What are the different types of hypoxia?

Types of Hypoxia

Tissue hypoxia is classified into four distinct types: hypoxaemic (hypoxic), anaemic, stagnant (circulatory), and histotoxic hypoxia, each requiring different therapeutic approaches beyond supplemental oxygen. 1

Core Classification Framework

The British Thoracic Society provides the definitive classification system for understanding hypoxia, distinguishing it from hypoxaemia (low blood oxygen) by emphasizing that hypoxia represents insufficient oxygen supply to meet tissue metabolic demands. 1

1. Hypoxaemic Hypoxia (Hypoxic Hypoxia)

This type results from reduced partial pressure of oxygen in arterial blood, causing low oxygen content. 1

Mechanisms include:

• Ventilation-perfusion (V/Q) mismatch 1
• Right-to-left cardiac or pulmonary shunts 1
• Alveolar hypoventilation 1
• Diffusion impairment across alveolar-capillary membrane 1
• High altitude exposure (reduced atmospheric oxygen) 1

Clinical significance: This is the only type of hypoxia that responds to supplemental oxygen therapy. 1 Type 1 respiratory failure (PaO2 <8 kPa or 60 mmHg with normal/low PaCO2) represents the clinical manifestation of hypoxaemic hypoxia. 1

2. Anaemic Hypoxia

This occurs when reduced haemoglobin levels or impaired oxygen-carrying capacity prevents adequate oxygen delivery despite normal PaO2 and SpO2. 1

Two primary mechanisms:

• True anaemia: Reduced haemoglobin concentration available for oxygen transport 1
• Functional anaemia: Carbon monoxide poisoning impairs haemoglobin's ability to bind oxygen, reducing oxygen-carrying capacity despite adequate haemoglobin levels 1

Critical pitfall: Pulse oximetry and arterial blood gas PaO2 will appear normal, masking tissue hypoxia. 2 Treatment requires blood transfusion or specific antidotes (e.g., for CO poisoning), not oxygen therapy alone. 2

3. Stagnant Hypoxia (Circulatory Hypoxia)

This results from inadequate blood flow delivering insufficient oxygen to tissues despite normal blood oxygen content. 1

Clinical scenarios:

• Global: Low cardiac output states (cardiogenic shock, severe heart failure) 1
• Regional: Peripheral vascular disease causing tissue ischemia and gangrene 1
• Cold exposure causing extremity hypoperfusion 1

Management focus: Improving cardiac output with fluids, inotropes, or vasopressors rather than increasing inspired oxygen. 2

4. Histotoxic Hypoxia

This represents cellular inability to utilize oxygen due to disrupted cellular metabolism, despite adequate oxygen delivery. 1

Primary mechanisms:

• Cyanide poisoning: Impairs cytochrome function, blocking cellular oxygen utilization 1
• Sepsis-related mitochondrial dysfunction: Decreased oxygen usage despite adequate delivery, termed "cytopathic dysoxia" 1

Treatment approach: Requires specific antidotes (e.g., hydroxocobalamin for cyanide) and treatment of underlying sepsis, not oxygen supplementation. 2

Critical Clinical Distinctions

Normal oxygen saturation does NOT exclude tissue hypoxia. 2 Patients with SpO2 >94% may have severe tissue hypoxia from anaemic, stagnant, or histotoxic mechanisms. 2

Oxygen therapy only corrects hypoxaemic hypoxia. 1 The other three types require alternative interventions targeting haemoglobin levels, perfusion, or cellular metabolism. 2

Assessment must extend beyond pulse oximetry to include lactate levels, mixed venous oxygen saturation, clinical perfusion signs, and haemoglobin concentration to identify non-hypoxaemic causes. 2

Practical Management Algorithm

For any suspected tissue hypoxia:

1. Measure SpO2 and arterial blood gas to determine if hypoxaemia exists 2
2. If PaO2 <60 mmHg (8 kPa): Treat as hypoxaemic hypoxia with supplemental oxygen targeting SpO2 ≥94% (or 88-92% if COPD risk) 2
3. If PaO2 normal but tissue hypoxia suspected: Check haemoglobin, lactate, and perfusion status 2
   • Low haemoglobin (<70 g/L in critical illness): Consider transfusion for anaemic hypoxia 2
   • Poor perfusion/low cardiac output: Optimize hemodynamics for stagnant hypoxia 2
   • Suspected toxin/sepsis with metabolic dysfunction: Treat underlying cause for histotoxic hypoxia 2

Common error: Escalating oxygen therapy or pursuing intubation when the problem is non-hypoxaemic hypoxia requiring entirely different interventions. 2