Dear Editor,
I read with interest the article by Matthews (2018) detailing the function and use of pulse oximetry technology. I would question Matthews' assertion that the Milliken Oximeter in 1940, was the first true oximeter. While Milliken is probably the father of clinical oximetry, it must be acknowledged that his work owed a lot to the work of a number of German scientists in the 1930s who demonstrated colour change of haemoglobin when oxygenated or deoxygenated, and constructed the first device which applied two wavelengths of light to the earlobe to demonstrate the oxygen saturation levels of a human subject (Severinghaus and Astrup, 1986).
I would also like to expand slightly on Matthews' work with some brief points which may further the understanding of paramedics in relation to the physics behind the pulse oximeter and help them anticipate causes of spurious readings in their clinical practice.
Laws of Light
Firstly, it is useful to recognise that the functionality of pulse oximeter technology (and indeed many arterial blood gas analysers and other laboratory equipment) relies upon the Beer-Lambert Law, which states that; The absorbance of light/radiation is proportional to the concentration (or density) of the tissue through which it passes, and to the distance the light/radiation must travel through.
Achieved Accuracy
It is also important to highlight limitations in pulse oximeter accuracy, as Matthews touched upon, specifically due to the ethical limitations of calibrating oximeters and gaining reference ranges during severe hypoxeamia. Due to a lack of calibration data during severe hypoxaemia, clinical pulse oximeters are only calibrated to be accurate between saturation readings of 70–100% (+/- 2%). Below 70%, the accuracy is not guaranteed and therefore the figure produced cannot be relied on.
Burns and Bruises
In relation to the dangers and limitations associated with pulse oximetry, it is important to note that there have been reports of pulse oximeters causing both burns and pressure damage due to prolonged use (Wille et al, 2000). While this is unlikely to occur during a short hospital transfer, for paramedics working in a more critical care role, this is certainly an important consideration.
The Penumbra Problem
It is also worth highlighting the Penumbra effect as a source of error (Kelleher and Ruff, 1989). This is a source of error well known to paramedics, but probably not by this name. It occurs when the probe is not symmetrically placed. This results in the distance between the two LEDs and photoreceptors being unequal leading to one wavelength being ‘overloaded’. Repositioning of the probe should produce a rapid resolution and return to accurate readings. It is important to note that the waveform may appear normal when the penumbra effect is causing significant error.
Presented here are some further issues related to the use of pulse oximetry in clinical practice, but these and the excellent article by Matthews (2018) are not exhaustive. Further sources of reading are available in various texts and also an excellent article by Chan et al (2013).
Yours Sincerely,
Dr T Mallinson
