Contribution of cerebrospinal fluid to rheoencephalographic waveforms during hypoxic and +Gz stress

Acceleration (G) forces generated by high performance aircraft induce a redistribution of blood and cerebrospinal fluid (CSF) in the head resulting in decreased visual function, and may lead to G-induced loss of consciousness (G-LOC). CSF provides a critical support function to the brain by equalizing the pressure changes occurring throughout the skull under G-stress, particularly to the venous system. While it has been acknowledged that understanding this role of the CSF system is essential in order to enhance G-tolerance, no such studies have been conducted since the 1940's, due to technical difficulties. We have shown that these can be surmounted through the development of rheoencephalography (REG), or impedance plethysmography of the head, to noninvasively monitor shifts in CSF under both hypoxic (oxygen deprivation in the laboratory) and actual +Gz stress (small centrifuge) conditions. Using surgical and physiologic techniques on New Zealand White rabbits, we have established the following: 1) REG contains information concerning the function of the vascular, CSF, and respiratory systems as they influence both beat-to-beat and bulk movement of cephalic fluids; 2) respiratory effects on cerebral blood volume and CSF pressure can be monitored with the REG; 3) REG waveforms obtained from rabbits under laboratory and +Gz stress conditions were similar to those obtained during human +Gz centrifuge exposures; 4) using (i) brief occlusions of blood flow into the head and (ii) withdrawals of CSF, it was estimated that the relative volumetric contributions of blood and CSF to the REG were 70% and 30%, respectively; and 5) physiologic responses to stress are reflected in changes in the frequency content of the REG.(ABSTRACT TRUNCATED AT 250 WORDS)