title: "Shock Pathophysiology"

Shock Pathophysiology

Shock = Inadequate cellular oxygen delivery relative to metabolic demand.

The Core Equation

DO₂ = CO × CaO₂

Where:

DO₂ = Oxygen delivery (mL/min)
CO = Cardiac output (L/min)
CaO₂ = Arterial oxygen content

CaO₂ = (Hb × 1.34 × SaO₂) + (0.003 × PaO₂)

The dissolved oxygen component (0.003 × PaO₂) is negligible under normal conditions.

This means oxygen delivery depends on:

Haemoglobin concentration - anaemia reduces DO₂
Oxygen saturation - hypoxaemia reduces DO₂
Cardiac output - low CO reduces DO₂

You can have adequate MAP but still have shock if DO₂ is insufficient!

Classification of Shock

Type	Primary Problem	CO	SVR
Hypovolaemic	Low preload	↓	↑
Cardiogenic	Pump failure	↓	↑
Distributive	Vasodilation	↑ or N	↓↓
Obstructive	Mechanical obstruction	↓	↑

Compensatory Mechanisms

When DO₂ drops, the body compensates:

Increased O₂ extraction - tissues take more O₂ from blood
Catecholamine release - tachycardia, vasoconstriction
RAAS activation - fluid retention
Anaerobic metabolism - lactate production

Clinical Pearl

Lactate rises when oxygen extraction is maximal and tissues switch to anaerobic metabolism. Rising levels indicate worsening shock; falling levels suggest improving perfusion.

When Compensation Fails

Warning

Decompensated shock occurs when compensatory mechanisms are exhausted:

Hypotension becomes refractory
Lactate continues to rise
Multi-organ dysfunction begins
Mortality significantly increases

Clinical Assessment

Parameter	What It Tells You
Blood pressure	Late marker - can be maintained until late
Heart rate	Early compensatory tachycardia
Lactate	Cellular hypoxia, oxygen debt
ScvO₂	Balance between O₂ delivery and consumption
Urine output	End-organ perfusion
Mental status	Cerebral perfusion

Central venous oxygen saturation (ScvO₂) measured from a central line reflects how much oxygen remains after tissues have extracted what they need.

Normal: 65-75%
Low (under 65%): Increased extraction due to inadequate delivery (shock) or increased demand (sepsis, fever)
High (above 75%): May indicate inability to extract O₂ (mitochondrial dysfunction, cell death) - this is a bad sign in sepsis

ScvO₂ trending is more useful than single values.

Source: Critical Care Bible / ICU physiology.