Laboratory assessment of oxygenation in methemoglobinemia.
نویسندگان
چکیده
BACKGROUND This case conference reviews laboratory methods for assessing oxygenation status: arterial blood gases, pulse oximetry, and CO-oximetry. Caveats of these measurements are discussed in the context of two methemoglobinemia cases. CASES Case 1 is a woman who presented with increased shortness of breath, productive cough, chest pain, nausea, fever, and cyanosis. CO-oximetry indicated a carboxyhemoglobin (COHb) fraction of 24.9%. She was unresponsive to O(2) therapy, and no source of carbon monoxide could be noted. Case 2 is a man who presented with syncope, chest tightness, and signs of cyanosis. His arterial blood was dark brown, and CO-oximetry showed a methemoglobin (MetHb) fraction of 23%. ISSUES Oxygen saturation (So(2)) can be measured by three approaches that are often used interchangeably, although the measured systems are quite different. Pulse oximetry is a noninvasive, spectrophotometric method to determine arterial oxygen saturation (S(a)O(2)). CO-oximetry is a more complex and reliable method that measures the concentration of hemoglobin derivatives in the blood from which various quantities such as hemoglobin derivative fractions, total hemoglobin, and saturation are calculated. Blood gas instruments calculate the estimated O(2) saturation from empirical equations using pH and Po(2) values. In most patients, the results from these methods will be virtually identical, but in cases of increased dyshemoglobin fractions, including methemoglobinemia, it is crucial that the distinctions and limitations of these methods be understood. CONCLUSIONS So(2) calculated from pH and Po(2) should be interpreted with caution as the algorithms used assume normal O(2) affinity, normal 2,3-diphosphoglycerate concentrations, and no dyshemoglobins or hemoglobinopathies. CO-oximeter reports should include the dyshemoglobin fractions in addition to the oxyhemoglobin fraction. In cases of increased MetHb fraction, pulse oximeter values trend toward 85%, underestimating the actual oxygen saturation. Hemoglobin M variants may yield normal MetHb and increased COHb or sulfhemoglobin fractions measured by CO-oximetry.
منابع مشابه
An unusual electrophoresis.
1. Percy MJ, Lappin TR. Recessive congenital methaemoglobinaemia: cytochrome b(5) reductase deficiency. Br J Haematol 2008;141:298–308. 2. Pallais JC, Mackool BT, Bishop Pitman M. Case 7-2011: a 52-year-old man with upper respiratory symptoms and low oxygen saturation levels. N Engl J Med 2011;364:957–66. 3. Haymond S, Cariappa R, Eby CS, Scott MG. Laboratory assessment of oxygenation in methem...
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ورودعنوان ژورنال:
- Clinical chemistry
دوره 51 2 شماره
صفحات -
تاریخ انتشار 2005