A 23 year old female presents with snoring, unrefreshing sleep, and somnolence. Body mass index is 29kg/m2. Examination is unremarkable. She has a history of coeliac disease and unstable haemoglobin Köln (HbKöln).
Overnight respiratory polygraphy shows mean saturations of 88.6% and a saturation time <90% of 89.4% (figure 1).
What could be the cause of her low oxygen saturations?
Investigations Unremarkable chest radiograph, spirometry, and gas transfer
Capillary blood gas pH 7.44, pCO2 4.76 KPa, pO2 13.5 KPa, Hb 133, sO2 97.2%, FO2Hb 94%, FCOHb 1.6%, FMetHb 1.7%, sO2 96.7%,BE 0.3, HCO3 24.9. Simultaneous pulse oximeter: sO2 89%.
Oximeters compare absorption spectra of oxyhaemoglobin and deoxyhaemoglobin in red (660nm) and infra-red (940nm) light to estimate oxygen saturation (figure 2). HbKöln is an autosomal dominant haemoglobinopathy with absorption spectra shifted upwards in the range of 600–900nm,1 resulting in reduced saturations measured by oximetry compared to the blood gas.
Key learning points
Oximeters are imperfect They can be inaccurate in carbon monoxide poisoning, dyshemoglobinemia, dark skinned patients, low saturation ranges, in patients with nail varnish and other situations.2
Clinical context, and family history, are key. Modern blood gas analysers use co-oximetry to measure absorption at >100 wavelengths to accurately distinguish ‘normal’ haemoglobin from other species.
Verhovsek M, Henderson MPA, Cox G, Luo H-Y, Steinberg MH, Chui DHK. Unexpectedly low pulse oximetry measurements associated with variant hemoglobins: a systematic review. Am. J. Hematol. 2010;85:882–885.
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