 While participants are in the hospital, they will undergo frequent blood and urine collections, chest x-rays, and temperature measurements; parents of participants will complete questionnaires. When participants are ready to leave the hospital, study researchers will perform a physical and functional assessment. Twenty-eight days after the cardiac arrest, researchers will contact parents of participants to gather information on the participants health and medical condition. At Months 3 and 12, a child development expert will contact parents to gather medical information. At Month 12, participants will attend a study visit for a neurologic examination and testing with a psychologist trained in rehabilitation. 