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Understanding the principles of pulse oximetry can reduce errors

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VOL: 100, ISSUE: 47, PAGE NO: 44

Lucy Knox, is a nursing student, University of Leeds

One of the tasks that often fell to me as a student on my first placement was to check the observations of patients...

One of the tasks that often fell to me as a student on my first placement was to check the observations of patients on the respiratory ward that I was on. From day one I was under the impression that oxygen saturations were one of the 'easy' readings to take. Reading the article on the principles and limitations of oximetry by Allen (2004) made me realise that the monitoring of oximetry is a much more complex issue, which requires consideration of a range of factors if it is to be used successfully and false readings avoided.

I was interested to read that pulse oximetry does not measure the oxygen saturation of the blood, as I previously thought, but measures the saturation of haemoglobin in arterial blood. Allen explains that any reading on an oximeter has to be considered in relation to the patient's levels of haemoglobin. There is also the possibility the haemoglobin is saturated with carbon monoxide rather than oxygen, resulting in critically low oxygen levels yet an oximeter reading within normal limits.

The article reinforced the importance of considering other factors such as respiratory rate, recent events (such as exposure to carbon monoxide), blood supply to the area the probe is situated at, cleanliness of the probe and cleanliness of the finger (nicotine staining or nail varnish can cause false readings). Allen also highlighted the risk of pressure ulcers from oximeters and pointed to advice from the Medical Devices Agency (2001) that the position of the probe be changed at least two-hourly.

This new knowledge helped me make more sense of an incident on the respiratory ward that had left me feeling confused. I had been asked to feed a patient with chronic obstructive pulmonary disease who was being treated with non-invasive positive pressure ventilation (NIPPV) and had been told to put the patient's NIPPV mask back on if the reading on her oximeter fell to less than 80 per cent. The patient's saturations remained relatively high, but she seemed sleepy and pale. I informed the staff nurse and we put the NIPPV mask back on. Arterial blood gases were taken and she was found to have very low oxygen levels. I was confused as to how this could be the case when her oximeter readings had remained high but the article has helped me to understand that while saturation of haemoglobin may have been high this does not necessarily reflect oxygen levels. The article clarified that indicators such as drowsiness, cyanosis and dyspnoea can help to determine whether treatment is effective and readings reliable.

I aim to follow up this reflection by examining rationales for this practice through speaking to my mentors and other staff and through further reading. The article made it apparent that a lack of knowledge of good practice with regard to oximetry can have serious consequences for patients. I intend to make pulse oximetry the topic of a presentation to my fellow nursing students, and hope to raise their awareness of the principles of good practice in this area.

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