CONCOMITANT EEG, LACTATE, AND PHOSPHORUS CHANGES BY H-1 AND P-31 NMR-SPECTROSCOPY DURING REPEATED BRIEF CEREBRAL-ISCHEMIA

Pilots of high-performance aircraft are subject to transient loss of c onsciousness due to cerebral ischemia resulting from sudden high gravi tational stress. To assess the effects of gravitational stress-induced blackout on cerebral metabolism and electrical function, we developed an animal model in which global cerebral ischemia is produced repeate dly at short intervals. Rats were prepared by ligation of subclavian a nd external carotid arteries and the right carotid artery was cannulat ed bidirectionally to measure circle of Willis and systemic pressures. Ischemia was induced by inflation of an occluder about the left carot id artery. Interleaved P-31 and H-1 NMR spectra were acquired on a 4.7 -T Biospec system simultaneously with EEG recordings. We report result s from 20 experiments of 30-min duration in which rats were subject to 30 1-min ischemia:reflow cycles of 10I:50R, 20I:40R, 30I:30R, and 40I :20R [numbers are seconds of ischemia (I) and reflow (R) during each 1 -min cycle]. During ischemia the graded delivery of the ischemic insul t permitted direct correlations between 2- to 5- and 7- to 20-Hz EEG a ctivity and progressive changes in pH, lactate, ATP, phosphocreatine ( PCr) and P-i. The best correlations were found between EEG activity an d pH and PCr; correlation coefficients ranged from 0.93 to 0.95. A los s of EEG activity was observed without significant sustained energy lo ss in all but the most severe cycle.