Limitation of pulse oximetry - a case report

Pulse oximetry provides a simple non-invasive method of monitoring the oxygenation status of patients by indirectly measuring the percentage of hemoglobin that is saturated with oxygen, i.e. % oxygen saturation (SpO₂). Pulse oximetry has important limitations, including inaccurate readings in patients with inherited abnormality in hemoglobin structure (hemoglobinopathy). 

 This limitation of pulse oximetry is highlighted by a recently published case history report of a 62-year-old man scheduled for elective surgery. Routine preoperative monitoring revealed an unexpectedly low SpO₂ of 90 %, despite absence of any symptoms or signs of cardiac or respiratory disease. 

Arterial blood gas analysis provided evidence of normal oxygenation status, including normal arterial oxygen saturation, sO₂(aB) (99 %) and normal arterial oxygen tension, pO₂(aB) (11.8 kPa). It eventually transpired that the pulse-oximeter reading was falsely low because the patient had an undiagnosed hemoglobinopathy. 

Mass spectrometry analysis of the patient’s hemoglobin revealed that 7 % of alpha chains were abnormal, the precise abnormality being a single amino-acid substitution, valine for alanine, at position 62. Interestingly, this particular abnormality in structure has not been described and so constitutes a novel hemoglobinopathy, once confirmed. 

In discussion of this case history, the authors provide a useful short review of the reason for the association between hemoglobinopathies and erroneous pulse-oximeter readings.