Norm of Transcranial Near-Infrared Spectroscopy (NIRS, Cerebral Near-Infrared Spectroscopy, Transcranial Cerebral Oximetry)
Norms not well established. Some studies indicate that a decrease in cerebral oxygen saturation (COS) of more than 25% indicates potentially correctable impending cerebral ischemia and the need for intervention.
Usage of Transcranial Near-Infrared Spectroscopy (NIRS, Cerebral Near-Infrared Spectroscopy, Transcranial Cerebral Oximetry)
Used in conjunction with transcranial Doppler to monitor cerebral oxygen metabolism during cardiac and neurologic surgical procedures in anesthetized clients; used in conjunction with EEG to assess cerebral activity in clients thought to be in a coma. COS values primarily represent the venous oxygenation of the brain (75% weight), and to a lesser extent the arterial oxygenation (25%).
Description of Transcranial Near-Infrared Spectroscopy (NIRS, Cerebral Near-Infrared Spectroscopy, Transcranial Cerebral Oximetry)
Transcranial near-infrared spectroscopy (NIRS) is a bedside neuromonitoring technique for detection of cerebral hypoxia by identifying changes in COS. The technique involves measuring changes in the absorption of light as it is transmitted through the skull, bone, brain, and cerebrospinal fluid. COS values change as the proportion of oxygen supply to oxygen consumption changes. Cerebral oxygen metabolism can be affected by any of five variables: mean arterial pressure, hemoglobin level, peripheral oxygen saturation, partial pressure of carbon dioxide, and core temperature. Use of NIRS during surgery can alert the physicians to possible inadequate anesthetic in which the waking brain uses more oxygen, or to cerebral seizures. Both conditions cause the COS to drop.
Professional Considerations of Transcranial Near-Infrared Spectroscopy (NIRS, Cerebral Near-Infrared Spectroscopy, Transcranial Cerebral Oximetry)
Consent form NOT required.
- Obtain near-infrared light emitter and receiver.
- Verify that other monitoring systems are in place to co-assess the underlying variables that can affect COS: arterial monitoring, peripheral oxygen saturation, carbon dioxide monitoring, core temperature.
- For synchronous, bilateral monitoring, place sensors as high as possible on the forehead. Sensors should be shielded from ambient light.
- Monitoring is carried out using a light emitter and sensors positioned near the forehead. Light emitted is reflected back to the receiver, which produces a graphical tracing of COS.
- Remove sensors.
Client and Family Teaching
- This technique is used to help determine how well the brain is using oxygen.
Factors That Affect Results
- Endovascular procedures in which arterial vasospasm occurs cause unreliable results.
- Although decreased COS indicates impending ischemia, a stable COS does not necessarily signify intact cerebral processes.
- Placement of sensors affects results. Readings are only indicative of the status of cerebral oxygenation in the region of the brain located near the sensors. Positioning sensors laterally instead of high on the forehead omits data from the saggital sinus. Results are erroneous if sensors are placed near areas of damaged brain tissue or implants such as metal plates.
- Areas of localized pooling of blood within the cranium will affect results. If pooled blood is unoxygenated, the results are not useful. Superficial or deep hematomas can cause false-negative results.
- Factors that interfere with the validity of the results include the presence of strong ambient light in the room, use of electrocautery, recent injection of dyes in the client's bloodstream, abnormal hemoglobin levels, and abnormal bilirubin levels.
- Interpretation of NIRS changes requires complex knowledge of physiologic mechanisms and consideration of all variables affecting the COS.