Proton Magnetic Resonance Spectroscopy of the Brain during Acute Hypoxia-Ischemia and Delayed Cerebral Energy Failure in the Newborn Piglet

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Abstract
Studies of the brains of severely birth-asphyxiated infants using proton(1H) magnetic resonance spectroscopy (MRS) have shown changes indicating a rise in cerebral lactate (Lac) and a fall in N- acetylaspartate (Naa). The aim of this study was to test two hypotheses: 1) that these changes can be reproduced in the newborn piglet after transient reversed cerebral hypoxia-ischemia, and their time course determined; and 2) that changes in Lac peak-area ratios are related to changes in phosphorylation potential as determined by phosphorus(31P) MRS. Eighteen piglets aged <24 h were anesthetized and ventilated. Twelve underwent temporary occlusion of the carotid arteries and hypoxemia, and six served as sham-operated controls. 1H and 31P spectra were acquired alternately, both during the insult and for the next 48 h, using a 7-tesla spectrometer. During hypoxia-ischemia, the median Lac/total creatine (Cr) peak-area ratio rose from a baseline of 0.14 (interquartile range 0.07-0.27), to a maximum of 4.34 (3.33-7.45). After resuscitation, Lac/Cr fell to 0.75 (0.45-1.64) by 2 h, and then increased again to 2.43(1.13-3.08) by 48 h. At all stages after resuscitation Lac/Cr remained significantly above baseline and control values. Naa/Cr was significantly reduced below baseline and control values by 48 h after resuscitation. The increases in the Lac peak-area ratios were concomitant with the falls in the[phosphocreatine (PCr)*]/[inorganic phosphate (Pi)] ratio, during both acute hypoxia-ischemia and delayed energy failure. The maximum Lac/Naa during delayed energy failure correlated strongly with the minimum[nucleotide triphosphate (NTP)]/[exchangeable phosphate pool (EPP)](r = -0.94, p < 0.0001). We conclude that both hypotheses have been confirmed.