ANESTHETIC PROTECTION IN BRAIN HYPOXIA: A P31 NMR STUDY

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We propose an in vivo study of hypoxic and ischemic brain injury in anesthesized rats with 31P and 1H NMR spectroscopy. Our aim is to evaluate factors that predict and contribute to cerebral damage, and to compare the metabolic protection conferred by thiopental, isoflurane, halothane, and fentanyl. Ketamine, an anesthetic that does not protect the brain, will be studied and compared with the other agents. Four basic questions will be addressed: 1) How do the different anesthetic agents ameliorate hypoxic and ischemic brain injury? 2) How soon can in vivo NMR spectroscopy determine that irreversible injury has occurred? 3) Does hypoxia during anesthesia produce more injury than global ischemia? 4) When hypoxic and ischemic brain damage occur during anesthesia, how much injury during reperfusion? Our study will use a horizontal NMR spectrometer whose magnetic field strength is 5.6 Tesla. Anesthetized rats will be placed prone in a region of high magnetic field homogeneity, and will be studied using a 14 mm low noise surface coil detector. 31P NMR spectroscopy will be performed in vivo during hyperoxia, hypoxic hypoxia, and global ischemia (obtained using Pulsinelli's model), and used to determine intracellular pH and intracellular concentrations of ATP, PCr, Pi, and monophosphate sugars. 1H spectroscopy will be performed separately under similar conditions to study intracellular concentrations of lactate and N-acetylaspartate. Magnetization transfer methods will be used to measure the unidirectional fluxes and unidirectional rate constants for the creatine kinase (CPK) and ATP synthetase reactions. Deterioration and recovery of energy stores will be studied during hypoxia and ischemia, and correlated with invasive post-mortem measurments of membrane phospholipids and free fatty acids, done by collaborators using freeze extraction techniques and HPLC. In vitro studies indicate that metabolic and electrical activity can return to normal in CNS neurons that were deprived of oxygen and glucose for 20 to 60 minutes. Clinical situations involving more than a few minutes of insufficient oxygen or glucose supply often result in permanent damage. We hope that our in vivo studies of different anesthetics will suggest therapeutic strategies for humans that extend the brain's tolerance of oxygen deprivation and reduce the likelihood of intraoperative brain injury.