Question 1

Explain why the oxygen-haemoglobin saturation value derived by a pulse oximeter (SpO2) could be different from the measured arterial value (SaO2)

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College Answer

This question required candidates to identify that the measured arterial value (SaO2) was the gold standard to which the limitations of the pulse oximeter should be compared. A detailed description of the intrinsic and extrinsic factors of potential sources of difference of the SpO2 measurement was then expected. Intrinsic factors included wavelengths used, pulse added absorbance, derivation of the SpO2 value and time delays. Extrinsic factors where largely patient and environment related including light pollution, poor peripheral perfusion for various reasons, probe location variances and probe artefact.

Discussion

This question really seems to be about the sources of error in pulse oximetry, as the stem says "explain why different" rather than "compare and contrast". Trainees who decided to do this in a tabulated format would potentially have squandered some time on listing the limitations of the amperometric ABG electrode that measures oxygen tension, i.e there would have been no point delving into the distant possibility that the pulse oximeter value is right, and the ABG machine is wrong. Thus:

  • The measurement of SaO2 by the ABG machine is the gold standard
  • Pulse oximetry may give a different value as an error
  • Sources of error include:
    • Physical limitations of the measurement technique:
      • Processing: ABG machines lyse RBCs and will therefore have a slightly different saturation measurement because of the change in sample pH (it will be trivially right-shifted)
      • Temperature difference: the oximeter measures the patient's "true" saturation at whatever their temperature happens to be, whereas the ABG oximeter adjusts its results to a blood temperature of 37 degrees, which can lead to a discrepancy (though this also is not an inaccurate reading, strictly speaking)
    • Sources of error in signal measurement
      • Ambient light: as discussed above, ambient light can interfere with oximetry, although it would have to be flickering light of a certain (high) frequency
      • Nail polish
      • Oedema
    • Error due to a failure to detect a pulse
      • Poor signal quality due to poor pulsatility of flow
        (shock, tourniquet, VA ECMO, etc)
      • Choice of probe site
      • Uninterpretable pulsatile signal due to
        • Erratic flailing movements  of the patient
        • Arrhythmia
      • Venous pulsation (eg. severe TR)
    • Interference with absorbance
      • Presence of haemoglobins other than standard vanilla haemoglobin
        • Carboxyhaemoglobin
        • Methaemoglobin
        • Foetal haemoglobin
      • Intravascular presence of dye, eg. methylene blue, indigo carmine,
        indocyanine green and fluorescein
    • Sources of error in signal processing
      • The calibration table does not go down to the lowermost saturation ranges, as this is difficult to measure in a population of volunteers
      • The calibration table is also racially biased: dark skin causes falsely low readings (Feiner et al, 2007)

References

Sinex, James E. "Pulse oximetry: principles and limitations." The American journal of emergency medicine 17.1 (1999): 59-66.

Jubran, Amal. "Pulse oximetry." Critical care 3.2 (1999): R11.

Jubran, Amal. "Pulse oximetry." Applied Physiology in Intensive Care Medicine 1. Springer, Berlin, Heidelberg, 2012. 51-54.

Jubran, Amal. "Pulse oximetry." Critical Care 19.1 (2015): 272.

Kamat, Vijaylakshmi. "Pulse oximetry." Indian J Anaesth 46.4 (2002): 261-8.

Ralston, A. C., R. K. Webb, and W. B. Runciman. "Potential errors in pulse oximetry: I. Pulse oximeter evaluation." Anaesthesia 46.3 (1991): 202-206.

Webb, R. K., A. C. Ralston, and W. B. Runciman. "Potential errors in pulse oximetry: II. Effects of changes in saturation and signal quality.Anaesthesia 46.3 (1991): 207-212.