Tissue oxygenation indices of cerebrovascular autoregulation in healthy volunteers : a comparison of two NIRS devices

Background: Correlation coefficients between blood pressure and cerebral oxygen saturation measured using near-infrared spectrometry may be used to derive the tissue oximetry index of cerebral autoregulation. Cerebral oxygen saturations demonstrate poor agreement between near-infrared spectrometers however it is unclear if measurements of autoregulation are similarly specific to the equipment used. Methods: Cerebral oxygen saturation was monitored bilaterally in 74 healthy volunteers using both the FORE-SIGHT and EQUANOX monitors in random order. The tissue oximetry index was calculated during changes in blood pressure induced by isometric handgrip manoeuvres and the mean bias and limits of agreement were calculated. Results: Tissue oximetry index measured by FORE-SIGHT was higher than EQUANOX (0.21 ± 0.16 versus 0.15 ± 0.17, P < 0.001) and limits of agreement were −0.24 to 0.36. Baseline cerebral oxygen saturation by FORE-SIGHT was lower than EQUANOX by 1.48% (CI95% 0.63–2.33) and limits of agreement ranged from −11.8% to 8.8%. Conclusions: The substantial bias and wide limits of agreement for the tissue oximetry index as a measure of cerebral autoregulation indicate that values must be treated as specific to models of near-infrared spectrometers.