In septic shock (warm phase), cardiac output is high and SVR is low — the opposite of cardiogenic and hypovolemic shock, where CO is low and SVR is high. The reason is mechanism: sepsis causes massive vasodilation that dramatically lowers afterload, and a suddenly unloaded left ventricle ejects more easily. CO rises even as blood pressure falls, because the circuit resistance has collapsed, not because the pump has failed.
The common mistake
On a hemodynamic shock profile question, Omar had correctly identified cardiogenic shock (low CO, high SVR, high PCWP) and hypovolemic shock (low CO, high SVR, low PCWP) on first attempt. When the septic shock pattern appeared, he picked "low CO, high SVR, low PCWP" — the same basic pattern as hypovolemic, without the PCWP distinction.
A lot of students default to low CO for all shock types. The intuition is: shock is a state of circulatory failure, circulatory failure means inadequate output, inadequate output means low CO. Septic shock fits every clinical description of a severely ill patient — so why would CO be high?
The answer requires understanding what kind of failure septic shock represents.
The actual mechanism
Cardiogenic shock is pump failure: the heart can't generate adequate output. CO is low because the pump is broken. SVR rises as a compensatory response — the body clamps down peripheral resistance to maintain perfusion pressure. PCWP is high because the failing LV backs blood up into the pulmonary circuit.
Hypovolemic shock is tank failure: the circulating volume is reduced. CO is low because there's nothing to pump. SVR rises for the same compensatory reason. PCWP is low because the circuit is underfilled, not backed up.
Septic shock (early/warm phase) is resistance failure: massive cytokine-driven vasodilation collapses SVR. With afterload suddenly near zero, the LV ejects with almost no resistance. CO rises — sometimes dramatically. The patient has a bounding pulse and warm, flushed extremities because blood is racing through a low-resistance circuit. Blood pressure is still low because pressure = CO × SVR, and even a high CO can't overcome a near-zero SVR.
The hemodynamic signature: - Cardiogenic: low CO, high SVR, high PCWP — failing pump, backed-up circuit - Hypovolemic: low CO, high SVR, low PCWP — empty tank, intact pump - Septic (warm): high CO, low SVR, low PCWP — intact pump, collapsed resistance
The key Step 1 anchor: septic shock is the only shock type where CO is high. If a hemodynamic table shows high CO, the answer is septic shock until proven otherwise.
PCWP is what separates cardiogenic from hypovolemic — both have low CO and high SVR, but cardiogenic has a backed-up pulmonary circuit (high PCWP) while hypovolemic has an empty circuit (low PCWP). When you're reading a hemodynamic table on Step 1, read CO first (is it septic?), then read PCWP (cardiogenic vs hypovolemic if CO is low). Understanding the SVR component of these patterns is closely tied to why SVR is afterload, and why confusing SVR with preload breaks the shock classification entirely.
How to remember it
All shock has low BP. Only septic has high CO.
The circuit melted — the pump is fine, but the pipes have no resistance.
Or think of it physiologically: sepsis = vasodilation = afterload drop = CO rises. The Fick equation follows: if CO is high, tissues extract less oxygen per pass, and mixed venous O2 saturation is actually elevated in early septic shock — another distinguishing feature on a hemodynamic profile.
Check yourself
A 58-year-old woman presents with fever, tachycardia, hypotension, and warm flushed skin. On Swan-Ganz catheterization: CO = 9.2 L/min (elevated), SVR = 400 dyne·sec/cm⁵ (low), PCWP = 6 mmHg (low).
Which shock type does this represent?
A) Cardiogenic shock
B) Hypovolemic shock
C) Obstructive shock
D) Septic shock (warm phase)
Answer: D. High CO, low SVR, and low PCWP are the hemodynamic signature of warm septic shock. The elevated CO distinguishes it from all other shock types. Cardiogenic shock shows low CO, high SVR, and high PCWP. Hypovolemic shock shows low CO, high SVR, and low PCWP.
Close the gap
The hemodynamic table pattern that Omar missed and then confirmed correctly on open-ended retrieval is exactly the kind of application work that moves a memorized table into usable clinical reasoning.