Septic shock – hemodynamic profile

Septic shock is the prototypical form of distributive shock: it is not the pumping capacity but the distribution of blood flow that is disturbed. Hemodynamically, systemic vascular resistance falls below 800 dyn·s·cm⁻⁵ while the cardiac index is normal or even elevated. This constellation of high flow at low resistance inverts the pattern of cardiogenic shock. Mixed venous saturation is typically normal or raised, because oxygen is no longer adequately used at cellular level and arteriovenous shunts bypass the capillary bed.

For educational use only. Not for clinical use.

Hemodynamic profile

ParameterDirectionTypicalNormal range
CI↑normal to > 4.0 L/min/m²2.5-4.0
SVR↓< 800 dyn·s·cm⁻⁵800-1200
PCWP↓normal to low6-15
ZVD↓normal to low2-6
SvO2↑normal to elevated (> 70 %)65-75

Differentiation

Differentiate fromDiscriminating parameterExplanation
Cardiogenic shockSVRSystemic vascular resistance separates the two: reduced in septic shock (< 800 dyn·s·cm⁻⁵), compensatorily elevated in cardiogenic shock. Venous saturation also moves in opposite directions.
Hypovolemic shockSVRBoth may show low filling pressures, but vascular resistance is high in hypovolemic shock and low in septic shock. Cardiac index in septic shock is typically normal or elevated.

Causes

  • Pneumonia
  • Intra-abdominal infection such as peritonitis or cholangitis
  • Urinary tract infection with urosepsis
  • Catheter-related bloodstream infection
  • Soft tissue infection and necrotising fasciitis
  • Endocarditis

Clinical context

Early on the extremities are often warm and well perfused — the classic picture of warm shock — while lactate and oxygen extraction already indicate tissue hypoxia. A high venous saturation therefore does not exclude shock; in a septic context it is precisely what is expected. Septic cardiomyopathy may supervene, lowering the cardiac index and shifting the picture towards mixed shock. PCWP is usually normal or low, though it may rise with concurrent volume therapy.

Frequently asked questions

Which values characterise septic shock hemodynamically?
A systemic vascular resistance below 800 dyn·s·cm⁻⁵ with a normal or elevated cardiac index, often above 4.0 L/min/m², and a normal to raised mixed venous saturation.
Why is venous saturation high in septic shock?
Cells can no longer use the oxygen delivered to them, and arteriovenous shunts divert blood past the capillary bed. Oxygen therefore returns to the venous system unconsumed.
How do you distinguish septic from cardiogenic shock?
By vascular resistance and venous saturation, which move in opposite directions in the two: low SVR with high saturation suggests sepsis, high SVR with low saturation a cardiogenic cause.
Can the cardiac index be low in septic shock?
Yes. With septic cardiomyopathy or inadequate volume filling the cardiac index may fall. A low SVR with a low CI then suggests a mixed picture rather than arguing against sepsis.
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