Canonical Question
Resp – Oximetry
Master answer
Methods of Measuring oxygen saturation
Co-oximetry
- Blood gas analyzers measure arterial SaO2 by co-oximetry. (gold standard of SaO2 measurement)
- measures concentrations of oxygenated hemoglobin (oxyHb), deoxygenated hemoglobin (deoxyHb or reduced Hb), carboxyhemoglobin (COHb), and methemoglobin (MetHb) as a percentage of the total hemoglobin concentration in the blood sample
- Accurate measure of oxygen saturation: low readings indicate true hypoxia, and high readings always represent true hypoxia
- Not confused by ambient light, absence of pulsatile flow, tricuspid regurgitation, methylene blue dye.
- Uses Beer-Lambert law to detect different Hb species.
- “Incidence of light is inversely proportional to the path distance and concentration of light absorbing particles within the path”
- multi-wavelength spectrophotometry (measures the absorption of light passing through blood from several dozens of wavelengths)
Pulse oximetry
- Light emitter
- Produces infra-red light at 660nm and 950nm
- Light detector
- Detects light at 660nm and 950nm
- Processor
- Calculates SpO2 using the ratio of absorbance at 660nm and 950nm
- Light detected (incidence) represents light passing through both pulsatile (arterial) blood and non-pulsatile elements (venous blood and other tissues)
- Processor differentiates light incidence during maxima and minima of pulse wave and calculates SpO2 from pulsatile blood only giving SaO2
SpO2 different from SaO2
| PATIENT FACTORS | ||
| Low or High SpO2 | Low SpO2 | Normal or High SpO2 |
| Met-Hb Sulph-Hb | Poor perfusion of finger Movement artifact Venous pulsations Fingernail polish Intravenous pigmented dyes Haemoglobinopathy Anaemia with co-existing hypoxia | Carbon Monoxide poisoning |
| EQUIPMENT FACTORS | ||
| – Ambient light interference – Poorly fitting probe – Assay calibrated using healthy volunteers only down to SpO2 80%. Unknown significance if tested SpO2 less than 80% | ||
| PHYSIOLOGICAL FACTORS | ||
| – Due to O2 dissociation curve, insensitive to changes above PaO2 80mmHg – Does not measure tissue oxygenation | ||
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2023B Q01 | Explain why the oxygen-haemoglobin saturation value derived by a pulse oximeter (SpO2) could be different from the measured arterial value (SaO2). | historical_member | — |