Critical Care and Intensive Care Medicine
Critical care questions ask you to classify severity, apply a protocol, and decide what to stop. The board favours definitions with numbers (Berlin, Sepsis-3), ventilator settings, and vasopressor sequencing.
Definitions and rules
- ARDS (Berlin): onset within one week, bilateral opacities not explained by heart failure, PEEP at least 5 cmH2O, and PaO2/FiO2 of 300 or less (mild 200 to 300, moderate 100 to 200, severe under 100).
- Sepsis-3: suspected infection plus a SOFA rise of 2 or more; septic shock is vasopressor-requiring hypotension with lactate above 2 mmol/L despite adequate fluids.
- Lung-protective ventilation: tidal volume 6 mL/kg predicted body weight, plateau pressure under 30 cmH2O, driving pressure under 15, permissive hypercapnia accepted, and prone positioning for 16 hours or more in severe ARDS.
- Noradrenaline is the first-line vasopressor; vasopressin or adrenaline is added for refractory shock. Fluids are given as 250 to 500 mL boluses guided by dynamic assessment, not static pressures.
- Sedation targets RASS -2 to 0 with daily interruption; delirium is screened with CAM-ICU; daily spontaneous breathing trials shorten ventilation.
Worked example
A 175 cm man with pneumonia has PaO2 80 mmHg on FiO2 0.6 and PEEP 8. The P/F ratio is 80/0.6 = 133, so this is moderate ARDS. Predicted body weight = 50 + 0.91 x (175 - 152.4) = 50 + 20.6 = about 70.6 kg. Initial tidal volume = 6 x 70.6 = about 420 mL. Reasoning: use predicted, never actual, body weight, and calculate the ratio only when PEEP is at least 5 and the imaging fits.
Traps that cost marks
- Setting tidal volume from actual weight in an obese patient, causing volutrauma.
- Calling any hypoxaemia ARDS without the PEEP and imaging criteria.
- Relying on a single lactate value rather than lactate clearance to judge resuscitation.
- Using high-dose steroids routinely in ARDS, where benefit is limited to early severe disease.