Shock Classification: Cardiogenic vs Septic vs Hypovolemic vs Obstructive
Clinical Reasoning · 8 min read · Published 2026-09-21
Introduction
"Shock" is inadequate tissue perfusion, not low blood pressure - a patient can be hypertensive and still in shock. Step 1 and Step 2 CK love to hand you vitals, a CVP or PA catheter reading, and a one-line vignette, then ask which category of shock you are dealing with. The fastest path to the answer is to stop memorizing lists and start reading the hemodynamic fingerprint.
The Four Categories
All shock ultimately reduces cardiac output or vascular tone. The four classic types differ in where that failure sits:
- Hypovolemic - not enough preload (hemorrhage, burns, diarrhea, vomiting)
- Cardiogenic - the pump itself fails (MI, myocarditis, arrhythmia, valve failure)
- Obstructive - the pump is fine, but something blocks filling or outflow (tamponade, tension pneumothorax, massive PE)
- Distributive - the pipes leak and dilate (sepsis, anaphylaxis, neurogenic)
The Hemodynamic Fingerprint
Each type leaves a characteristic signature across four variables: preload (CVP/PCWP), cardiac output, SVR, and mixed venous oxygen saturation.
| Parameter | Hypovolemic | Cardiogenic | Obstructive | Distributive |
|---|---|---|---|---|
| CVP / PCWP | Low | High | High | Low / normal |
| Cardiac output | Low | Low | Low | High (early) |
| SVR | High | High | High | Low |
| SvO2 | Low | Low | Low | High |
| Extremities | Cold, clammy | Cold, clammy | Cold | Warm, flushed |
The trick is that three of the four look identical - cold, low-output, vasoconstricted. Only the filling pressure separates hypovolemic from cardiogenic and obstructive: low preload in hypovolemia, high preload in cardiogenic (the failing pump backs blood up) and obstructive (blood cannot enter or leave the right heart).
Separating Cardiogenic from Obstructive
Both show high CVP and low output, so read the vignette for the mechanism:
- Tamponade: pulsus paradoxus, muffled heart sounds, equilibration of diastolic pressures (right atrial, right ventricular, PCWP all converge)
- Tension pneumothorax: tracheal deviation, absent breath sounds, hyperresonance
- Massive PE: acute dyspnea with clear lungs, elevated pulmonary pressures but normal PCWP
A pearl worth knowing: in tamponade and tension pneumothorax, both right- and left-sided filling pressures equalize. In massive PE, the PCWP stays normal because the left atrium is unaffected - the blockage is upstream of it.
Distributive Shock: The Septic Signature
Septic shock early on shows the paradox of hypotension despite high cardiac output: warm extremities, wide pulse pressure, low SVR, and a high SvO2 because tissues cannot extract oxygen efficiently (cytopathic hypoxia). Late sepsis can depress the myocardium and drift toward a low-output picture, which is a favorite trap on Step 2 CK.
Anaphylaxis follows the same hemodynamic pattern (low SVR, high output) but adds urticaria, bronchospasm, and a clear trigger. Neurogenic shock (high spinal cord injury) is the exception that mimics hypovolemia's low pressure but pairs it with bradycardia and warm, dry skin - hypovolemia produces reflex tachycardia, so a "shocked" patient with a slow heart rate after trauma should push you toward the cord.
Common Pitfalls
- Treating hypotension as the definition of shock - lactic acidosis and end-organ dysfunction define it
- Forgetting that early sepsis can look hyperdynamic; do not rule out sepsis because the extremities are warm
- Missing that cardiogenic and obstructive shock both raise CVP - the vignette must give you a mechanism clue
- Confusing neurogenic shock (bradycardic, warm) with hemorrhagic shock (tachycardic, cold)
Clinical Pearl
If the question gives you a mixed venous saturation, use it: a low SvO2 means tissues are grabbing every molecule of oxygen (low flow states), while a high SvO2 in a hypotensive patient means oxygen is being delivered but not consumed - the distributive signature. It is one of the few lab values that points in opposite directions across shock categories.
Summary
Classify shock by two questions: Is the preload low or high? Is the SVR low or high? Low preload plus vasoconstriction is hypovolemia; high preload plus vasoconstriction is cardiogenic or obstructive (split them by the vignette's mechanism); low SVR with preserved or high output is distributive. Build the habit of reading the four-variable fingerprint, and these questions become pattern recognition.