Shock, Sepsis & Multisystem · Topic 1 of 14

Shock: Classification & Hemodynamics

Shock & resuscitation · classification

Shock: Classification & Hemodynamics

Shock is tissue hypoperfusion, not just low blood pressure. Name the mechanism from cardiac output, filling pressures and SVR, then treat the cause.

Key numbers

Hypoperfusion: mentation, skin, urine, lactateLactate > 2 mmol/L

01 · What it isFour types: hypovolemic, cardiogenic, obstructive, distributive

Classify by mechanism: hypovolemic (lost volume), cardiogenic (pump failure), obstructive (blocked filling or outflow — tamponade, tension pneumothorax, massive PE) and distributive (vasodilation — sepsis, anaphylaxis, spinal cord injury). Types can coexist.

02 · Don't confuseOnly distributive shock has low SVR

Hypovolemic, cardiogenic and obstructive shock are low-output states with compensatory high SVR; distributive shock has low SVR with normal or high cardiac output, although output can fall in late sepsis.

03 · Look forHypoperfusion: mentation, skin, urine, lactate

Recognize hypoperfusion by altered mentation, cool clammy or mottled skin with capillary refill > 2 seconds, urine output < 0.5 mL/kg/h and lactate > 2 mmol/L; follow the trend, since no single value is diagnostic.

What it is

Core idea

Shock is hypoperfusion, not just hypotension must knowShock is acute circulatory failure with inadequate oxygen delivery and cellular dysfunction. Hypotension (SBP < 90 or MAP < 65 mmHg) is common but not required; early or distributive shock can be normotensive.

Presentation

What you'll see

Warm, flushed skin suggests distributive shockDistributive shock may give warm, flushed skin, normal capillary refill and bounding pulses; hypovolemic, cardiogenic and obstructive shock give cool, pale or dusky skin with prolonged capillary refill from vasoconstriction.

Neurogenic shock: hypotension with bradycardia, warm skinNeurogenic shock follows cervical or high thoracic cord injury (usually above T4): loss of sympathetic tone causes hypotension with bradycardia and warm, dry skin — unlike other shock states.

Tamponade: distended neck veins, muffled heart soundsCardiac tamponade presents with jugular venous distention, muffled heart sounds and pulsus paradoxus; with hypotension it is obstructive shock needing immediate drainage.

Do right now

In this order

First moves

Hypoperfusion: mentation, skin, urine, lactateLactate > 2 mmol/L

→

Large-bore access, oxygen, arterial line, ICU

→

Point-of-care ultrasound identifies the shock type

→

Fluids first in hypovolemic and distributive shock

Why each step

Hypoperfusion: mentation, skin, urine, lactateRecognize hypoperfusion by altered mentation, cool clammy or mottled skin with capillary refill > 2 seconds, urine output < 0.5 mL/kg/h and lactate > 2 mmol/L; follow the trend, since no single value is diagnostic.

Large-bore access, oxygen, arterial line, ICUPlace two large-bore (14–16 gauge) peripheral IVs (intraosseous or central if they fail), give oxygen when SpO₂ is < 94%, monitor blood pressure with an arterial line, and admit to the ICU.

Point-of-care ultrasound identifies the shock typeBedside ultrasound (e.g., the FALLS protocol) rapidly sorts the type: pericardial effusion, dilated RV or pneumothorax (obstructive), poor LV function with B-lines (cardiogenic), small collapsing IVC (hypovolemic).

Fluids first in hypovolemic and distributive shockStart hypovolemic and distributive shock with crystalloid (sepsis: ≥ 30 mL/kg balanced crystalloid within 3 hours); give 250–500 mL boluses with raised neck veins or acute MI, and no fluid challenge with pulmonary edema.

Management

Orders

Suspected cardiogenic shock: ECG, troponin, echocardiogramSuspect a cardiac cause with chest pain, a new systolic murmur (septal rupture, mitral regurgitation) or pulmonary edema; obtain an ECG, cardiac biomarkers and an echocardiogram. Staging and support are on the Cardiogenic Shock sheet.

Obstructive shock: relieve the obstruction immediatelyTreat obstruction directly: needle-decompress tension pneumothorax (5th intercostal space, midaxillary line) then place a chest tube; pericardiocentesis for tamponade (surgery if traumatic); thrombolysis or embolectomy for massive PE.

Hemorrhage: control bleeding and transfuse 1:1:1In hemorrhagic shock, surgical control of bleeding comes first; give red cells, plasma and platelets about 1:1:1 as soon as possible rather than large crystalloid volumes.

Norepinephrine for distributive shock despite fluidsDistributive shock that stays hypotensive after initial crystalloid gets norepinephrine (septic shock target MAP 65 mmHg) plus cause-specific therapy: antibiotics and source control in sepsis, epinephrine in anaphylaxis.

Hypovolemic or distributive (septic) shock?

Commonly confused

Cardiac output	Low
SVR	High
CVP and wedge	Low
Skin	Cool, pale, delayed capillary refill
First moves	Volume — blood if hemorrhage, crystalloid if fluid loss

More detail

Reference

Show 6 reference facts

A low central venous oxygen saturation reflects increased oxygen extraction when cardiac output falls and can drop before lactate rises.

Mixed shock is common: septic and anaphylactic shock usually carry a hypovolemic component, and septic shock can later depress cardiac output.

When oxygen delivery falls short, cells switch to anaerobic metabolism and lactate rises; sympathetic vasoconstriction, tachycardia, renin and cortisol release initially preserve flow to the heart and brain.

Nonhemorrhagic hypovolemia: vomiting, diarrhea, burns, diuretics, osmotic diuresis (diabetes, diabetes insipidus) and capillary leak into tissues (sepsis, pancreatitis, crush injury).

Persistent shock and reperfusion injury lead to multiple organ dysfunction, most often after septic shock, in which the lungs (ARDS) are the most frequent target.

Even with treatment, mortality is about 40–50% for cardiogenic shock after MI and 30–40% for septic shock; untreated shock is usually fatal.

Pearl

Three shocks are cold and squeezed (low output, high SVR); only distributive shock is warm and dilated.

Sources · Merck Manual Professional — Shock (reviewed Jun 2026) · Hemodynamic profiles related to circulatory shock in cardiac care units. Arch Cardiol Mex 2020;90(1) (Table 1: shock hemodynamic profiles) · Merck Manual Professional — Spinal Trauma (neurogenic vs spinal shock) · Merck Manual Professional — Intravenous Fluid Resuscitation (2026) · Surviving Sepsis Campaign: International Guidelines for Management of Sepsis and Septic Shock 2026 — SCCM recommendations page · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).