The impact of extracranial contamination on regional cerebral oxygen saturation: a comparison of three cerebral oximetry technologies
Cerebral oximetry is a non-invasive technology using near-infrared spectroscopy (NIRS) to measure regional cerebral oxygen saturation (rSO2). Although NIRS cerebral oximetry is being increasingly used in many clinical settings, inter-device technologic differences suggest potential variation in the ability to accurately acquire brain oxygenation signals. The primary objective of this study was to determine if NIRS-derived rSO2 measurements accurately account for oxygen saturation contamination from extracranial tissue. Twelve healthy volunteers had three (the FORE-SIGHT, INVOS, and EQUANOX) NIRS devices randomly applied to the forehead. After this, a circumferential pneumatic head cuff was positioned such that when inflated, hypoxia-ischemia would be produced in the scalp extracranial tissue beneath the NIRS cerebral oximeters. Comparisons among the three devices were made of the NIRS measurements prior to and following hypoxia-ischemia produced in the scalp tissue with inflation of the head cuff. The induction of extracranial hypoxia-ischemia resulted in a significant reduction in rSO2 measurements in all three NIRS devices studied. At 5 minutes post-inflation of the pneumatic head cuff, the INVOS demonstrated a 16.6± 9.6% decrease from its baseline (p = 0.0001), the FORE-SIGHT an 11.8± 5.3% decrease from its baseline (p = 0.0001) and the EQUANOX a 6.8± 6.0% reduction from baseline (p = 0.0035). Therefore, extracranial contamination appears to significantly affect NIRS measurements of cerebral oxygen saturation. Although the clinical implications of these apparent inaccuracies require further study, it suggests that the oxygen saturation measurements provided by cerebral oximetry do not solely reflect that of the brain alone.