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>Functional and Fractional Oxygen Saturation-A Technical Overview
Summary
Smiths Medical PM, Inc’s pulse oximeters measure functional oxygen saturation. Functional oxygen saturation refers to the ratio of oxyhemoglobin to all hemoglobin that is capable of carrying oxygen. Laboratory co-oximeters commonly are capable of measuring fractional oxygen saturation. Fractional saturation refers to the ration of oxyhemoglobin to all hemoglobin measured, including dyshemoglobins. However, a laboratory co-oximeter requires a sample of whole blood and is not non-invasive like a pulse oximeter. When dyshemoglobins are present, it can be expected that the saturation values obtained from a co-oximeter and a pulse oximeter may be different.
Types of Hemoglobin
Some basic understanding of the types of hemoglobin is necessary to fully comprehend the difference between functional and fractional saturation. Hemoglobin (Hb) is responsible for transporting 97-98% of the body’s oxygen to the tissues for cellular metabolism. Hemoglobin is referred to as functional or dysfunctional. Functional Hb is capable of transporting oxygen. Hb that are capable of, and are indeed transporting oxygen are called oxyhemoglobin. Hb that we capable of, but are not actually transporting oxygen are called deoxyhemoglobin. Dysfunctional Hb, or otherwise referred to as dyshemoglobin, are not able to carry oxygen. Dyshemoglobin occurs when the Hb molecule is bound by other than oxygen, or it is otherwise impaired. Dyshemoglobins caused by carbon monoxide are called carboxyhemoglobin (COHb). Dyshemoglobins caused by the oxidation of the iron in Hb are called methemoglobin (MetHb).
Types of Saturation
Functional saturation is the ratio of oxyhemoglobin to all other functional hemoglobin. Fractional saturation is the ratio of oxyhemoglobin to all hemoglobin measured, even dyshemoglobin. Functional saturation can be obtained non-invasively from a pulse oximeter or invasively from an arterial blood gas sample. Fractional saturation can be obtained by a co-oximeter with an invasive blood sample. Functional saturation measurements provide clinicians with fast, continuous and non-invasive means of monitoring oxygenation. When abnormal amounts of dyshemoglobins are suspected, a fractional saturation measurement is necessary.
Functional vs. Fractional Technology
Pulse oximeters in general measure functional saturation by transmitting two wavelengths of light that are so differently absorbed by oxyhemoglobin and deoxyhemoglobin, commonly referred to as SpO2. An aterial blood gas sample is an invasive procedure that indirectly calculates functional saturation from various other parameters, commonly referred to as ScO2. A co-oximeter utilized four wavelengths of light to measure fractional saturation (SaO2). These four wavelengths of light differentiate between oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin and methemoglobin. Because of the differences in these two technologies, it can be expected that the results may be different when significant amounts of dyshemoglobin are present.
Example:
A person has 15 grams of Hb per 100 milliliters of blood
11 grams are oxyhemoglobin
2 grams are deoxyhemoglobin
1 gram is carboxyhemoglobin
1 gram is methemoglobin
1. Functional Saturation: Pulse Oximetery
(Oxyhemoglobin) 11g divided by (Oxyhemoglobin + Deoxyhemoglobin) 11g + 2g
11 divided by 11+2 = 85% SpO2
2. Fractional Saturation: Co-Oximetry
(Oxyhemoglobin) 11g divided by (Oxyhemoglobin + Deoxyhemoglobin + Carboxyhemoglobin + Methemoglobin) 11g + 2g + 1g + 1g
11 divided by 11 + 2 + 1 + 1 = 73% SaO2
Information provided by Smiths Medical BCI at
http://www.smiths-medical.com/Upload/products/product_relateddocs/How%20can%20SpO2%20readings%20differ.pdf |
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