The four classes of shock
Separates hypovolaemic, cardiogenic, obstructive and distributive shock by their preload, afterload and cardiac-output profile, with the first-line management of each.
Shock is inadequate tissue perfusion — sort it by the numbers
Every shock is a failure of oxygen delivery. What separates the four types is the haemodynamic fingerprint: preload (filling / CVP / PCWP), the pump (cardiac output), and afterload (systemic vascular resistance, SVR). Read those three and the cause — and the treatment — follow.
| Type | Preload (CVP/PCWP) | Cardiac output | Afterload (SVR) | Skin | First-line management |
|---|---|---|---|---|---|
| Hypovolaemic | ↓↓ | ↓ | ↑ (compensatory) | Cold, clammy | Stop the loss + rapid crystalloid / blood |
| Cardiogenic | ↑ (PCWP ↑) | ↓↓ | ↑ | Cold, clammy | Inotropes (dobutamine), revascularise; cautious fluids |
| Obstructive | ↑ (right-sided) | ↓ | ↑ | Cold | Relieve the obstruction (needle/pericardiocentesis, thrombolysis) |
| Distributive | ↓ / normal | ↑ (early) | ↓↓ | Warm (early) | Fluids + vasopressors (noradrenaline); treat the cause |
Reason through each type
Hypovolaemic
Loss of intravascular volume — haemorrhage or fluid (burns, GI losses). Low preload, high SVR as the body clamps down. The surgical priority is control the bleeding; fluids buy time but do not fix an open vessel. Note the ATLS classes: tachycardia appears in Class II, but hypotension is late (Class III, >30% loss) because young patients compensate.
Cardiogenic
The pump fails — commonly large anterior MI, arrhythmia, or acute valve failure. High filling pressures (congested lungs) with a low output. Fluids can worsen pulmonary oedema, so support the pump: inotropes, treat the ischaemia, consider mechanical support.
Obstructive
The heart and volume are fine but something mechanically blocks filling or output: tension pneumothorax, cardiac tamponade, massive PE. The fix is decompression, not drugs — needle thoracostomy, pericardiocentesis, or thrombolysis/embolectomy.
Distributive
Pathological vasodilation: septic, anaphylactic, neurogenic. SVR collapses; early output is high and the patient is warm — the one type that breaks the "cold and clammy" rule. Neurogenic shock uniquely shows hypotension with bradycardia (lost sympathetic tone). Management is fluids then vasopressors, plus the cause (antibiotics, adrenaline for anaphylaxis).
Pearl: A warm, vasodilated, hypotensive patient is distributive until proven otherwise; cold with raised JVP and a quiet/hyperresonant chest is obstructive — do not reach for fluids alone in either.
Anchor
Three questions — is the tank empty (hypovolaemic), the pump failing (cardiogenic), the pipes blocked (obstructive), or the pipes too wide (distributive)? Preload, output and SVR answer all three.
Sources
- Bailey & Love's Short Practice of Surgery, 28e
- Harrison's Principles of Internal Medicine, 21e