CARDIAC ARREST AND RESUSCITATION--MECHANISMS OF BRAIN INJURY

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Severe hypoxia and complete asphyxia are the most common causes of cardiac arrest in newborn and infant children and can result in cerebral palsy, mental retardation and severe seizures. We have developed a model of asphyxic cardiac arrest in the immature pig that resembles the clinical course of birth asphyxia with selective injury to basal ganglia, cortex and thalamus and with emergence of clinical seizures one day after resuscitation. Our preliminary data indicate that injury to neurons and astrocytes, accompanied by loss of their respective glutamate transporter isoforms, is dense in putamen by 24 hours, whereas neurodegeneration is largely delayed until 48 hours in primary sensorimotor cortex and until 96 hours in thalamic sensory nuclei. Our goal is to understand the mechanism of injury at each step of recovery so that specific therapeutic modalities can be designed to prevent the maturation of injury at each location. We will determine if decreased capacity of the glutamate reuptake transporters occurs during the early hours of reoxygenation when bursts of electrical activity are seen. Mild hypothermia and the glutamate release inhibitor lamotrigine will be used to reduce the overflow of glutamate into the extracellular space as monitored by microdialysis during the first day of recovery. We will determine if suppressing glutamate overflow after resuscitation reduces early neuronal and astrocyte loss in putamen, delayed loss in cortex and thalamus, and neurobehavioral deficits. A component of the delayed neuronal loss in sensorimotor cortex and thalamus may be apoptotic in nature resulting from a) target deprivation secondary to loss of other neurons in the sensorimotor axis, and b) seizures. We will use various morphological and biochemical markers of apoptosis to determine when and where programmed cell death occurs. Phenobarbital loading, as used clinically to suppress birth asphyxia seizures in newborns, will be used to determine the role of seizure activity in the progression of the injury process. The integrative approach of systems neuropathology, immunocytochemical localization, microdialysis and EEG spectral analysis will generate unique mechanistic insights in a model of neonatal brain injury. In mature brain, transgenic mice have been useful for investigating focal ischemic injury, but this approach has not been applied to cardiac arrest. In a model of cardiac arrest and resuscitation in mice, we will test the hypothesis that the combination of neuronal nitric oxide (NO) synthase gene deletion with overexpression of Cu, Zn- superoxide dismutase provides better neuroprotection and improve memory and learning than either gene alteration alone. This study provides a novel approach for understanding the role of NO and oxygen radicals in delayed neuronal injury after cardiac arrest/CPR.