Overview
The oxygen saturation is a reflection of the percentage of haemoglobin that is bound to oxygen.
Normal Range
- >90%
Reduced Oxygen Saturation
The level of haemoglobin saturated by oxygen may be reduced in the context of reduced oxygen, right shift of the oxygen saturation curve, displacement of oxygen by carbon dioxide, or in the setting of haemoglobinopathies.
Causes of Reduced Oxyhaemoglobin
Reduced Environmental Oxygen
- High altitude
Hypoventilation
- CNS - drugs, trauma, encephalopathy, motor neurone disease, Guillain-Barré
- Muscular / neuromuscular - myaesthenia, paralytics, myopathy, fatigue, malnutrition
- Airway obstruction - foreign body, asthma, COPD, bronchiectasis
- Decreased compliance - interstitial lung disease
- Chest wall abnormalities
Ventilation / Perfusion Mismatch
- Physiological shunt - atelectasis, pulmonary oedema, pneumonia, ARDS
- Anatomical shunt - cardiac shunt e.g. tetralogy of Fallot
- Dead space - asthma, COPD, pulmonary embolus, heart failure
Other
- Right shift of the oxygen dissociation curve - acidaemia, hyperthermia, high 2,3-DPG
- Carbon monoxide poisoning
- Haemoglobinopathy - methaemoglobinaemia, foetal haemoglobin
The Oxygen Dissociation Curve
The oxyhaemoglobin dissociation curve describes the relationship between PaO₂ and HbO₂. At high PaO₂ (alveolar-capillary membrane) oxygen readily binds to haemoglobin. At low PaO₂ (systemic capillaries) oxygen is readily released from haemoglobin.
Significance
- As the curve shifts to the left, oxygen will more readily bind to haemoglobin. As the curve shifts to the right, oxygen will more readily be released from haemoglobin.
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