Guide — Pulmonary Diseases
ARDS Basics
Acute respiratory distress syndrome is stiff, leaky, refractory lungs — and the ventilator that supports them can also injure them. This guide covers the Berlin definition, the pathophysiology, and the lung-protective strategy the RT is responsible for implementing and protecting.
11 min read · Pulmonary Diseases
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
ARDS is an acute, diffuse inflammatory lung injury that produces severe, refractory hypoxemia. It is a syndrome with many triggers — pneumonia, sepsis, aspiration, trauma, pancreatitis — but a common final pathway: diffuse alveolar damage, a leaky alveolar-capillary membrane, and protein-rich pulmonary edema that floods and collapses alveoli.
The defining diagnostic framework is the Berlin definition, which sets the timing, imaging, edema-origin, and oxygenation criteria and grades severity by the PaO₂/FiO₂ ratio. For the respiratory therapist, the central task is delivering ventilation that supports oxygenation without inflicting additional injury on already-fragile lung.
Key Concepts
The Berlin definition rests on four criteria, with oxygenation graded by the P/F ratio measured on at least 5 cmH₂O of PEEP.
| Criterion | Requirement |
|---|---|
| Timing | Within 1 week of a known clinical insult or new/worsening respiratory symptoms |
| Imaging | Bilateral opacities not fully explained by effusion, collapse, or nodules |
| Origin of edema | Failure not fully explained by cardiac failure or fluid overload |
| Oxygenation | P/F ratio with PEEP ≥ 5 cmH₂O — see severity grades below |
| Severity | P/F Ratio (PEEP ≥ 5) |
|---|---|
| Mild | 200 – 300 mmHg |
| Moderate | 100 – 200 mmHg |
| Severe | ≤ 100 mmHg |
- The hypoxemia is shunt. Flooded, collapsed alveoli are perfused but not ventilated, so blood passes the lung without picking up oxygen. That is why the hypoxemia is refractory — raising FiO₂ alone does little for true shunt, which is the rationale for PEEP and recruitment.
- The lungs are small, not just stiff. So much lung is consolidated that only a fraction remains available for ventilation — the “baby lung.” A normal-sized tidal volume delivered into that small compartment overdistends it, which is the entire reason for the 6 mL/kg strategy.
- Decreased compliance. Edema and collapse stiffen the lung, so it takes more pressure to deliver each breath — making plateau pressure and driving pressure the parameters that matter most.
Assessment & Findings
- Refractory hypoxemia. Oxygenation that fails to improve as expected when FiO₂ is increased — the bedside signature of shunt physiology.
- Bilateral infiltrates. Diffuse, bilateral opacities on chest imaging that are not explained by effusion, collapse, or a cardiac cause.
- A falling P/F ratio. Trending the PaO₂/FiO₂ ratio grades severity and tracks the trajectory — a drop below 150 is the usual trigger to consider proning.
- Rising pressures and falling compliance. An increasing plateau pressure for the same tidal volume signals worsening lung mechanics and a need to reassess the strategy.
- A known precipitant. Sepsis, pneumonia, aspiration, or trauma in the preceding days supports the diagnosis and frames the timing criterion.
RT Priorities / Interventions
The lung-protective strategy is the core of ARDS care, and the RT owns its delivery and protection.
| Parameter | Target | Why |
|---|---|---|
| Tidal volume | 6 mL/kg PBW (range 4–8) | Sized to predicted body weight, never actual weight |
| Plateau pressure | ≤ 30 cmH₂O | Reflects alveolar distending pressure — the injury driver |
| pH / CO₂ | Permissive hypercapnia, pH ≥ ~7.20 | Accept a high CO₂ to keep volume and pressure low |
| PEEP / FiO₂ | Titrated per ARDSNet tables | Higher PEEP recruits lung and reduces shunt in moderate–severe disease |
- Implement and protect the protective strategy. Set tidal volume on predicted body weight — use the ideal body weight calculator to derive it from height — and defend the 6 mL/kg target when the temptation is to raise it for a high CO₂.
- Monitor plateau and driving pressure. Check the plateau on an inspiratory hold and watch the trend; the compliance calculator helps quantify the worsening or improving mechanics behind those numbers.
- Trend the P/F ratio. Recalculate the P/F ratio as oxygenation changes to grade severity and flag the threshold for proning.
- Assist proning and the adjuncts. Help execute prone positioning in moderate-to-severe disease, support conservative fluid management, and be ready for neuromuscular blockade, recruitment maneuvers, or an ECMO referral in refractory cases per your center’s protocol.
Common Pitfalls
- Setting tidal volume on actual body weight. The most consequential ARDS error: an overweight patient’s 6 mL/kg looks safe on actual weight but is a massive, lung-injuring breath. Always use predicted body weight.
- Chasing a normal CO₂ at the cost of pressure. Cranking up tidal volume or rate to normalize the gas abandons lung protection. Permissive hypercapnia exists precisely so you do not have to.
- Under-recognizing ARDS as “just” pneumonia. Bilateral infiltrates with refractory hypoxemia after an insult meet the definition. Missing the label means missing the protective strategy that improves survival.
- Treating refractory hypoxemia with FiO₂ alone. Pure shunt barely responds to oxygen. The answer is PEEP, recruitment, and proning — not simply turning the FiO₂ to 100%.
Board Exam Pearls
- Know the Berlin P/F cutoffs cold: mild 200–300, moderate 100–200, severe ≤ 100, all measured on at least 5 cmH₂O of PEEP.
- The lung-protective numbers are exam staples: 6 mL/kg of predicted body weight and a plateau pressure ≤ 30 cmH₂O. If a stem offers a large tidal volume on actual weight, that is the wrong answer.
- Refractory hypoxemia that barely improves with rising FiO₂ points to shunt — the indication for PEEP, not more oxygen.
- Prone positioning improves mortality in moderate-to-severe ARDS, typically with a P/F below 150 — a survival intervention, not just a rescue trick.
FAQ
What is the Berlin definition of ARDS?
ARDS is defined by four elements: acute onset within one week of a known insult; bilateral opacities on chest imaging not fully explained by effusion, lobar collapse, or nodules; respiratory failure not fully explained by cardiac failure or fluid overload; and impaired oxygenation graded by the PaO₂/FiO₂ ratio with a minimum PEEP of 5 cmH₂O — mild 200–300, moderate 100–200, and severe ≤ 100.
Why is tidal volume set on predicted body weight, not actual weight?
Lung size scales with height and sex, not with how much the patient weighs. Using actual body weight in an overweight or edematous patient would set a tidal volume far too large for the available lung, driving up alveolar pressure and worsening injury. Predicted body weight, derived from height and sex, sizes the breath to the lung — the basis of the 6 mL/kg lung-protective target (range 4–8 mL/kg).
What is permissive hypercapnia and why is it accepted?
Holding tidal volume and plateau pressure low to protect the lung means minute ventilation may be insufficient to keep PaCO₂ normal, so CO₂ rises. Permissive hypercapnia is the deliberate acceptance of that elevated CO₂ — tolerating a pH down to roughly 7.20 — because preventing ventilator-induced lung injury matters more than a normal gas. The priority is lung protection, not a normal CO₂.
When is prone positioning used in ARDS?
Prone positioning improves survival in moderate-to-severe ARDS and is generally applied when the PaO₂/FiO₂ ratio is below 150 despite optimized ventilator settings. Proning improves oxygenation by recruiting dependent lung and making ventilation and perfusion more uniform. It is an evidence-based mortality intervention, not merely a rescue maneuver for refractory hypoxemia.
Put it to work
ARDS severity is a single ratio. Run a PaO₂ and FiO₂ through the calculator to grade the lung injury and see where the proning threshold falls.
Open the P/F Ratio Calculator →Related Resources
Sources
- ARDS Definition Task Force; Ranieri VM, Rubenfeld GD, Thompson BT, et al. Acute respiratory distress syndrome: the Berlin definition. JAMA. 2012;307(23):2526-2533.
- Acute Respiratory Distress Syndrome Network; Brower RG, Matthay MA, Morris A, et al. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000;342(18):1301-1308.
- Guérin C, Reignier J, Richard JC, et al. Prone positioning in severe acute respiratory distress syndrome. N Engl J Med. 2013;368(23):2159-2168.