Definitions

Hypoxia is defined as a lack of O₂ (usually meaning lack of O₂ delivery to tissues or cells). Hypoxaemia is a lack of O₂ in arterial blood (low PaO₂). Hypoventilation is inadequate breathing leading to an increase of CO₂ (hypercapnia) and hypoxaemia. Apnoea means cessation of breathing in expiration.

Classification of Hypoxia

• Hypoxic hypoxia: reduced O₂ entering the blood.
  
• Hypaemic/anaemic hypoxia: reduced capacity of blood to carry O₂.
  
• Stagnant hypoxia: poor oxygenation due to poor circulation.
  
• Histotoxic hypoxia: inability of cells to use O₂.

Common Causes

Postoperative Causes (Usually Hypoxic Hypoxia)

• CNS depression, e.g. post-anaesthesia.
  
• Airway obstruction, e.g. aspiration of blood or vomit, laryngeal oedema.
  
• Poor ventilation, e.g. abdominal pain, mechanical disruption to ventilation.
  
• Loss of functioning lung, e.g. V/Q mismatch (pulmonary embolism, pneumothorax, collapse/consolidation).

General Causes

• Central respiratory drive depression, e.g. opiates, benzodiazepines, CVA, head injury, encephalitis.
  
• Airway obstruction, e.g. facial fractures, aspiration of blood or vomit, thyroid disease or head and neck malignancy.
  
• Neuromuscular disorders (MS, myasthenia gravis).
  
• Sleep apnoea (obstructive, central or mixed).
  
• Chest wall deformities.
  
• COPD.
  
• Shock.
  
• Carboxyhaemoglobinaemia, methaemoglobinaemia.

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