Oxygen Saturation a Guide to Laboratory Assessment by SHANNON HAYMOND, PHD

CLN’S LAB 2006: SUPPORTING CLINICAL DECISIONS SERIES Oxygen Saturation A Guide to Laboratory Assessment BY SHANNON HAYMOND, PHD uman life depends on the oxygen transport by hemoglobin. In healthy patients, the major- ity of molecular oxygen (O2) is bound to hemoglobin and only a small fraction is dissolved in blood. But in patients with respiratory problems or certain metabolic and genetic disorders, the fraction of oxygenated hemoglobin can fall to dangerously low values. Therefore, laboratory assessment of oxygen saturation (SO2)—the percentage of hemoglobin saturated with oxygen—providesH an important indicator of a patient’s cardio-respiratory status and is frequently used in the emergency department, during general and regional anesthesia, and in intensive care settings. Although the measured parameters are quite different for each, the three major analytical methods for measuring oxygen saturation—arterial blood gas analyzers, pulse oximetry, and CO-oximetry—are frequently used interchangeably by health care workers. Arterial blood gas analyzers calculate estimated oxygen saturation (O2sat) in a blood sample based on empirical equations using pH and PO2 values, while pulse oximeters monitor arterial blood oxygen saturation, commonly referred to as SpO2 or SaO2, noninvasively by passing selected wavelengths of light through an area of the body, such as a finger. Both are measures of oxygen saturation. The Clinical Laboratory Standards In- leading results, so health care professionals mutations, such as the thalessemias, reduce stitute (CLSI) defines O2sat as an estimated need to understand the differences between the quantity of α- or ß-chain synthesis in the value based on a calculation, whereas SaO2 these methods and the limitations of each. hemoglobin subunit or lower the solubility and SpO2 refer to the arterial saturation This article will review the basics of oxygen- of hemoglobin—as occurs with HbS (sickle cell hemoglobin) or HbC (hemoglobin C), for example—but gene mutations rarely alter the O2 affinity of hemoglobin. Hemoglobins can be divided into two classes: normal hemoglobin that is capable of binding O2, and dyshemoglobins—hemoglobin derivatives that are incapable of binding O2. The normal hemoglobins include oxyhemoglobin (O2Hb) and deoxy- hemoglobin (HHb), while the dyshemoglobins include carboxyhemoglobin (COHb), methemoglobin (MetHb), and sulfhemo- globin (SHb). COHb forms when a person is exposed to CO fumes and CO replaces O2 in hemoglobin, which can result in death. SHb forms through a reaction of sulfa-containing compounds with the heme moiety; cases of sulfhemoglobinemia are rare and usually result from extensive use of sulfa-containing drugs. MetHb represents the oxidized, deoxy form (Fe(III)-Hb) of hemoglobin to which O2 cannot bind. The likely etiologies of met- hemoglobinemia include exposure to highly oxidizing drugs or the presence of genetic hemoglobin variants such as HbM. To understand how hemoglobin carries and releases oxygen, the oxygen dissociation curve (ODC) serves as an important tool (Figure 1). In the lungs, where partial pres- measured spectrophotometrically. (To avoid ation of hemoglobin and will provide guid- sures of O2 are high, O2 binds to hemoglo- further confusion, I will use only O2sat ance for laboratorians on how to accurately bin to form O2Hb. Erythrocytes carrying and SO2 for the remainder of this article.) assess oxygenation saturation values. O2Hb then circulate in the blood and release CO-oximetry, a more complex and reliable O2 in response to decreased partial pressures An Oxygen Saturation Primer method, measures the concentration of he- of O2 in the tissues. The cooperativity of O2 moglobin derivatives in the blood, the results Hemoglobin, the O2 transport protein in binding—an allosteric phenomenon where- of which are then used to calculate various blood, is comprised of four subunits: two by binding of oxygen by one hemoglobin parameters such as hemoglobin derivative α subunits and two non-α subunits, for subunit enhances the ability of the remain- fractions (e.g., FO2Hb), total hemoglobin, example ß, γ, or δ. Each subunit contains a ing subunits to bind oxygen—produces the and oxygen saturation. porphyrin heme iron (Fe) moiety and seven sigmoidal shape of the curve. As O2 binds to In most patients, the results—O2sat, SO2, helices. The heme Fe exists in the Fe(II) or the second and third subunits of hemoglo- and FO2Hb—from these three methods are Fe(III) oxidation state, but only the Fe(II) bin, binding increases incrementally so that virtually identical. But in cases of dyshemo- state is capable of binding O2. Most clini- the four subunits of hemoglobin all become globinemia, some methods can yield mis- cally important dyshemoglobinemia gene fully saturated at the normal O2 tension in 10 CLINICAL LABORATORY NEWS FEBRUARY 2006 Figure 1 Figure 2 Oxygen Dissociation Curve Extinction Curves of Purified Of Hemoglobin Hemoglobin Derivatives OF OHMEDA COURTESY 10 100 R IR t n e i 1 MetHb 80 c i f f e o O Hb C 2 n n o i t o HHb c i 60 n t i .1 t a x E r u t a S 40 COHb .01 % 600 640 680 720 760 800 840 880 920 960 1000 Wavelength (nm) 20 The graph shows the light absorption relationship of HHb, O2Hb , COHb, and MetHb. Vertical lines indicate the red (R) and infrared (IR) monitor- P50 ing wavelengths used in most pulse oximeters, at 660 nm and 940 nm, respectively. Modified from Journal of Clinical Monitoring, 4(4), 1988, p. 0 292. Pulse Oximetry: Analysis of Theory, Technology, and Practice. M.W. 0 20 40 60 80 100 Wukitsch et al. With permission from Kluwer Academic Publishers. Oxygen tension (mmHg) The percent saturation of hemoglobin with oxygen at different oxygen SO2, expressed as a percentage, is typically tion, the altered molecular structure of the 94%–98%. Blood gas analyzers report an heme moiety in the various hemoglobin tensions is depicted by the sigmoidal curves. The P50, indicated by the dashed lines, is about 27 mm Hg in normal erythrocytes. Modifications of estimated saturation, O2sat, which is based derivatives gives rise to unique absorption hemoglobin function that increase oxygen affinity shift the curve to the on measurement of pH, PO2, and hemoglo- spectra, making it possible to determine the left, whereas those that decrease oxygen affinity shift the curve to the bin values and utilization of empirical equa- concentrations of each derivative present in right. Reprinted with permission from Kelley’s Textbook of Internal Medi- tions. blood. (Figure 2) cine, 4th ed., 2000, figure 241.2. Lippincott Williams & Wilkins. Fractional oxyhemoglobin. Only instru- Pulse oximetry. Pulse oximeters assess arte- ments with a multi-wavelength spectro- rial oxygen saturation (SO2) by measuring photometer, such as CO-oximeters or some light transmission through a well-perfused lung alveoli. The same process works in re- temperature and 2,3-DPG concentration, modern blood gas analyzers, are capable of area of the body such as a finger or an ear verse in the tissues; once fully loaded hemo- or decreases in pH, can shift the ODC to the measuring FO2Hb. The value, as calculated lobe. Light sources, typically light-emit- globin releases one O2 molecule, it releases right, increasing the P50 and indicating de- from the formula below, represents the frac- ting diodes, shine two wavelengths of light the next more easily. creased O2 affinity. tion of oxyhemoglobin in relation to the to- through the tissue —visible red (R) light The ODC also explains what happens to tal hemoglobin present, including the non- (600 to 750 nm) and infrared (IR) light (850 patients when oxygen cannot bind hemoglo- Definitions of Oxygenation Status oxygen-binding hemoglobins. to 1000 nm). Deoxygenated hemoglobin bin. For example, in the presence of CO or CLSI has defined the three key terms used Fractional oxyhemoglobin allows more infrared light to pass through when the heme Fe is oxidized to the Fe(III) to describe oxygenation status: oxygen sat- FO2Hb = cO2Hb and absorbs more red light than oxygen- state, O2 cannot bind one of the hemoglo- uration (SO2), fractional oxyhemoglobin ctHb ated hemoglobin, while highly oxygenated bin subunits. These conditions decrease O2 (FO2Hb), and estimated oxygen saturation FO2Hb is usually expressed as a percent- hemoglobin allows more red light to pass capacity in addition to inhibiting O2 trans- (O2sat). The Institute recommends the use age and typically ranges from 90%–95% in through and absorbs more infrared light. port by blood. In addition, CO and oxidized of the term “oxygen saturation” to indi- healthy individuals. Reporting FO2Hb alone After calculating the absorption at the two heme Fe alter hemoglobin’s conformation cate the amount of hemoglobin capable of is of limited value because the dyshemoglo- wavelengths, the instrument uses a calibra- in a way that decreases the O2 affinity for transporting O2 and “fractional O2Hb” to bin fractions also play an important role in tion curve programmed into the device to the remaining heme Fe groups, shifting the represent the fraction of hemoglobin that the analysis of oxygen carrying capacity. The compute the SO2. ODC to the right. The net biological effect is oxygenated. The terms “fractional satura- fraction of any of the hemoglobin deriva- For example, on most pulse oximeters is decreased O2 delivery to tissues. Tem- tion” and “functional saturation” refer to the tives may be calculated in the same manner an absorbance ratio (AR/AIR) of 0.43 cor- perature, pH, and 2,3-diphosphoglycerate FO2Hb and SO2, respectively. as that used for oxyhemoglobin. responds to 100% oxygen saturation, and a (2,3-DPG) concentration also affect the O2 Oxygen saturation. The following empiri- As demonstrated by the above equations, ratio of 3.4 corresponds to 0% oxygen satu- affinity of hemoglobin, shifting the ODC as cal equations are used to determine oxygen SO2 and FO2Hb are not equivalent terms.

Load more