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Mechanical ventilation and ARDS
Lung-protective ventilation means a small volume by predicted body weight (6 mL/kg), a plateau pressure up to 30 cmH2O and adequate PEEP; in moderate-to-severe ARDS prone positioning is added.
Why it matters in the exam
The exam asks for the Berlin definition of ARDS and the severity grades by PaO2/FiO2, the correct tidal volume and how predicted body weight is calculated, when to prone, and how to interpret a rise in airway pressure: a high peak with a normal plateau (resistance — kinked tube, bronchospasm, secretions) versus a high peak and plateau (low compliance — pneumothorax, endobronchial intubation, distended abdomen). It also asks about auto-PEEP in asthma and COPD.
The Berlin definition: onset within one week of a known cause, bilateral opacities on imaging, oedema not fully explained by heart failure, and a PaO2/FiO2 of 300 or less with a PEEP of at least 5. Protective ventilation is based on predicted body weight — from height and sex — not on actual weight. In theatre too, a volume of 6–8 mL/kg of predicted weight with PEEP is recommended.
Beyond the ventilator settings, ARDS care includes treating the cause (sepsis, pneumonia, aspiration, pancreatitis, multiple transfusions), keeping a restrictive fluid balance once the patient is stable, and minimal sedation that allows a daily assessment for weaning. Remember also the prevention of ventilator-associated pneumonia: head up, a daily sedation break and a daily check of whether the tube can come out.
Key points
- ARDS by Berlin: a week, bilateral opacities, not cardiac, PaO2/FiO2 of 300 or less with PEEP 5.
- 6 mL/kg of predicted weight, plateau up to 30, driving pressure below 15.
- Prone positioning with PaO2/FiO2 below 150.
- High peak with normal plateau = resistance; both high = compliance.
- Auto-PEEP: lengthen expiration; in collapse — disconnect.
Summary table
| Issue | Rule | Exam point |
|---|---|---|
| ARDS severity | Mild 201–300; moderate 101–200; severe 100 or less (PaO2/FiO2, with PEEP of at least 5) | Within a week; not due to heart failure alone |
| Volume and pressures | 6 mL/kg of predicted weight (4–8); plateau up to 30 cmH2O; driving pressure below 15 | Predicted weight from height, not actual weight |
| Prone positioning | With PaO2/FiO2 below 150, at least 12 hours a day | Reduced mortality in the PROSEVA trial |
| High peak, normal plateau | A resistance problem: kink, secretions, bronchospasm | Suction, bronchodilators |
| High peak and plateau | A compliance problem: pneumothorax, endobronchial intubation, atelectasis, distended abdomen | Check the side, the tube, a film |
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Sources
- ARDS Definition Task Force: the Berlin Definition, 2012 (with the 2024 global definition update) — Definition and severity
- ATS/ESICM/SCCM Clinical Practice Guideline: Mechanical Ventilation in Adult Patients with ARDS, 2017 — Low tidal volume, PEEP and prone positioning
- Miller's Anesthesia, 9th edition (2020) — Critical Care: Mechanical Ventilation
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.