Shock - Reloaded

SHOCK

Shock = Failure of oxygen delivery relative to demand

• Not hypotension

• Not MAP <65

• Not vasopressor deficiency

Core equation: DO₂ = CO x CaO₂

Shock = FLOW failure ± CONTENT failure → tissue hypoxia

Pressure opens the gate. Flow saves organs. Microcirculation proves it.

WHEN SHOCK EXISTS?

Shock is present if ANY of the following are seen (BP irrelevant):

1. Lactate ≥ 2 mmol/L or rising

2. CRT > 3 seconds

3. Cold/mottled extremities

4. Altered mentation

5. U OP < 0.5 ml/kg/hr

Normal BP does NOT exclude shock

OXYGEN DELIVERY — PRIORITY LADDER

Determinants of tissue oxygenation (in order of importance):

1. Cardiac Output (FLOW)

2. Hb

3. SaO₂

4. PaO₂ (minimal contribution)

Lungs don’t deliver oxygen. Blood flow does.

SHOCK MANAGEMENT — PHASED, IN ORDER

PHASE 1 — OPEN THE PRESSURE GATE (MACRO, ENTRY ONLY); Timeframe: first 30–60 min

Goal: allow perfusion to occur

Targets (not endpoints):

• MAP ≈ 65 mmHg

• SBP ≥ 90 mmHg

Tools: Fluids (guided), Norepinephrine, Early blood if clearly needed

MAP is a gatekeeper, not the destination

PHASE 2 — RESTORE FLOW (CORE PHASE)

This is where shock is won or lost

FLOW = Cardiac Output, NOT EF

CO = HR × Stroke Volume

Stroke Volume depends on: 1. Preload, 2. Contractility, 3. Afterload, & 4. Rhythm

Bedside assessment of FLOW (no calculations needed)

A. Clinical

1. Narrow pulse pressure → low SV

2. Cold extremities, prolonged CRT

3. Oliguria, altered mentation

B. Echo (answer ONE question only): Is forward flow adequate?

Patterns:

1. Hyperdynamic + small LV → preload failure

2. Hyperdynamic + normal LV → distributive / high VO₂

3. Sluggish LV → contractility failure

4. RV dilated, LV compressed → obstructive

C. ScvO₂ trend

1. <60% → delivery failing

2. Rising after intervention → flow/content improved

3. Falling despite MAP → pressor-induced flow strangulation

FLOW FAILURE — FOUR MECHANISMS

1. Preload failure → fluids (stop if no perfusion response)

2. Contractility failure → inotrope

3. Excess afterload → reduce pressor + add inotrope

4. Rhythm failure → fix rhythm first

You cannot resuscitate shock through an arrhythmia

PHASE 3 — FIX CONTENT (CaO₂ = OXYGEN CARGO)

Priority order:

1. Hemoglobin

2. SaO₂

3. PaO₂ (least important)

Transfuse when: 

• Hb < 7

• Hb < 8 with ongoing shock, rising lactate, MI, TBI, persistent tachycardia

One unit of blood often improves DO₂ more than FiO₂ 100%

Avoid hyperoxia → causes vasoconstriction → worsens microflow

PHASE 4 — TRUST MICRO-PERFUSION (FINAL JUDGE)

If microperfusion is bad, shock is ongoing — MAP be damned

Primary endpoints (trend, not single value):

• CRT < 3 sec (best bedside marker)

• Lactate ↓ ≥10–20% in 2–4 hours

• Warm extremities

• Improving mentation

• Improving urine output

Adjuncts:

• Temperature gradients

• Mottling score (knee window)

• PPI (if available)

ACID–BASE AND SHOCK (CRITICAL ADDITION)

Acidosis kills shock resuscitation even if MAP is “normal”

• pH < 7.25 → catecholamine failure

• pH < 7.15 → myocardial depression

• pH < 7.10 → mitochondrial failure

Mechanisms:

• Capillary shutdown

• RBC rigidity → micro-shunting

• Mitochondrial inhibition

If lactate isn’t clearing, shock is ongoing — regardless of pressure

DO₂–VO₂ PHYSIOLOGY (WHY LACTATE RISES)

Lactate rises when:

1. DO₂ < VO₂ (true hypoxia)

2. β-adrenergic drive (aerobic glycolysis)

3. Mitochondrial dysfunction

Two zones:

• Supply-independent: lactate normal

• Supply-dependent: lactate rises steeply (true shock)

Resuscitation can work by:

• Increasing DO₂ (flow, content)

• OR decreasing VO₂ (often missed)

VO₂ MANIPULATION — THE MISSING HALF

Ways to reduce VO₂ :

• Fever control (biggest lever)

• Analgesia + sedation

• Control work of breathing (ventilation)

• Treat agitation, seizures, and shivering

Sedation in shock is metabolic resuscitation, not comfort care

SHOCK PHENOTYPES (FICK-BASED, NOT MAP-BASED)

1. Cold shock (low flow)

• CO ↓, ScvO₂ ↓, lactate ↑

• Cardiogenic, hypovolemic, obstructive, late sepsis

Treat: flow (fluids/inotrope), avoid vasoconstrictor overkill

2. Warm shock (extraction failure)

• CO ↑/N, ScvO₂ ↑, lactate ↑

• Early sepsis

Treat: source control, NE, time

3. Hypoxic/content shock

• CO may be normal

• Hb / SaO₂ low

Treat: blood, oxygen, and remove toxins