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- Elizabeth Mack, MD*
- *Dr Mack wrote this article when she was a third-year resident at Palmetto Richland Hospital in Columbia, SC. She is now a pediatric critical care fellow at Cincinnati Children’s Hospital, Cincinnati, Ohio
Introduction
A co-oximeter is a blood gas analyzer that, in addition to the status of gas tensions provided by traditional blood gas measurements, measures concentrations of oxygenated hemoglobin (oxyHb), deoxygenated hemoglobin (deoxyHb or reduced Hb), carboxyhemoglobin (COHb), and methemoglobin (MetHb) as a percentage of the total hemoglobin concentration in the blood sample. Use of co-oximetry is indicated when a history is consistent with toxin exposure, hypoxia fails to improve with the administration of oxygen, there is a discrepancy between the Pao2 on a blood gas determination and the oxygen saturation on pulse oximetry (Spo2), or the clinician suspects other dyshemoglobinemias such as methemoglobinemia or carboxyhemoglobinemia.
Pulse oximetry measures the oxygen saturation (Sao2) of hemoglobin in arterial blood or the average amount of oxygen bound to each hemoglobin molecule. Blood gas analyzers calculate oxygen saturation from the measured parameters Po2 and pH on the basis of standard oxygen-dissociation curves. Unfortunately, pulse oximetry, a noninvasive procedure, does not distinguish among the different types of hemoglobins. For example, in the case of methemoglobinemia, pulse oximetry may read 88%, but desaturation can be demonstrated with co-oximetry, recording 70% oxyHb and 30% MetHb.
Each of the dyshemoglobins has a unique absorption spectrum, and the concentration can be derived from the Beer-Lambert law by measuring absorption at …
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