Optimum therapeutic hypothermia temperature after perinatal asphyxia: a magnetic resonance spectroscopy biomarker and immunohistochemistry study in the newborn piglet

Abstract Background Therapeutic hypothermia for neonatal encephalopathy improves survival with normal neurological outcome. Despite treatment, however, 50% of infants have adverse outcomes; customising cooling with more precision might provide further benefit. The specific brain temperature for optimum neuroprotection is unknown. We have previously shown in the piglet that optimum neuroprotection by delayed cooling occurs at different temperatures in the cortical (33°C) and deep grey (35°C) matter. Magnetic resonance biomarkers, including lactate:creatine ratio (Lac/Cr), predict neurodevelopmental outcome after neonatal encephalopathy. We aimed to assess the optimum temperature for regional neuroprotection in a proton magnetic resonance system (1H MRS). Methods 28 male piglets aged less than 24 h were anaesthetised with isoflurane and underwent trachaeal intubation, ventilation, continuous physiological monitoring, and intensive care treatment to maintain sedation and physiological homoeostasis. Within the bore of our 9·4 T magnetic resonance system all piglets underwent a hypoxic-ischaemic insult for 12·5 min, measured with real-time phosphorous MRS, before resuscitation. After this insult piglets were randomised (seven in each group) to normothermia (38·5°C), hypothermia (35°C), hypothermia (33·5°C), and hypothermia (30°C). Serial MRS assessments were acquired before, during, and up to 48 h after transient cerebral hypoxia-ischaemia. Areas under the curve (AUC) for the 1H MRS Lac/Cr peak-area ratio in ventromedial forebrain (vmFB, predominantly grey matter) and dorsal subcortical (dsc, predominantly white matter) voxels were calculated. Programmed cell death at 48 h was measured with terminal deoxynucleotidyl transferase dUTP nick-end labelling (TUNEL) staining on paraffin-embedded tissue in corresponding regions. Findings Compared with normothermia, cooling to 35°C and 33·5°C produced consistent 50–75% reduction in density of TUNEL-positive cells with significant decreases in insular cortex, hippocampus, subcortical white matter, thalamus, and putamen (p