Evaluating the impact of skin perfusion on cerebral hemoglobin concentration measurement using modified Beer-Lambert and Spatially Resolved Spectroscopy: Preliminary findings

Abstract

Background: The impact of potential extracranial interference with the interpretation of cerebral Near-Infrared Spectroscopy (NIRS) values remains a matter of debate.

Objective: To examine the influence of skin perfusion variation on NIRS parameters in the NIROTM 200NX device (Hamamatsu), using an occlusive head band setup. We specifically investigated the alleged difference in sensitivity to extracranial blood flow of the parameters calculated using the Modified Beer-Lambert law [MBL: oxyhemoglobin (O2Hb), deoxyhemoglobin (HHb) and total hemoglobin (tHb)] versus those using Spatially Resolved Spectroscopy [SRS: Tissue Oxygenation Index (TOI and normalized Tissue Hemoglobin Index (nTHI)]. The manufacturer suggests that SRS values are less influenced by skin perfusion than MBL- based parameters. The NIRO device uses both methods (MBL and SRS) in its calculations of cerebral oximetry parameters.

Design: Prospective observational clinical study.

Setting: Single-centre, tertiary academic medical centre; inclusions February to December 2022.

Methods: Thirty-one patients undergoing elective cardiac surgery were planned to be included. After inclusion of the first two thirds of patients (n=21), we analyzed the data for this preliminary report. In a controlled setup, while recording cerebral NIRS and plethysmography, we obstructed extracranial blood flow for one minute under general anesthesia using an adjustable head band.

Main outcome measures: MBL-based (tHb) vs. SRS-based (nTHI) NIRS measurements of hemoglobin concentration, before and during application of an occlusive head band.

Results: Head band occlusion resulted in a wide and diverging range of SRS- as well as MBLbased parameter effects, but without a significant trend for either nTHI or tHB. Only nTHI rose significantly when the head band was released (p = 0,017). When comparing the differences of normalized data the Wilcoxon signed rank test showed no significant difference between changes in tHb and nTHI before and after attaching the head band (p-value = 0.06, confidence interval (CI): -0,05; 0.95). Intraclass correlation was poor at 0.055 (CI -0.37 – 0.46). These results were confirmed by Bland-Altman analysis pointing to a weak agreement between nTHI and tHb, with again no clear trend.

Conclusions: In this preliminary analysis, our data cannot confirm that NIRO parameters using SRS are less influenced by extracranial contamination than those measured using MBL when producing a diminishment of extracranial circulation using an occlusive head band.