Noninvasive Assessment of Brain Injury in a Canine Model of Hypothermic Circulatory Arrest Using Magnetic Resonance Spectroscopy

Background Studies have confirmed the neuroprotective effect of diazoxide in canines undergoing hypothermic circulatory arrest (HCA). A decreased N-acetyl-asparate:choline (NAA:Cho) ratio is believed to reflect the severity of neurologic injury. We demonstrated that noninvasive measurement of NAA:Cho with magnetic resonance spectroscopy facilitates assessment of neuronal injury after HCA and allows for evaluation of neuroprotective strategies. Methods Canines underwent 2 hours of HCA at 18°C and were observed for 24 hours. Animals were divided into three groups (n = 15 in each group): normal (unoperated), HCA (HCA only), and HCA+diazoxide (pharmacologic treatment before HCA). The NAA:Cho ratios were obtained 24 hours after HCA by spectroscopy. Brains were immediately harvested for fresh tissue NAA quantification by mass spectrometry. Separate cohorts of HCA (n = 16) and HCA+diazoxide (n = 23) animals were kept alive for 72 hours for daily neurologic assessment. Results Cortical NAA:Cho ratios were significantly decreased in HCA versus normal animals (1.01 ± 0.29 versus 1.31 ± 0.23; p = 0.004), consistent with severe neurologic injury. Diazoxide pretreatment limited neurologic injury versus HCA alone, reflected in a preserved NAA:Cho ratio (1.21 ± 0.27 versus 1.01 ± 0.29; p = 0.05). Data were substantiated with fresh tissue NAA extraction. A significant decrease in cortical NAA was observed in HCA versus normal (7.07 ± 1.9 versus 8.54 ± 2.1 μmol/g; p = 0.05), with maintenance of normal NAA levels after diazoxide pretreatment (9.49 ± 1.1 versus 7.07 ± 1.9 μmol/g; p = 0.0002). Clinical neurologic scores were significantly improved in the HCA+diazoxide group versus HCA at all time points. Conclusions Neurologic injury remains a significant complication of cardiac surgery and is most severe after HCA. Magnetic resonance spectroscopy assessment of NAA:Cho ratios offers an early, noninvasive means of potentially evaluating neurologic injury and the effect of neuroprotective agents.