Pulse oximetry and genetic hemoglobinopathies

An unusual presentation of chronic cyanide toxicity from self-prescribed apricot kernel extract

Hypoxia under general anaesthesia is a potentially life-threatening condition. A seemingly well 67-year-old man appeared hypoxic with peripheral pulse oximetric measurement during routine anaesthesia. Postoperatively, the patient admitted to daily self-prescription of apricot kernel extract for a period of 5 years. Apricot kernel is a commonly taken extract used for a range of ailments, and is associated with cyanide toxicity, which was confirmed through blood analysis. Our explanation for the hypoxic measurement was the presence of free cyanide interfering with functioning of the peripheral pulse oximeter. On cessation of the apricot kernel extract, peripheral oxygen saturations returned to normal. Cardiac and respiratory causes together with rare haemoglobinopathies were excluded. This case illustrates how chronic dosing of complementary medicines can result in harmful toxicities, which may carry potential for serious consequences and how these chronic toxicities may present to physicians in atypical ways.

Diagnosis and clinical presentation of hemoglobin Kirksey

More than 1000 hemoglobin (Hb) variants have been identified. Hb Kirksey, a rare Hb variant involving mutation at codon 94 of α2-globin, is associated with low oxygen affinity. To our knowledge, there is no case report on Hb Kirksey in the literature, and, accordingly, the clinical features of patients with Hb Kirksey are currently unknown. We report here the case of a neonate who presented with asymptomatic low oxygen saturation on pulse oximetry (SpO2 ) just after birth, together with clinical analysis of the sister and cousin. Based on the presentations, Hb Kirksey does not seem to be associated with clinical abnormalities. Discrepancy between SpO2 and partial pressure of arterial oxygen (PaO2 ) detected on arterial blood gas analysis provided a clue to the diagnosis. Hb variants should be suspected and arterial blood gases should be measured in pediatric cases of unexplained low SpO2 .

Pulse oximetry: understanding its basic principles facilitates appreciation of its limitations

Pulse oximetry has revolutionized the ability to monitor oxygenation in a continuous, accurate, and non-invasive fashion. Despite its ubiquitous use, it is our impression and supported by studies that many providers do not know the basic principles behind its mechanism of function. This knowledge is important because it provides the conceptual basis of appreciating its limitations and recognizing when pulse oximeter readings may be erroneous. In this review, we discuss how pulse oximeters are able to distinguish oxygenated hemoglobin from deoxygenated hemoglobin and how they are able to recognize oxygen saturation only from the arterial compartment of blood. Based on these principles, we discuss the various conditions that can cause spurious readings and the mechanisms underlying them.

Unexpectedly low pulse oximetry measurements associated with variant hemoglobins: a systematic review

Pulse oximetry estimates arterial blood oxygen saturation based on light absorbance of oxy- and deoxy-hemoglobin at 660 and 940 nm wavelengths. Patients with unexpectedly low SpO₂ often undergo cardio-pulmonary testing to ascertain the cause of their hypoxemia. However, in a subset of patients, a variant hemoglobin is responsible for low SpO₂ measurements. The extent of this problem is unclear. We performed a systematic literature review for reports of low SpO₂ associated with variant hemoglobins. We also reviewed unpublished cases from an academic hemoglobin diagnostic reference laboratory. Twenty-five publications and four unpublished cases were identified, representing 45 patients with low SpO₂ and confirmed variant hemoglobin. Fifty-seven family members of patients had confirmed or suspected variant hemoglobin. Three low oxygen affinity variant hemoglobins had concordantly low SpO₂ and SaO₂. Eleven variant hemoglobins were associated with unexpectedly low SpO₂ measurements but normal SaO₂. Hemoglobin light absorbance testing was reported in three cases, all of which showed abnormal absorption spectra between 600 and 900 nm. Seven other variant hemoglobins had decreased SpO₂, with unreported or uncertain SaO₂. Twenty-one variant hemoglobins were found to be associated with low SpO₂. Most variant hemoglobins were associated with spuriously low SpO₂. Abnormal absorption spectra explain the discrepancy between SpO₂ and SaO(2) for some variants. The differential diagnosis of possible variant hemoglobin ought to be considered in asymptomatic patients found to have unexpectedly low SpO₂. The correct diagnosis will help to spare patients from unnecessary investigations and anxiety.