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Gravity-induced loss of consciousness (G-LOC) is caused by loss of cerebral blood flow during high +Gz (head-to-foot inertial forces). The resistance of the jugular vein is a significant factor in decrease in cerebral blood flow. Ultrasonography of thoracic inlet veins, including internal jugular vein, is feasible to visualize the internal jugular vein and hemodynamic information. Anti-gravity straining maneuver (AGSM) was widely recognized as one of the important factors in preventing G-LOC. The purpose of this study was to evaluate the relationship between the ultrasonographic shape and size of internal jugular vein during AGSM and G-LOC. 47 trainee pilots who participated in human centrifuge education program were enrolled. They were all men, and their mean age was 23.9 +/- 1.38 years. Questionnaire sheets were used to collect information about well-being sensation, smoking, drinking, height, and weight. Using ultrasonography, we monitored shape and size of internal jugular vein during AGSM. After ultrasonographic examination, 47 subjects underwent human centrifuge on the same day. The protocol of human centrifuge training was maximal 6G with sustaining time of 30 s. G-LOC occurred to ten out of 47 subjects in human centrifuge. To find presumptive variable associated with G-LOC, we performed logistic regression analysis. Concave contour and smaller cross-sectional area of internal jugular vein during AGSM were associated with G-LOC.

It has been reported that hypobaric hypoxia exposure by high altitude is responsible for neuropsychological impairment. In the present study, we examined an effect of hypobaric hypoxia on the writhing test. The ICR mice were exposed in hypobaric chamber with several altitudes (5000, 10,000 or 20,000 ft) for 1 or 2 h, and then immediately injected intraperitoneally (i.p.) with 1% acetic acid for writhing test. Our results show that both 10,000 ft and 20,000 ft exposure induce antinociceptive effect in writhing test, but 5,000 ft does not. In addition, this antinociceptive effect was abolished by L-NAME (nitric oxide synthase inhibitor) pre-treated intraperitoneally, but not naloxone (non-specific opioid receptor antagonist). Furthermore, we examined that neuronal NOS immunoreactivities in the hypothalamus (paraventricular nucleus and arcuate nucleus) were increased by hypobaric hypoxic exposure (10,000ft). These results suggest that hypobaric hypoxic-induced antinociception may be associated with neuronal NOS IR in the hypothalamus.