High-yield map
Key points before the detail
- 01
Ask separately whether the patient is oxygenating, ventilating and sustaining the work of breathing.
- 02
SpO2 does not measure ventilation; severe hypercapnia can coexist with acceptable saturation, especially on supplemental oxygen.
- 03
Low-tidal-volume ventilation is based on predicted/ideal body weight rather than actual body weight.
- 04
Ventilation is organ support: pneumonia, sepsis, aspiration, oedema, trauma or another cause still needs definitive treatment.
Oxygenation, ventilation and work of breathing
| Question | Useful clues |
|---|---|
| Is oxygenation failing? | SpO2/PaO2 relative to inspired oxygen, escalating oxygen requirement, cyanosis as a late sign |
| Is ventilation failing? | PaCO2, pH, minute/alveolar ventilation, reduced respiratory drive or severe airflow obstruction |
| Is the patient tiring? | Work of breathing, accessory muscles, falling tidal volumes, altered consciousness, paradoxical movement |
| Can the airway be protected? | Conscious level, bulbar function, secretion load, aspiration risk |
Why gas exchange fails
Low V/Q units are perfused but inadequately ventilated; shunt is the extreme in which perfusion reaches an unventilated unit; dead space is ventilation without effective perfusion. V/Q mismatch commonly causes hypoxaemia with a normal or low PaCO2 because minute ventilation rises. Hypercapnia more often signals global hypoventilation, severe obstruction, weakness or exhaustion.
ARDS as a syndrome
ARDS develops after a recognised pulmonary or systemic insult such as pneumonia, aspiration, sepsis, pancreatitis, major trauma or transfusion-related lung injury. Increased alveolar-capillary permeability causes protein-rich pulmonary oedema, reduced aerated lung volume, low compliance and severe V/Q mismatch/shunt. Diagnosis requires acute timing, bilateral lung abnormalities and exclusion of cardiac failure/fluid overload as the complete explanation; exact criteria should be applied using the current critical-care definition and imaging context.
Lung-protective ventilation
Ventilator-induced lung injury can arise from excessive stretch, repeated opening/closing and injurious pressures. FICM/ICS quality standards published in 2024 state that patients with or at risk of ARDS should receive tidal volumes up to 6 mL/kg ideal body weight and plateau airway pressures of 30 cmH2O or lower. FICM teaching also frames the tidal-volume target using predicted body weight derived from height.
- Use predicted/ideal body weight rather than actual body weight, especially in obesity.
- FiO2 and PEEP mainly support oxygenation; respiratory rate and delivered tidal volume/pressure influence minute ventilation and PaCO2.
- Every ventilator change should have a purpose and be followed by reassessment of gas exchange, haemodynamics and airway pressures.
Prone positioning and escalation
FICM/ICS guidance supports prone positioning in appropriate moderate-to-severe ARDS as part of lung-protective care. Proning can improve dorsal recruitment and V/Q matching and reduce injurious regional stress. It requires a trained critical-care team because airway, line, pressure-injury and haemodynamic risks must be managed. Severe refractory hypoxaemia should prompt specialist critical-care review and, when appropriate, discussion with a severe respiratory failure/ECMO service.
Active recall
Close the notes and answer these
Try each question from memory before revealing the answer. These public prompts are a small preview of the integrated retrieval system inside SurgAtlas.
01What does a normal SpO2 fail to tell you?
It does not tell you whether alveolar ventilation is adequate or whether PaCO2 is rising.
02Why is tidal volume based on predicted/ideal rather than actual body weight?
Lung size relates more closely to height than body mass; using actual weight in obesity risks excessive tidal volume and volutrauma.
03What UK lung-protective targets are highlighted by FICM/ICS standards?
Tidal volume up to about 6 mL/kg ideal/predicted body weight and plateau airway pressure at or below 30 cmH2O for patients with or at risk of ARDS.
Sources & editorial basis
References & editorial basis
- SurgAtlas production chapter — ARDS and Mechanical Ventilation; Breathing and Acute Respiratory Failure. Primary SurgAtlas source. Guideline-dependent statements in this public lesson were separately checked against the current authoritative sources listed below.
- Faculty of Intensive Care Medicine — GPICS 9.4 ARDS best practice and measures. UK intensive-care standards supporting low-tidal-volume ventilation, plateau-pressure limitation and prone positioning in appropriate ARDS. Source ↗
- Intensive Care Society — ARDS guideline. FICM/ICS ARDS management recommendations, supported by the British Thoracic Society. Source ↗
This lesson is derived from the corresponding SurgAtlas production teaching material. Where the source makes current management or guideline claims, the public lesson uses the cited contemporary guidance. It is written for education and examination preparation, not as patient-specific clinical advice.
Editorial details
Medical Doctor (MD) · MRCS Part A · Physician · Surgical Educator
SurgAtlas is an educational resource. For patient care, verify current national guidance, local antimicrobial and transfusion policies, specialty pathways and individual patient factors.
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