What is an oximeter
Pulse oximeters are unreliable in dark skinned people. I learnt some worrying news at the 2022 Winter British Thoracic Society meeting . This relates to pulse oximeters that are in common use to assess people with heart or lung diseases. These are the small devices that fit on a fingertip measure the amount of oxygen circulating in the bloodstream. These devices measure the oxygen level by shining a light through the fingernail.
Oxygen is essential for life – how are levels measured in your body
Two methods are generally used to measure oxygen levels in the blood. One is by using a pulse oximeter described above, and the level is summarised as SpO2. The other method is by measuring arterial blood gas by drawing a small ammount of blood from an artery (usually in the arm) and this measurement is depicted as SaO2. The arterial blood gas (SaO2) being the more accurate measurement. So the ‘gold standard’ measurement of blood oxygen levels is by arterial blood gas measurement – however this is an invasive test involving an experienced person taking blood from an artery and then taking it, packed in ice, quickly to a laboratory or a machine in a hospital ward if available to do the measurement. The reason why pulse oximetry has become so popular is that its very convenient and patients as well as health care professionals and assistants can easily do the measurements.
The function of the lungs is to get oxygen from the air into the blood via the lungs, and also to expel (get rid of) carbon dioxide which is harmful if it builds up in the body. Pulse oximetry only measures oxygen levels while arterial blood gas measures both oxygen and also carbon dioxide levels and the latter is therefore more reliable in determining if someone is going into or in respiratory (or lung) failure.
Usually when a person is well, the oxygen readings are between 95 and 100%. When someone is unwell, for example with a lung disease like pneumonia or during an asthma attack, doctors usually get concerned if the readings drop down towards and below 94% and they get very worried if these are below 91%.
Now, Ive always been aware that the pulse oximetry readings (SpO2) can be slightly innacurate and also that the readings may be very inaccurate if someone has nail polish. However, what I knew a little about, but not how serious, was the size of the inaccuracy is in people with dark skins.
Oxygen levels are misleading in people with dark skins
The problem in everyone, irrespective of the colour of their skin, is that the pulse oximetry level is always higher than the true arterial levels. is that the readings may be falsely high in people with dark skin. There has been some research that shows that the difference between the true oxygen level in the blood (SaO2) and the pulse oximetry reading (SpO2) in dark skinned people can be as high as 11.7% compared with those with white skin which can be 3.6%. One study showed that the mean over-estimate of oxygen saturation is 1.4% ( 95% Ci +0.5 to +2.3) in people with non-white skin compared with white skin. What this means in real life is that there is a big risk that doctors may underestimate the severity of oxygen depletion in dark skinned people if they only rely on pulse oximetry.
Possible dangerous implications of reliance on oximeters alone
This has implications for people having asthma attacks and therefore, medical personnel as well as people with asthma need to be taught not to ONLY rely on pulse oximetry levels when deciding on the severity of an asthma attack (or severity of pneumonia or other conditions that result in reduced blood oxygen levels). Other measurements need to be taken into account, for example in the case of asthma, symptoms (cough, wheeze, shortness of breath, or difficulty breathing) respiratory rate, peak expiratory flow (PEF) or FEV1 with a spirometer, as well as pulse rate and blood pressure, And Therefore, if there are signs of worsening asthma despite ‘normal’ SpO2 people with asthma should seek medical assistance.
When assessing a patient with dark skin, be aware that pulse oximetry will overestimate the true blood oxygen level (the arterial oxygen level). Therefore – dont only rely on the pulse oximetry reading when assesing a person having an asthma attack (or a chest infection). Use the clinical history, other vital signs (NB Respiratory rate, pulse, blood pressure), other indictions of respiratory failure (drowsiness, altered breathing, cyanosis) and response to treatment. Also refer early to a specialist and test the arterial oxygen level early in people with dark skin and pulse oximetry readings of 94% and lower.