Concomitant EEG, lactate, and phosphorus changes by 1H and 31P NMR spectroscopy during repeated brief cerebral ischemia

Pilots of high-performance aircraft are subject to transient loss of consciousness due to cerebral ischemia resulting from sudden high gravitational 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 repeatedly at short intervals. Rats were prepared by ligation of subclavian and external carotid arteries and the right carotid artery was cannulated bidirectionally to measure circle of Willis and systemic pressures. Ischemia was induced by inflation of an occluder about the left carotid artery. Interleaved 31P and 1H NMR spectra were acquired on a 4.7-T Biospec system simultaneously with EEG recordings. We report results 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 insult permitted direct correlations between 2- to 5- and 7- to 20-Hz EEG activity and progressive changes in pH, lactate, ATP, phosphocreatine (PCr) and Pi. The best correlations were found between EEG activity and pH and PCr; correlation coefficients ranged from 0.93 to 0.95. A loss of EEG activity was observed without significant sustained energy loss in all but the most severe cycle.