ARDS for Surgeons
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Direct answer
A ratio of arterial oxygen partial pressure to inspired oxygen of 300 or less, with bilateral infiltrates not fully explained by cardiac failure, within a week of a known insult — that is the Berlin definition of ARDS (mild 201–300, moderate 101–200, severe 100 or below, all at PEEP of 5 cm H2O or more), and for the surgeon the definition is only the entry point. The surgical causes are everywhere: sepsis and pneumonia (the commonest), massive transfusion and TRALI, trauma, aspiration, pancreatitis, and near-drowning. Management that changes survival is remarkably concentrated: lung-protective ventilation with 6 mL per kilogram of predicted body weight and plateau pressures under 30 cm H2O, prone positioning for 16 or more hours daily in the moderate-to-severe range, a conservative fluid strategy once shock resolves, and referring the refractory to extracorporeal support.
What you must remember
- Berlin criteria: onset within 1 week of a known clinical insult; bilateral opacities on imaging not fully explained by effusion, collapse or nodules; respiratory failure not fully explained by cardiac failure or fluid overload; and hypoxaemia graded by PaO2/FiO2 at PEEP of at least 5 cm H2O — mild 201–300, moderate 101–200, severe 100 or less (the old "acute lung injury" label was abolished).
- Lung-protective ventilation: tidal volume 6 mL/kg predicted body weight (formulas based on height and sex, not actual weight), plateau pressure under 30 cm H2O, permissive hypercapnia tolerated (pH above about 7.20–7.25); this single strategy from the ARDS Network trial remains the proven mortality reducer.
- Prone positioning: for PaO2/FiO2 below about 150, 16 or more hours per day, reduces mortality in severe ARDS; it requires a trained team.
- Fluid strategy: conservative (FACTT trial) once the patient is out of shock — shorter ventilation and ICU stay; the surgical instinct of "filling well" must be re-examined daily.
- Neuromuscular blockade: an early 48-hour infusion had support in one trial, but a later trial found no mortality benefit — reserve it for dyssynchrony and severe oxygenation failure.
- Steroids are not routine; dexamethasone's proven role is in COVID-19 ARDS, and other indications remain debated.
- ECMO: consider referral for severe refractory hypoxaemia (commonly PaO2/FiO2 below 80 despite optimisation) in potentially recoverable patients.
- Surgical specifics: TRALI (new lung injury within hours of transfusion), abdominal sepsis as a driver, and intra-abdominal hypertension worsening oxygenation — source control is ARDS therapy.
How to work through it
Day 3 after an emergency Hartmann's operation for faecal peritonitis, a 55-year-old develops increasing oxygen requirements; the chest radiograph shows bilateral infiltrates, the PaO2/FiO2 on 10 cm H2O PEEP and FiO2 0.7 is 130, and echocardiography shows no elevation of filling pressures. Moderate ARDS on a septic background — but the first question is surgical, not ventilatory: is the source controlled? A collection under the diaphragm drives the lung, so a CT drain may be the definitive "ventilator" adjustment. In parallel: ventilation reset to 6 mL/kg predicted body weight with plateau checks, sedation deepened for synchrony, and prone positioning planned for 16 hours daily since the ratio sits below 150. Fluids are restricted to a conservative target once vasopressors are weaning; nutrition is enteral. Later risks include ventilator-associated pneumonia, delirium and ICU-acquired weakness. If oxygenation collapses despite all this, the referral conversation about extracorporeal support happens early, not as a last gasp. When the process is TRALI rather than sepsis — a fresh transfusion hours earlier, fever, hypoxaemia, bilateral infiltrates — management is stopping the transfusion, supportive oxygen and notifying the blood bank, with improvement typically within 48–96 hours.
Where students slip
The commonest technical slip is quoting tidal volume per kilogram of actual weight: the trial used predicted body weight from height and sex, and in an obese or oedematous surgical patient the difference is clinically large. The second is definitional currency: "acute lung injury" as a separate category was retired by the Berlin definition, and PaO2/FiO2 bands are graded at a minimum PEEP of 5 — a stem describing a ratio of 250 on 0 PEEP needs re-testing at PEEP before labelling. The third is aetiological: candidates treat ARDS purely as a lung disease and forget the surgical drivers — undrained sepsis, pancreatitis, transfusion — where the therapeutic lever is the operation or the drain.
Frequently asked questions
What are the Berlin criteria for ARDS?
Onset within one week of a recognised insult, bilateral opacities not fully explained by effusions or collapse, non-cardiogenic respiratory failure, and PaO2/FiO2 of 300 or less at PEEP of at least 5 cm H2O, graded mild, moderate or severe.
What ventilation settings are proven to reduce mortality?
Low tidal volumes of 6 mL per kilogram of predicted body weight with plateau pressures under 30 cm H2O and permissive hypercapnia — the ARDS Network approach — remain the core evidence-based strategy.
When is prone positioning indicated?
For PaO2/FiO2 below about 150, applied 16 or more hours per day; it improves oxygenation and reduces mortality in severe disease.
What is TRALI and how does it present?
Transfusion-related acute lung injury — new bilateral infiltrates with hypoxaemia and fever or hypotension within about six hours of transfusion; treatment is stopping the transfusion, supportive oxygen or ventilation, and reporting to the blood bank.
Why does the surgeon matter in ARDS management?
Because sepsis source control, drainage of collections, pancreatitis management and transfusion practice address the drivers of lung injury — no ventilator mode substitutes for a controlled abdominal source.