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Physiologic monitors designed primarily for automated continuous measuring and display in real time the amount of oxygen that is attached to hemoglobin found in the peripheral circulation (i.e., tissue oxygen saturation [StO2] in subcutaneous tissue). Light at several frequencies, typically in the near infrared spectrum, is transmitted through the skin into the subcutaneous tissue and the value of the absorbed light is determined based on the measurement of the reflected light. These monitors typically consist of portable electronic units that facilitate movement from one location to other; the monitor may be connected to the line and/or powered by internal batteries. The main unit typically includes controls and a display; it also includes an attached probe that is fixed to the skin and includes the light sources and a light detector to measure the scattered light reflected to the skin surface. The monitor software includes algorithms to calculate the absorbed light at several different frequencies and give as a result the oxygen status of the hemoglobin in the tissues. Tissue oximetry monitors are used mainly for noninvasive identification and tracking of the hemodynamic conditions in critically ill patients.
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