Mild Hypothermia after Severe Transient Hypoxia-Ischemia Reduces the Delayed Rise in Cerebral Lactate in the Newborn Piglet

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Abstract
This study tested the hypothesis that mild hypothermia after severe transient hypoxia-ischemia reduces the subsequent delayed rise in cerebral lactate peak-area ratios as determined by proton (1H) magnetic resonance spectroscopy (MRS) in the newborn piglet. Nine piglets aged <24 h underwent temporary occlusion of the common carotid arteries and hypoxemia. Resuscitation was started when cerebral [phosphocreatine]/[inorganic phosphate] had fallen close to zero and [nucleotide triphosphate(NTP)]/[exchangeable phosphate pool (EPP)] was below about a third of baseline. On resuscitation rectal and tympanic temperatures were lowered to 35°C for 12 h after which normothermia (38.5 °C) was resumed. 1H MRS data collected over 48 or 64 h after resuscitation were compared with concurrently established data from 12 piglets similarly subjected to transient cerebral hypoxia-ischemia, but maintained normothermic, and six shamoperated controls. The severity of the primary insult (judged from the time integral of depletion of [NTP]/[EPP]) was similar in the hypothermic and normothermic groups. The maximum lactate/N-acetylaspartate ratio observed between 24 and 48 h after resuscitation in the hypothermic group was 0.10 (0.05-0.97), median (interquartile range), which was significantly lower than that observed in the normothermic group, 1.28 (0.97-2.14), and not significantly different from that observed in the control group, 0.08 (0.06-0.11). Similar results were obtained for lactate/choline and lactate/total creatine. We conclude that mild hypothermia after a severe acute cerebral hypoxic-ischemic insult reduces the delayed elevation in lactate peak-area ratios, thus reflecting reduced lactate accumulation.

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