Canonical Question
Resp – Oximetry
Master answer
Pulse oximeters assumes that only oxy-Hb and deoxy-Hb are present in the blood. It cannot differentiate other species of haemoglobin such as carboxyhaemoglobin (carboxy-Hb) or methaemoglobin (met-Hb) because it only uses two wavelengths of light (660 nm and 940 nm).
Co-oximeter:
- A co-oximeter is a blood gas analyzer that, in addition to the status of gas tensions provided by traditional blood gas measurements, 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
- Use of co-oximetry is indicated:
- when a history is consistent with toxin exposure
- hypoxia fails to improve with the administration of oxygen
- there is a discrepancy between the Pao2 on a blood gas determination and the oxygen saturation on pulse oximetry (Spo2)
- clinician suspects other dyshemoglobinemias such as methemoglobinemia or carboxyhemoglobinemia.
- Different haemoglobin species have different absorption spectra.
Mechanism / Measurement
- 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)
- Complex, but straightforward internal computations
- enables the instrument to distinguish between oxy-Hb, deoxy-Hb, and carboxyhemoglobin,-COHb, methemoglobin -metHb, other hemoglobin moieties and ‘background’ light-absorbing species
- This is reported as the fractional oxyhaemoglobin content (FO2Hb), which is defined as:
\[ FO_2Hb \; = \; {{oxy-Hb} \over {oxy-Hb \; + \; deoxy-Hb \; + \; carboxy-Hb \; + \; met-Hb}} \]
- The FO2Hb measure provides a more accurate picture of the availability of oxygen to the tissues in the presence of haemoglobin variants.
- If the original blood sample contained no carboxy-Hb or met-Hb, then the values for FO2Hb and SaO2 would be identical.
Advantages:
- Accurate measure of oxygen saturation: low readings indicate true hypoxia, and high readings always represent true hypoxia
- Able to detect different Hb species, like deoxy-Hb, carboxy-Hb, met-Hb
- Not confused by ambient light, absence of pulsatile flow, tricuspid regurgitation, methylene blue dye.
Limitations of co-oximetry:
- Expensive (depending on device
- Invasive (Blood sample needed)
- Variations with different devices, especially due to Limitations on Quality control
- Possible false values with severe hypoxemia, and extremes of Hb concentrations (better with newer generation devices)
- Not available in all blood gas analyzers
- Continuous monitoring not commercially availabe yet
Limitations of pulse oximetry:
| 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 | ||
JC 2020
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2018A Q08 | Outline the principle of co-oximetry (40% of marks), describe what a co-oximeter is able to measure (30% of marks), and compare its limitations to those of a pulse oximeter (30% of marks). | historical_member | — |