Extra-cerebral oxygenation influence on near-infrared-spectroscopy-determined frontal lobe oxygenation in healthy volunteers: a comparison between INVOS-4100 and NIRO-200NX

INTRODUCTION: Frontal lobe oxygenation (Sc O2 ) is assessed by spatially resolved near-infrared spectroscopy (SR-NIRS) although it seems influenced by extra-cerebral oxygenation. We aimed to quantify the impact of extra-cerebral oxygenation on two SR-NIRS derived Sc O2 .

METHODS: Multiple regression analysis estimated the influence of extra-cerebral oxygenation as exemplified by skin oxygenation (Sskin O2 ) on Sc O2 in 21 healthy subjects exposed to whole-body exercise in hypoxia (Fi O2 = 12%; n = 10) and normoxia (n = 12), whole-body heating, hyperventilation (n = 21), administration of norepinephrine with and without petCO2 -correction (n = 15), phenylephrine and head-up tilt (n = 7). Sc O2 was assessed simultaneously by NIRO-200NX (Sniro O2 ) and INVOS-4100 (Sinvos O2 ). Arterial (Sa O2 ) and jugular bulb oxygen saturations (Sj O2 ) were obtained.

RESULTS: The regression analysis indicated that Sinvos O2 reflects 46% arterial, 14% jugular, 35% skin and 4% oxygenation of tissues not interrogated. Sinvos O2 follows a calculated estimate of cerebral capillary oxygenation (r = 0·67; P<0·0001). In contrast, the NIRO-200NX-determined Sc O2 did not correlate with the estimate of cerebral oxygenation (r = 0·026; P = 0·71).

CONCLUSION: For all interventions, 35% of the INVOS-4100 signal reflected extra-cerebral oxygenation while, on the other hand, NIRO-200NX did not follow changes in a calculated estimate of cerebral capillary oxygenation. Thus, the NIRO-200NX and INVOS-4100 do not provide for unbiased evaluation of the cerebral signal.