Near infrared spectroscopy (NIRS) has potential to offer a fast and non-invasive method of assessing cerebral saturation in a clinical setting, however, there are concerns that NIRS brain measures suffer contamination from superficial tissues. This study used the Valsalva manoeuver (VM) to determine whether NIRS could differentiate between superficial (from somatic tissue) and neurological changes in the context of traumatic brain injury. A potent vasopressor was used to assess the effect of reducing total haemoglobin concentration in the superficial regions of the forehead. Frequency domain NIRS measurements during the VM pre and post vasoconstrictor injection, combined with simulation data, conclusively show that NIRS can detect neurological changes, in both haemoglobin content and saturation, when positioned on the forehead. The effect of superficial contamination in this instance appeared to be insignificant, with no statistically significant change in saturation over 8 patients, even with a drop in superficial haemoglobin concentration due to the vasoconstrictor, confirmed by laser Doppler. Nevertheless, simulations indicated that the absolute values of the recovered NIRS parameters are not quantitatively accurate; however a direct comparison with invasive measures is needed to confirm this.
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