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J Appl Physiol 103: 1326-1331, 2007. First published July 19, 2007; doi:10.1152/japplphysiol.01191.2006
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Frontal cortical oxygenation changes during gravity-induced loss of consciousness in humans: a near-infrared spatially resolved spectroscopic study

¹Department of Hygiene and Preventitive Medicine, Showa University School of Medicine, Shinagawa-ku, and ²Aeromedical Laboratory, Japan Air Self-Defense Force, Tachikawa-shi, Tokyo, Japan

Submitted 10 February 2006 ; accepted in final form 13 July 2007

Gravity (G)-induced loss of consciousness (G-LOC), which is presumably caused by a reduction of cerebral blood flow resulting in a decreased oxygen supply to the brain, is a major threat to pilots of high-performance fighter aircraft. The application of cerebral near-infrared spectroscopy (NIRS) to monitor gravity-induced cerebral oxygenation debt has generated concern over potential sources of extracranial contamination. The recently developed NIR spatially resolved spectroscopy (SRS-NIRS) has been confirmed to provide frontal cortical tissue hemoglobin saturation [tissue oxygenation index (TOI)]. In this study, we monitored the TOI and the standard NIRS measured chromophore concentration changes of oxygenated hemoglobin and deoxygenated hemoglobin in 141 healthy male pilots during various levels of +G_z (head-to-foot inertial forces) exposure to identify the differences between subjects who lose consciousness and those who do not during high +G_z exposure. Subjects were exposed to seven centrifuge profiles, with +G_z levels from 4 to 8 G_z and an onset rate from 0.1 to 6.0 G_z/s. The SRS-NIRS revealed an 15% decrease in the TOI in G-LOC. The present study also demonstrated the TOI to be a useful variable to evaluate the effect of the anti-G protection system. However, there was no significant difference found between conditions with and without G-LOC in subjects with terminated G exposure. Further studies that elucidate the mechanism(s) behind the wide variety of individual differences may be needed for a method of G-LOC prediction to be effectively realized.

hypergravity; loss of consciousness; near-infrared spectroscopy; cerebral oxygenation; oxygenated hemoglobin; deoxygenated hemoglobin