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Pulmonary artery oximetry represents a combination of the technology of reflection spectrophotometry with that of pulmonary artery catheterization. Pulmonary oximeter catheters, first developed in the early 1980's, employ fiberoptic bundles to transmit light at two or three discrete wavelengths to and from the tip of the catheter. The amount of light reflected from red cells surrounding the catheter tip is a function of the hemoglobin saturation. A microprocessor in the oximeter compares reflected light intensities at the different wavelengths and calculates mixed venous hemoglobin saturation. Some of the sources of error in this calculation will be discussed. Mixed venous hemoglobin saturation is a physiological variable which reflects the oxygen supply versus demand balance of the body. By solving the Fick equation for mixed venous saturation (Sv02), we see that it is a function of arterial saturation (5a02), oxygen consumption (V02), hemoglobin (Hb), and cardiac output (C.O.): Sv02 = Sa02 - 100(V02)/(13.8 x Hb x C.O.) Whenever Sv02 changes, the clinician must decide on the basis of other evidence which of these variables is responsible for the change. Processes that will decrease Sv02 include hypoxemia, anemia, and decreased cardiac output. Sv02 will also decrease following any increase in oxygen consumption (V02), which may result from fever, malignant hyperthermia, thyroid storm, exercise or agitation, and shivering. Increases in Sv02 above the normal range of 68% to 77% result from decreased oxygen uptake in relation to cardiac output, for example: sepsis, Paget's disease of bone, cyanide poisoning, hypothermia, or excessive use of inotropes. Continuous Sv02 monitoring can therefore provide an early warning of many pathologic processes, but it is very nonspecific. The physician must use his clinical expertise in conjunction with the other available data to determine the cause of the change in Sv02 values. Sv02 is a global oxygen parameter; that is, it may not detect changes in the oxygen supply-demand balance of specific organs or regions of the body.
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