- The oxygen saturation is a reflection of the percentage of haemoglobin that is bound to oxygen.
- Most patients: >95%
- Chronically hypoxic patients: 88 - 92%
Measuring Oxygen Saturations
- Oxygen saturations are usually measured by placing a probe on the patient's finger; other sites include the toes, earlobes or forehead. Oxygen saturation probes may be on a trolley or portable.
- The oxygen saturation probe works by emitting light at fixed wavelengths corresponding to oxygenated and deoxygenated haemoglobin. A detector then measures the difference in transmitted light, and this is used to calculate the percentage of oxyhaemoglobin.
Factors that Reduce Accuracy
- Peripheral shutdown
- Nail polish
- Skin hyperpigmentation
- Excessive movement
- Severe hypoxia (<80%)
TroubleshootingIf the pulse oximeter is not producing an accurate measurement, several measures can be attempted:
- Remove any nail polish and clean the area
- Warm the skin
- Attempt at a different site
- Attempt with a different probe or machine
- If unable to measure oxygen saturations and none of these measures are working, consider that the patient may in fact be severely hypoxic - look for cyanosis!
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
- 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
- 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.
- 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.