MULTIPLE +GZ EXPOSURES CAUSE BRAIN EDEMA IN RATS

The most serious effect of high sustained +Gz (head-to-foot inertial load) known to occur in pilots of high performance aircraft is +Gz-induced loss of consciousness (G-LOC), which may result in pilot incapacitation and subsequent loss of life. G-LOC is believed to occur due to a critical reduction in cerebral blood flow (CBF). Recently, using a small 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 brain water content or edema, the present study was undertaken. Rats were exposed to six exposures of either +25 Gz (30 s each) or +10 Gz (2 min 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 these animals were removed 15 min to 24 h after the +Gz exposure and analyzed 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 during +Gz exposure. Edema formation was observed 15 min after six exposures of either +10 or +25 Gz. A slight but significant decrease in CBV was also observed in rats exposed to six +10 Gz exposures. Edema formation 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 accumulation of lactate during ischemia.