Shock: Classification & Hemodynamic Profiles
A boards-focused walkthrough of the four shock categories — hypovolemic, cardiogenic, obstructive, and distributive — anchored to their signature preload, cardiac output, SVR, and SvO2 profiles, with the classic vignette buzzwords and the correct immediate next step for each.
Framing shock as a perfusion problem
Shock is a state of inadequate tissue perfusion causing cellular hypoxia and, if uncorrected, multi-organ failure — it is not simply a low blood-pressure number. Perfusion pressure is set by MAP = cardiac output (CO) x systemic vascular resistance (SVR), while oxygen delivery (DO2 = CO x arterial O2 content) also depends on hemoglobin and O2 saturation — so shock can arise from a failing pump, lost volume, mechanical obstruction, or misdistributed flow. Boards organize shock into four hemodynamic categories — hypovolemic, cardiogenic, obstructive, and distributive — each with a signature pattern of preload (CVP/PCWP), cardiac output, SVR, and mixed venous O2 saturation (SvO2). Because early compensation (tachycardia, peripheral vasoconstriction) can hold systolic pressure near-normal, the tested skill is recognizing the profile, not waiting for frank hypotension.
- MAP = CO x SVR; shock = inadequate perfusion, not one BP cutoff
- Four classes: hypovolemic, cardiogenic, obstructive, distributive
- Cold, clammy skin = low-output shock (hypovolemic, cardiogenic, obstructive) with compensatory high SVR; warm, flushed skin = distributive/septic warm phase (low SVR, high CO)
- Among the cold-skin shocks, neck veins split them: flat/collapsed -> hypovolemic; distended (high CVP) -> cardiogenic or obstructive
- Low SvO2 = increased O2 extraction (low-output states); high/normal SvO2 = impaired extraction/shunting (distributive/septic)
- Narrow pulse pressure -> hypovolemic/cardiogenic; wide, bounding pulse -> distributive
- Hypotension + bradycardia = neurogenic shock (nearly every other shock -> tachycardia)
- Elevated serum lactate signals anaerobic metabolism / hypoperfusion and can be raised in any shock type
Hemodynamic profiles of the four categories
| Parameter | Hypovolemic | Cardiogenic | Obstructive | Distributive (septic, warm) |
|---|---|---|---|---|
| Preload (CVP/PCWP) | down | up | up* | down / normal |
| Cardiac output | down | down | down | up (early) |
| SVR | up | up | up | markedly down |
| SvO2 (mixed venous) | down | down | down | up / normal |
| Skin | cold, clammy | cold, clammy | cold, clammy | warm, flushed |
The distributive (vasodilatory) group
Distributive shock is vasodilatory: the primary lesion is a collapse in SVR, so cardiac output is typically high or normal even as perfusion pressure fails. Septic shock (the most common cause, and the most common cause of shock in the ICU) shows a warm, hyperdynamic early phase — bounding pulses, wide pulse pressure, warm extremities, high CO, low SVR, and high/normal SvO2 from impaired O2 extraction and microvascular shunting — that can progress to a cold, hypodynamic late phase with septic myocardial depression. Anaphylactic shock is IgE/mast-cell/histamine-mediated vasodilation with urticaria, angioedema, and bronchospasm. Neurogenic shock follows spinal cord injury above ~T6: loss of sympathetic outflow drops SVR, and loss of the cardiac accelerator fibers (T1–T4) leaves vagal tone unopposed, causing bradycardia — the one shock pairing hypotension with a slow heart rate and warm, dry skin. Distinguish it from spinal shock, a transient areflexia/flaccidity below the lesion that is a neurologic, not hemodynamic, phenomenon.
- Hypovolemic — trauma hemorrhage, GI bleed, vomiting/diarrhea, burns; tachycardia, narrow pulse pressure, oliguria, flat neck veins -> IV fluids/blood, control bleeding
- Cardiogenic — post-MI, cold/clammy, pulmonary edema, elevated JVP, S3 -> revascularization (definitive in MI) + dobutamine for low output, norepinephrine if hypotensive; avoid fluid overload
- Cardiac tamponade (obstructive) — Beck triad (hypotension, distended neck veins, muffled heart sounds), pulsus paradoxus, electrical alternans, low-voltage ECG -> pericardiocentesis
- Tension pneumothorax (obstructive) — absent breath sounds, hyperresonance, tracheal deviation away, distended neck veins -> immediate needle decompression (do not wait for CXR)
- Massive PE (obstructive) — sudden dyspnea/hypoxia, elevated JVP, acute RV strain (S1Q3T3) -> anticoagulation +/- thrombolysis (for hemodynamic instability)
- Septic — fever, infection source, warm extremities, wide pulse pressure, high lactate -> cultures, broad-spectrum antibiotics, IV fluids, then norepinephrine if fluid-refractory
- Anaphylactic — allergen exposure, urticaria/angioedema/wheeze -> IM epinephrine
- Neurogenic — cord injury, hypotension + bradycardia, warm dry skin -> fluids, vasopressors, atropine for symptomatic bradycardia
Distributive subtypes side-by-side
| Feature | Septic (warm) | Anaphylactic | Neurogenic |
|---|---|---|---|
| Trigger | Infection / SIRS | Allergen (IgE) | Cord injury above ~T6 |
| Heart rate | up (tachycardia) | up (tachycardia) | down (bradycardia) |
| Skin | warm, flushed | urticaria, angioedema | warm, dry |
| Classic clue | fever, high lactate, wide pulse pressure | wheeze, hives, hypotension | hypotension + bradycardia |
| First-line Rx | fluids + antibiotics + norepinephrine | IM epinephrine | fluids, vasopressors, atropine |
- SvO2/ScvO2 is a diagnostic lever: low in hypovolemic/cardiogenic/obstructive (increased extraction); high or normal in septic/distributive (impaired extraction, shunting)
- First-line pressor/inotrope: septic -> norepinephrine; anaphylaxis -> epinephrine (IM); cardiogenic -> dobutamine for low output plus norepinephrine if hypotensive, with revascularization the definitive fix in MI
- Fluids help hypovolemic and distributive; use cautiously in cardiogenic (raises PCWP -> worsens pulmonary edema) and only as a brief bridge in tamponade while awaiting drainage
- Lactate and lactate clearance track severity and adequacy of resuscitation in septic shock
- Pulse pressure: narrow (low stroke volume + vasoconstriction) vs wide/bounding (low SVR)
Bedside two-step to nail the profile
A rapid bedside triage narrows the diagnosis before invasive numbers return. First: warm or cold extremities? Warm, well-perfused, bounding pulses point to distributive/septic shock (low SVR); cold, clammy, mottled skin points to a low-output state (hypovolemic, cardiogenic, obstructive). Second: what are the neck veins / right-sided pressures doing? Flat neck veins with cold skin -> hypovolemic; distended neck veins with cold skin -> cardiogenic or obstructive (tamponade, tension pneumothorax, massive PE). Confirm with the hemodynamic signature (preload, CO, SVR, SvO2) and treat the mechanism directly — volume and hemorrhage control, inotropes/revascularization, relief of the obstruction, or vasopressors plus source control.
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