Near-infrared spectroscopy to measure brain oxygenation: A comparison of measurements on the skin, skull and dura mater.

Abstract

The reliability of near-infrared spectroscopy (NIRS) for measuring cerebral oxygenation (ScO₂ ) is controversial due to the possible contamination from extracranial tissues. We compared ScO₂ measured with the NIRS optode on the forehead, the skull and the dura mater in anaesthetised patients undergoing craniotomy. We hypothesised that ScO₂ measured directly on the skull and the dura mater would differ from ScO₂ measured on the skin. This prospective observational study included 17 adult patients scheduled for elective craniotomy. After induction of general anaesthesia, ScO₂ was measured on the forehead skin, as well as on the skull and on the dura mater in the surgical field. The primary comparison was the difference in ScO₂ measured on the dura mater and on ScO₂ measured on the skin; secondary comparisons were the differences in ScO₂ on the skull and ScO₂ on the skin and the dura mater, respectively. Data were described with median (5%-95% range) and analysed with the Wilcoxon signed-rank test. ScO₂ values on the dura mater were obtained in 11 patients, and median ScO₂ (48%, 29%-95%) did not differ significantly from ScO₂ on the skin (73%, 49%-92%; p = .052), median difference -25% (-35.6% to -1.2%). ScO₂ on the skull (N = 16) was lower than that on the skin (63% [43%-79%] vs. 75% [61%-94%]; p = .0002), median difference -10% (-20.8 to -3.0). In adults undergoing craniotomy, NIRS-based ScO₂ measured on the dura mater did not reach statistically significantly lower values than ScO₂ measured on the skin, whereas values on the skull were lower than on the skin, indicating a contribution from scalp tissue to the signal.