Cerebral hemodynamics and brain oxygen changes related to gravity-induced loss of consciousness in rhesus monkeys.

The aim of the present experiment was to study the influence of +Gz acceleration (head-to-foot inertial forces) onset on cerebral oxygenation changes (cerebral oxy- and deoxy-hemoglobin) and cerebral blood volume (CBV) in order to evaluate the role of cerebral hypoxemia and ischemia in the appearance of +Gz-induced loss of consciousness (G-LOC). We used five rhesus monkeys which were equipped with near infrared spectroscopy optodes fixed onto the parietooccipital cranial bone. G-LOC (isoelectric electrocorticogram) was detected with silver balls electrodes in contact with the dura matter. The animals were centrifuged up to +12 Gz with two onset rates (0.1 and 3 G/s). Cerebral deoxy-hemoglobin increased significantly (max: +30 +/- 6% of control, P < 0.01) only during the 0.1 G/s run. At G-LOC, CBV changes were not related to G-onset rate (P = 0.30; mean change: -32 +/- 6% of control). We conclude that cerebral ischemia is the main mechanism in the occurrence of G-LOC.