General Intensive Care Medicine: Physiology, Resuscitation and Organ Support
Core concepts
Candidates must be fluent in oxygen delivery and consumption (DO2/VO2), the oxygen-haemoglobin dissociation curve and its shifts, and the determinants of mean arterial pressure. Understand shock phenotypes (hypovolaemic, cardiogenic, distributive, obstructive) and the bedside tools that separate them: lactate, central venous oxygen saturation, capillary refill, echocardiography, and dynamic measures of fluid responsiveness such as pulse pressure and stroke volume variation.
Know the physiology and complications of mechanical ventilation: lung-protective tidal volumes of 6 mL/kg predicted body weight, plateau pressure targets, driving pressure, permissive hypercapnia, auto-PEEP and its haemodynamic consequences. Be able to interpret arterial blood gases systematically, including compensation rules and the anion gap with the delta ratio. Familiarity with sepsis bundles, vasopressor choice, sedation and delirium, glycaemic control, renal replacement therapy indications, and nutrition in the critically ill is expected.
Traps in questions
- Confusing oxygen saturation with oxygen content; saturation can be normal with profound anaemia.
- Assuming a normal central venous pressure excludes hypovolaemia, or that a raised CVP predicts fluid responsiveness.
- Forgetting that high peak pressures may reflect resistance rather than compliance; always check plateau pressure.
- Missing mixed acid-base disorders by accepting a single diagnosis without checking expected compensation.
- Attributing a raised lactate solely to hypoperfusion when beta-agonists, seizures, liver failure and malignancy also raise it.
Worked example
A patient has pH 7.22, PaCO2 30 mmHg, HCO3 12 mmol/L, Na 138, Cl 100. Expected HCO3 for acute respiratory alkalosis is a fall of about 2 per 10 mmHg CO2, which cannot explain a bicarbonate of 12. The anion gap is 138 − (100 + 12) = 26, so a metabolic acidosis with raised anion gap is present alongside a respiratory alkalosis. Rule of thumb: always calculate the anion gap and delta ratio before concluding the acid-base picture.