Chapter 29
Therapeutic Hypothermia in Cardiac Arrest
Nainesh C. Patel, MD, FACC, Sudip Nanda, MD, FACP, Barbara Tate Unger, RN, BS, FAACVPR, Michael Mooney, MD, FACC

It is estimated that approximately 350,000 cardiac arrests occur annually in the United States. Greater than 80% of these cardiac arrests will occur out-of-hospital. More than 50% of deaths are related to previously undiagnosed heart disease. The time-sensitive model of cardiac arrest describes three separate phases. The first, or electrical phase, is up to 4 or 5 minutes from the onset of cardiac arrest. Myocardial energy depletion is not complete and defibrillation is usually successful in generating a perfusing rhythm. The second phase, also called the circulatory phase, lasts from 4-5 minutes to 10 minutes after cardiac arrest. Myocardial energy stores are mostly depleted. Immediate defibrillation is usually unsuccessful, and results in asystole or pulseless electrical activity (PEA). Chest compressions are imperative prior to defibrillation. Restoring blood flow to the brain is critical for neurologic recovery. It is in this second phase of cardiac arrest that hypothermia has been shown to have beneficial effects. The third phase, the metabolic phase, extends beyond 10 minutes of cardiac arrest. Defibrillation during the third phase is universally unsuccessful. New innovative treatments will be needed to improve outcomes in this phase.