StO2: Tissue Oxygen Saturation
StO2 is the quantification of the ratio of oxygenated hemoglobin to total hemoglobin in the microcirculation of a volume of illuminated tissue and is an absolute number. The measurement of StO2 is taken with a noninvasive, fiber optic light that illuminates tissues below a sensor placed on the skin. StO2 correlates well with other accepted means of measuring oxygen saturation and the results of these studies demonstrate that StO2 is a valid measure of hemoglobin oxygen saturation.
How StO2 Differs from Other Measures of Oxygenation
Previously, there were two basic kinds of oxygenation measurements—hemoglobin oxygen saturation in the blood and partial pressure of oxygen. Hemoglobin oxygen saturation in the blood (SO2, SaO2, SpO2), expressed as a percent, is the oxygen present on the hemoglobin in circulating blood divided by the total possible oxygen that could be carried by the hemoglobin. Transcutaneous pO2 measures the partial pressure of oxygen in the skin only.
How StO2 Differs from SpO2
StO2 measurements differ from the SpO2 measurements provided by pulse oximetry. StO2 is a measure of oxygen saturation in the microcirculation where oxygen is exchanged with tissue and is therefore a local measure. Pulse oximetry, which also uses near infrared light, measures the systemic oxygen saturation of arterial blood.
Measurements of StO2 will therefore change with changes in the local conditions of supply and consumption in the tissue, and SpO2 will not. Because no oxygen is exchanged between the thick walls of arteries and the tissue, SpO2 is fairly constant regardless of whether the measurement site is the earlobe, finger, or big toe. In addition, SpO2 requires a pulsatile flow, while StO2 readings do not.
How StO2 Differs from Transcutaneous pO2 (TcpO2)
NIRS can be used to measure oxygenation at various depths of tissue—skin, subcutaneous tissue, and muscle. Transcutaneous pO2 measures the partial pressure of oxygen in the skin only.