Changes in cerebral oxygen saturation and cerebral blood flow velocity under mild +Gz hypergravity.

We previously reported that cerebral blood flow (CBF) was reduced by even mild +Gz hypergravity. Regional cerebral oxygen saturation as measured by near-infrared spectroscopy (C-rSO) has been widely used to detect cerebral ischemia in clinical practice. For example, decreases in C-rSO reflect reduced CBF or arterial oxygen saturation. Thus it was hypothesized that C-rSO would decrease in association with reduced CBF during mild hypergravity. To test this hypothesis, we measured CBF velocity by transcranial Doppler ultrasonography and C-rSO during mild +Gz hypergravity while participants were in a sitting position. Among 17 male participants, 15 completed 21 min of exposure to +1.5 Gz generated by short-arm centrifuge. C-rSO and mean CBF velocity in the middle cerebral artery (MCBFV) during centrifugation were averaged every 5 min and compared with pre-hypergravity (+1.0 Gz). C-rSO did not change significantly throughout centrifugation, although MCBFV gradually decreased from the beginning (-1.2% at 0-5 min), and significantly decreased at 5-10 min (-4.8%), 10-15 min (-6.7%), and 15-20 min (-7.4%). Contrary to our hypothesis, decreases in C-rSO were not detected, despite reductions in CBF velocity during hypergravity. Since some assumptions, such as unaltered arteriovenous volume ratio, hemoglobin concentration, extracranial blood flow, and brain activity, need to be satisfied to monitor cerebral ischemia by C-rSO, the present results suggest that these necessary assumptions for near-infrared spectroscopy are not always applicable, and that cerebral oxygenation may not precisely reflect decreases in CBF under mild +Gz hypergravity. To our knowledge, this is the first study to evaluate simultaneously cerebral oxygenation monitored by near-infrared spectroscopy and cerebral blood flow (CBF) monitored by transcranial Doppler under +1.5 Gz hypergravity. Contrary to our hypothesis, there was no significant correlation between CBF velocity and regional cerebral oxygen saturation (C-rSO). However, an incomplete case nearly involving syncope suggests the possibility that C-rSO can detect a remarkable decrease in CBF with development of presyncope during +Gz hypergravity.