EDIC Part I: Acute Respiratory Failure

Acute respiratory failure questions test ABG interpretation, ARDS Berlin criteria, lung-protective ventilation and non-invasive support.

Core rules

• Type 1 hypoxaemic failure: PaO2 <8 kPa on air. Type 2 hypercapnic failure: PaCO2 >6 kPa. A–a gradient = PAO2 − PaO2, where PAO2 = FiO2(Patm − 47) − PaCO2/0.8.
• ARDS: onset within 1 week, bilateral opacities, not primarily cardiac or fluid overload, PEEP ≥5 cmH2O, P/F ≤300 mmHg. Mild 201–300, moderate 101–200, severe ≤100.
• Ventilation: 6 mL/kg predicted body weight, plateau ≤30 cmH2O, permissive hypercapnia, prone positioning if P/F <150.
• NIV is established for COPD hypercapnia and cardiogenic pulmonary oedema; high-flow nasal oxygen is used for de novo hypoxaemia.

Worked example

PaO2 80 mmHg, FiO2 0.60, PEEP 10 cmH2O gives P/F = 80/0.60 = 133 mmHg, i.e. moderate ARDS. If plateau is 32 cmH2O, reduce tidal volume by 1 mL/kg steps and check driving pressure (plateau − PEEP = 22 cmH2O).

Traps

• Do not use SpO2 as PaO2; altitude and methaemoglobinaemia alter saturation.
• Peak pressure is not plateau pressure; sedate or paralyse to measure plateau.
• Cardiogenic oedema can mimic ARDS; echo and fluid balance matter.
• NIV in de novo hypoxaemic failure may increase mortality if intubation is delayed.