Pulse oximetry in Children - consider variant haemoglobin.
We read with interest the recent review by Sinha et al regarding physiological background, technological basis and limitations of pulse oximetry. The factors listed by the authors that may affect the accuracy of pulse oximetry include motion artifact, inadequate perfusion, nail polish, and high-ambient infrared light.
We would like to add to that list structural variants of haemoglobin. Over 1,000 variant haemoglobins have been described , and while the majority are not associated with abnormal SpO2 readings, a reduced SpO2 may in some instances be the main finding associated with variant haemoglobin. Variant haemoglobins with low SpO2 as measured by pulse oximetry may be associated with either reduced SaO2 or normal SaO2, as measured by arterial blood gas analysis, as reviewed by Verhovsek et al .
The finding of an unexplained reduced SpO2 can give rise to extensive cardiopulmonary investigations. Diagnosis of variant hemoglobin should be considered early on in the investigation of patients who are found to have unexpectedly low oxygen saturation but do not have clinical evidence of cardiopulmonary disease. Arterial blood gas analysis (which may in some cases of variant haemoglobin show a normal SaO2) or the simple expedient of carrying out pulse oximetry on parents (as haemoglobin variants are autosomally transmitted) may direct investigations towards a haemoglobin variant and spare the patient unnecessary cardiopulmonary investigations. Furthermore, with the increasing recommendations for use of pulse oximetry as a screening tool for detecting congenital heart disease , it is worth remembering haemoglobin variants as potential cause of unexplained low SpO2.
Conflict of Interest: