MULTIPLE +GZ EXPOSURES CAUSE BRAIN EDEMA IN RATS

The most serious effect of high sustained +Gz (head-to-foot inertial l oad) known to occur in pilots of high performance aircraft is +Gz-indu ced loss of consciousness (G-LOC), which may result in pilot incapacit ation and subsequent loss of life. G-LOC is believed to occur due to a critical reduction in cerebral blood flow (CBF). Recently, using a sm all animal centrifuge (SAC), we showed that +Gz exposure causes global cerebral ischemia in a rodent animal model. Since ischemia, depending upon the severity and duration, has been associated with increased br ain water content or edema, the present study was undertaken. Rats wer e exposed to six exposures of either +25 Gz (30 s each) or +10 Gz (2 m in each) in the SAC at +20 Gz . s-1 G onset rate. The appearance of G- LOC was monitored by the flattening of the electroencephalography (EEG ) brain wave recording. G-LOC was observed at 101 +/- 46 and 19.2 +/- 5 s during +10 and +25 Gz exposures, respectively. The brains from the se animals were removed 15 min to 24 h after the +Gz exposure and anal yzed for edema formation (increase in the percentage of tissue water), metabolites, and cerebral blood volume (CBV). A significant decrease in glucose and an increase in lactate concentration were observed duri ng +Gz exposure. Edema formation was observed 15 min after six exposur es of either +10 or +25 Gz. A slight but significant decrease in CBV w as also observed in rats exposed to six +10 Gz exposures. Edema format ion was transient and resolved within 24 h. We concluded that multiple exposures of either +25 Gz, short duration or +10 Gz, long duration, that resulted in G-LOC, can cause cytotoxic brain edema which probably results from tissue hyperosmolality due to metabolic changes and accu mulation of lactate during ischemia.