Cerebral Oximetry in Children: So NIRS Yet So Far.

April 2019 • Volume 128 • Number 4 www.anesthesia-analgesia.org 605 DOI: 10.1213/ANE.0000000000002844 Near-infrared spectroscopy (NIRS) is a noninvasive technique that relies on the relative transparency of biological tissue to transmitted light in nearinfrared range (700–950 nm wavelength) to measure the unique absorption spectra of this light by physiologically important chromophores such as oxyhemoglobin (HbO2), deoxyhemoglobin, and cytochrome c oxidase (also referred to as cytochrome aa3) in underlying tissue. In current clinical practice, NIRS technology is primarily used to measure cerebral oxygenation. Commercially available cerebral oximeters (CerOx, EQUANOX, FORE-SIGHT, INVOS series, NIRO series, Oxymon) use continuouswave, spatially resolved spectroscopy to measure the ratio of HbO2 to total hemoglobin (HbO2/[HbO2 + deoxyhemoglobin]) to provide cerebral hemoglobin oxygen (O2) saturation. The optode array of these devices uses proprietary arrangements of closely spaced emitters and detectors to measure light attenuation in a curvilinear photon path through extracranial and superficial cerebral tissue as a function of source–detector separation.1,2 Open and proprietary algorithms are then used to derive a scaled absolute hemoglobin concentration. So what do cerebral NIRS monitors or cerebral oximeters actually measure? Cerebral oximeters are calibrated and validated by comparison of the NIRS-measured cerebral O2 saturation (SctO2) with a “field” or reference O2 saturation (REF CX) calculated as a weighted average from arterial and jugular bulb O2 saturations. The basic equation3 describing this relationship, regardless of manufacturer, is as follows: