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Near Infrared Spectroscopy (NIRS)
Near infrared spectroscopy (NIRS) uses specific, calibrated wavelengths of near infrared light to noninvasively illuminate the tissue below a sensor placed on the skin. These wavelengths of light scatter in the tissue and are absorbed differently dependent on the amount of oxygen attached to hemoglobin in the microcirculation.  Light that is not absorbed is returned as an optical signal and analyzed to produce a ratio of oxygenated hemoglobin to total hemoglobin, expressed as percent StO2.
In practice, near infrared light penetrates tissues such as skin, bone, muscle and soft tissue where it is absorbed by chromophores (hemoglobin and myoglobin) that have absorption wavelengths in the near infrared region (approximately 700-1000nm). These chromophores vary in their absorbance of NIRS light, depending on changes in oxygenation. Complex algorithms built into the InSpectra™ StO2 Tissue Oxygenation Monitor differentiate the absorbance contribution of the individual chromophores.
Hutchinson Technology's InSpectra StO2 Tissue Oxygenation Monitor uniquely measures hemoglobin oxygen saturation in the microcirculation, where oxygen is exchanged with tissue.

The depth of tissue measured by the InSpectra StO2 Tissue
Oxygenation Monitor is directly related to the distance between the illumination fibers and detection fibers: studies have shown that with 15mm sensor spacing, approximately 95% of the detected optical signal is from a depth of

The InSpectra™ StO2 Tissue Oxygenation Monitor provides a direct, absolute measurement of hemoglobin oxygen saturation in tissue (StO2), providing trauma teams the ability to measure tissue oxygenation and monitor it during resuscitation. It is the only tissue oxygenation monitor designed for trauma environments. The InSpectra StO2 Tissue Oxygenation Monitor uses near infrared light to illuminate tissue, and then analyzes the returned light to produce a quantitative measurement of oxygen saturation in the tissue's microcirculation.

The StO2 Trauma Study researched the role that tissue oxygen saturation monitoring could play in hemorrhagic shock and resuscitation. Study results demonstrate that StO2 measurements less than 75% may indicate serious hypoperfusion in trauma patients and that StO2 functions as well as base deficit in indicating hypoperfusion in trauma patients.