# Acute Respiratory Distress Syndrome > ARDS for NEET-PG Medicine: Berlin definition, PaO2/FiO2 grading, lung-protective ventilation 6 mL/kg, prone positioning and fluid strategy. - Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/ards - Exam / course: NEET-PG · Subject: Medicine - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Acute Respiratory Distress Syndrome", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/ards ## Direct answer Bilateral white-out on a chest radiograph within a week of sepsis, pneumonia, aspiration, trauma, pancreatitis or transfusion — with refractory hypoxaemia and no cardiogenic explanation — signals acute respiratory distress syndrome (ARDS), an inflammatory lung injury producing non-cardiogenic pulmonary oedema. The Berlin definition requires onset within one week of a known insult, bilateral opacities not explained by effusion or collapse, non-cardiogenic origin, and PaO2/FiO2 of 300 or less with PEEP of at least 5 cm of water. Management centres on treating the cause and lung-protective mechanical ventilation with low tidal volumes. ## What you must remember - Berlin severity by PaO2/FiO2: mild 201 to 300, moderate 101 to 200, severe 100 or less, all with PEEP at least 5; mortality rises with severity. - Common causes: sepsis (the leading trigger), pneumonia, aspiration of gastric contents, major trauma, pancreatitis, massive transfusion (TRALI), burns, drowning and drug overdose. - Diagnostic anchor: exclude cardiogenic pulmonary oedema — no elevated filling pressures required clinically; echocardiography helps when the picture is mixed. - Ventilation bundle: tidal volume 6 mL per kg predicted (ideal) body weight, plateau pressure below 30 cm of water, adequate PEEP, permissive hypercapnia accepted; this strategy lowers mortality. - Prone positioning for at least 16 hours daily improves oxygenation and survival in moderate-to-severe ARDS (PaO2/FiO2 below 150); neuromuscular blockade may be considered early in severe disease. - Conservative fluid strategy after resuscitation reduces lung water; treat the precipitant vigorously — source control in sepsis, antibiotics in pneumonia. - Rescue for refractory hypoxaemia: inhaled nitric oxide or prostacyclin, extracorporeal membrane oxygenation in selected patients, and recruitment manoeuvres with caution. - Steroids have a role in specific phases and causes (for example moderate-to-late ARDS or COVID-19 pneumonia under trial protocols) but are not routine blanket therapy. ## Setting the ventilator, step by step A 55-year-old with faecal peritonitis develops PaO2 of 90 on FiO2 0.8, bilateral infiltrates and an echocardiographically normal heart — moderate ARDS by Berlin (PaO2/FiO2 about 112). Take a height of 170 cm: ideal body weight is close to 70 kg for a man, so the tidal volume is 6 mL per kg of that predicted weight, roughly 420 mL — regardless of his actual 92 kg, because the lungs are sized to height, and an overweight patient over-ventilated on actual weight is the classic unit error. Set PEEP adequately, accept permissive hypercapnia, and keep plateau pressure below 30 cm of water, checking it with an inspiratory hold. Sedate as needed and consider early neuromuscular blockade in severe disease. Treat the precipitant in parallel — source control for the sepsis, targeted antibiotics — since no ventilator setting cures the trigger. When PaO2/FiO2 stays below 150, prone the patient for sessions of at least 16 hours with a trained team, which improves survival, not just oxygenation numbers. Run a conservative fluid strategy once shock has resolved to limit lung water. If hypoxaemia remains refractory, escalate to inhaled nitric oxide or prostacyclin as a bridge, cautious recruitment manoeuvres, and extracorporeal membrane oxygenation in selected patients. ## How the exam frames ARDS The numbers carry the question: Berlin cut-offs of 300, 200 and 100 with PEEP of 5, tidal volume 6 mL per kg of ideal body weight, plateau below 30, proning threshold of PaO2/FiO2 below 150, and sepsis as the leading trigger. Two distinctions separate the prepared. First, ARDS versus cardiogenic oedema: clear heart size or normal echocardiography, refractory hypoxaemia, protein-rich fluid and no response to diuretics alone, against a picture that improves with diuresis and afterload reduction. Second, the timeline question — acute interstitial pneumonia or bilateral pneumonia can mimic the radiograph, but the one-week insult-to-failure interval and the PEEP condition of the Berlin criteria settle it. And when a stem mentions an obese patient, expect the tidal-volume trap: the calculation is on ideal, not actual, weight. ## Frequently asked questions ### What defines ARDS in the Berlin definition? Respiratory failure within one week of a recognised insult, bilateral opacities, non-cardiogenic origin, and PaO2/FiO2 of 300 or less with PEEP at least 5 cm of water. ### How is ARDS graded? Mild (201 to 300), moderate (101 to 200) and severe (100 or less) by PaO2/FiO2 ratio. ### What ventilation strategy reduces mortality? Lung-protective ventilation — tidal volume 6 mL per kg ideal body weight and plateau pressure below 30 cm of water with appropriate PEEP. ### When is prone positioning indicated? In moderate-to-severe ARDS, typically PaO2/FiO2 below 150, for sessions of at least 16 hours by a trained team. ### How are fluids managed in ARDS? Liberal early resuscitation for shock, then a conservative strategy to limit lung water once stable. ### Which trigger most often causes ARDS? Sepsis, with pneumonia the other leading precipitant — search for and treat the cause in every case.