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Mnemonic

Oxygen Content Versus Saturation

A memory aid for distinguishing oxygen tension, saturation and content.

Physiology Respiratory Respiratory Respiratory Physiology and Function Tests
Expansion

Tension drives diffusion, saturation describes haemoglobin, content determines delivery

Expansion

  • Partial pressure (tension) - the driving force for diffusion; measured by blood gas analysis. Reflects dissolved oxygen only
  • Saturation - the percentage of available haemoglobin binding sites occupied; measured by pulse oximetry
  • Content - the total amount of oxygen per litre of blood, and the only one that determines delivery

Content = (1.34 x haemoglobin x saturation) + (0.023 x PaO2)

The distinction matters because:

  • In anaemia, tension and saturation are normal but content is low
  • In carbon monoxide poisoning, tension is normal, pulse oximetry reads falsely high, and content is very low
  • In polycythaemia, content is high despite normal saturation

This is why pulse oximetry, though invaluable, can be dangerously reassuring: it reports the proportion of haemoglobin that is saturated, not how much haemoglobin there is nor how much oxygen is reaching the tissues.