min steps in intubated adults; it is not indicated for pediatric ICU sedation. Monitor for hypotension and serum triglycerides with prolonged use, and watch for propofol infusion syndrome (severe metabolic acidosis, rhabdomyolysis, hyperkalemia, rising CK) with high doses or prolonged infusion. See Propofol, Dexmedetomidine and Analgosedation.
• Neuromuscular blockade: the ATS 2024 guideline suggests it for early severe ARDS (under 48 hours from onset, PaO₂/FiO₂ at or below 100), conditional with low certainty. ROSE found no mortality benefit with early continuous NMBA, and the ESICM guideline recommends against routine use in moderate to severe ARDS, so treat this as protocol-dependent. Cisatracurium was used in the two largest trials; limit use to 48 hours when possible. Agent and infusion are per ICU protocol; see Rocuronium for the paralytic and monitoring principles.
• Corticosteroids: the ATS 2024 guideline suggests corticosteroids for ARDS (conditional, moderate certainty), and the SCCM 2024 focused update also suggests them for critically ill ARDS. Neither guideline names a specific agent or dose. The dexamethasone regimen below is from one multicentre trial in moderate to severe ARDS (P/F 200 or lower on PEEP of 10 or more and FiO₂ 0.5 or more, assessed 24 hours after onset), started at that point: dexamethasone 20 mg IV daily on days 1 to 5, then 10 mg daily on days 6 to 10. Off-label. Not a guideline dose; ESICM 2023 makes no corticosteroid recommendation for ARDS. In the trial, ventilator-free days rose by 4.8 and 60-day mortality fell (21% vs 36%); hyperglycemia occurred in 76% vs 70%, and the trial stopped early (277 of 314 planned). Monitor blood glucose and watch for new infection during the course; the trial excluded patients already on corticosteroids or immunosuppressants. See Dexamethasone.
• Cardiac arrest in the prone position: 2025 AHA Guidelines for CPR and ECC, Part 7, Section 7.1 (COR 2b, LOE C-LD): it may be reasonable to turn the patient supine before compressions. If the patient cannot be safely turned supine, rescuers may consider CPR in the prone position. See ED & ICU Management for the prone team turn.
• Fluids: do not give routine boluses for oxygenation alone; follow the conservative strategy in ED & ICU Management and treat hypotension with vasoactive support per protocol.

• Lung-protective ventilation: keep tidal volume at 4 to 8 mL/kg PBW. The ARDS Network protocol starts at 8 mL/kg PBW. It drops by 1 mL/kg every 2 hours or less, down to 6 mL/kg PBW. Plateau pressure stays below 30 cmH₂O. Use the ARDS Network PEEP and FiO₂ table in Mechanical Ventilation.
• Predicted body weight (PBW): male 50 + 0.91 × (height in cm − 152.4); female 45.5 + 0.91 × (height in cm − 152.4). Only height and sex go in, never actual weight. Try it: a 170 cm man has a PBW of 66 kg, so 6 mL/kg is about 400 mL. If plateau pressure stays at or above 30 cmH₂O, step tidal volume down by 1 mL/kg steps to a minimum of 4 mL/kg PBW.
• Oxygen and CO₂ targets (ARDS Network protocol): SpO₂ 88 to 95% or PaO₂ 55 to 80 mmHg. Aim for pH 7.30 to 7.45. At pH 7.15 to 7.30, raise the respiratory rate first, up to 35/min. Don't chase a normal PaCO₂ with tidal volume. See Pitfalls & Safety Checks.
• PEEP: the ATS 2024 guideline suggests higher PEEP without prolonged recruitment maneuvers in moderate to severe ARDS, conditional with low to moderate certainty. Prolonged recruitment, meaning PEEP at 35 cmH₂O or higher for more than 60 s, is strongly recommended against.
• Prone positioning: in severe ARDS, the ATS 2024 guideline strongly recommends prone positioning for more than 12 hours per day. PROSEVA ran sessions of at least 16 hours. Its 28-day mortality was 16.0% prone versus 32.8% with supine care.
• Prone team turn: rehearse it as a team, per local protocol. Secure the endotracheal tube, central and arterial lines, drains and pressure-injury pads before the turn. Recheck tube position and pressures afterward.
• Fluids: once shock has resolved, go conservative. FACTT found 60-day mortality of 25.5% versus 28.4%, and that difference was not significant. The conservative arm had better lung function and fewer ventilator and ICU days.
• Refractory hypoxemia: VV-ECMO is suggested in selected severe ARDS, conditional with low certainty. Early means under 7 days. Candidates have PaO₂/FiO₂ under 80, or pH under 7.25 with PaCO₂ of 60 or higher, despite optimal conventional care. They also have a reversible cause and few risk factors for futility. Facilities without ECMO should consider transfer to an ECMO center when feasible. See ECMO.
• High-frequency oscillation: not used routinely. The ATS 2024 guideline strongly recommends against it in moderate or severe ARDS.
• Observation: reassess plateau pressure at least every 4 hours and after every PEEP or tidal volume change.

• Before departure: record the current tidal volume, PEEP, FiO₂, plateau pressure and sedation score; confirm a bag and PEEP valve are at the bedside.
• Keep settings unchanged in transit unless the protocol directs a change. Recheck plateau pressure and SpO₂ after every move and after any position change.
• Prone patients: do not turn the patient in the vehicle without the full team and equipment. Continue prone care per the receiving unit's protocol.
• Neuromuscular blockade: monitor depth with a peripheral nerve stimulator, and keep sedation running. A paralyzed patient cannot show distress.
• Destination: go to a center that can deliver ICU care and, where indicated, VV-ECMO. See Flight Transport Physiology for altitude and cabin-pressure effects on oxygenation.

• Pediatrics: pediatric ARDS (PARDS) is defined and managed under PALICC-2 (2023). That guideline has its own ventilator and oxygenation targets. This page gives no pediatric numeric targets. See Respiratory Distress and Failure and weight-based dosing.
• Pregnancy: the same lung-protective principles apply as in non-pregnant adults. The targets differ, though. Fetal oxygenation needs higher maternal oxygenation targets. Normal pregnancy PaCO₂ runs lower, about 30 mmHg, so hypercapnia is less tolerated. Set targets with obstetric and critical-care input. Prone positioning, ECMO and delivery decisions are made with the obstetric team and local protocol.
• Older Adults: no ARDS-specific adjustment is established. Reserve ECMO and prolonged sedation for patients with a reasonable chance of recovery, per ICU protocol.
• Obesity: size tidal volume to predicted body weight, not actual weight. A high actual weight does not raise the safe volume.
• Renal Impairment: no specific adjustment is established for ARDS drugs. Follow each medication page.
• Patients on Anticoagulants: prone positioning and ECMO raise bleeding risk. Check the anticoagulant plan with the ICU team before the turn or cannulation.

• Error: sizing tidal volume to actual weight. Use PBW from height and sex. 6 mL/kg on actual weight can be far too large.
• Error: treating peak pressure as the limit. Peak pressure includes airway resistance. The lung-protective limit applies to plateau pressure, below 30 cmH₂O.
• Error: raising tidal volume to correct hypercapnia. Find the cause first. Raise respiratory rate first, up to 35/min. Per the ARDS Network protocol, consider a tidal volume increase only if pH stays below 7.15 at that rate, under physician direction. pH below 7.25 with PaCO₂ of 60 or higher despite optimal care is the ECMO referral trigger. It is not a reason to raise tidal volume.
• Error: calling cardiogenic edema ARDS. Check cardiac function and fluid balance before you use the diagnosis to set therapy.
• Error: continuing neuromuscular blockade past 48 hours without a documented reason. Reassess sedation and paralysis daily.
• Error: delaying prone positioning in severe ARDS because the turn is difficult. Plan the turn with the team early, before you need it.
• Error: leaving a patient on an open-ended recruitment maneuver. Prolonged recruitment is strongly recommended against.