Biology of Critical Illness and Support of Organ System Function
Core concepts
Critical illness is best understood as a failure of oxygen delivery and substrate use relative to demand, layered on a dysregulated inflammatory and neuroendocrine response. Candidates must be fluent in oxygen delivery (DO2 = cardiac output × arterial oxygen content), the determinants of oxygen content (hemoglobin, saturation, dissolved oxygen), and the balance with consumption (VO2). Shock states (hypovolemic, cardiogenic, distributive, obstructive) differ in preload, contractility, and vascular tone; mixed venous saturation and lactate clearance help distinguish adequacy of resuscitation from mere pressure targets.
Organ support requires mechanism-level knowledge: lung-protective ventilation (tidal volume ~6 mL/kg predicted body weight, plateau pressure limits, permissive hypercapnia), PEEP and recruitment, and the recognition that oxygenation and ventilation are separate problems. Renal replacement therapy indications (refractory acidosis, hyperkalemia, volume overload, uremic complications, toxin removal) are frequently tested. Nutritional support, glycemic control, stress ulcer prophylaxis, and venous thromboembolism prophylaxis are routine but high-yield supportive measures.
Traps
Questions love the distinction between pressure and flow (a normal blood pressure with high lactate), between oxygen saturation and oxygen content (anemic hypoxia with normal SpO2), and between permissive hypercapnia and dangerous acidosis. Another trap is treating a number rather than a patient - for example, transfusing to a hemoglobin threshold without considering ischemia.
Worked rule of thumb
A patient with septic shock has MAP 68 mmHg, SpO2 98%, hemoglobin 7 g/dL, and rising lactate. Despite adequate pressure and saturation, oxygen content is low because hemoglobin is the dominant carrier; consider transfusion and source control rather than escalating vasopressors alone. Rule: fix content, then flow, then pressure.
Exam strategy
Anchor each answer in physiology: what is delivery, what is demand, what is the failing organ, and what intervention changes the mechanism?