Induced TH is currently used in several medical conditions. However, it is by far more commonly employed in today's clinical practice to prevent or diminish hypoxic-ischemic encephalopathy (HIE) and its deleterious outcomes in postcardiac arrest patients.
HIE is a syndrome of acute global brain injury resulting from critical reduction or loss of blood flow and supply of oxygen and nutrients. Some of the terms used to describe this clinical syndrome include anoxic encephalopathy, postcardiac arrest brain injury, and other terms denoting a diminution of blood or oxygen to the whole brain. The pathologic injury to the brain results in cell injury and death from an intracellular energetic crisis that selectively affects vulnerable areas such as CA-1 of the hippocampus, caudate, putamen and neocortex, and relative sparing of brain stem. High-quality cardiopulmonary resuscitation (CPR) with prompt restoration of spontaneous circulation is crucial to survival. The mainstay of postresuscitation care is TH. ICU management also includes optimizing hemodynamic status, diagnosing and treating seizures, and other supportive care.
The most common cause of HIE is cardiac arrest, which is commonly secondary to cardiac etiologies (infarction or arrhythmia). Each year over half a million cardiac arrests occur in the United States. Approximately 380,000 of these occur outside of health care facilities, and another 210,000 occur in hospital every year.
Outcomes of Postanoxic Brain Injury After Cardiac Arrest
The outcome of patients experiencing SCA is poor. The primary cause of mortality after SCA is primarily related to the effects of anoxic brain injury and not necessarily from cardiac complications.
Outcomes differ based on initial rhythm found at scene of SCA. For example, outcomes are poorer with so-called “nonshockable” rhythms (asystole, pulseless electrical activity [PEA]) compared to “shockable” rhythms (ventricular tachycardia [VT] or ventricular fibrillation [VF]). Improved survival may be best when initial rhythm is VF, but still dependent on prompt delivery of effective CPR. Clinical factors identified as predictors of greater likelihood of survival to hospital discharge are witnessed arrest, VT or VF as initial rhythm, return to spontaneous circulation (ROSC) during first 10 minutes, and longer duration of overall resuscitation efforts.1
Since the development of modern CPR techniques in the 1950s, numerous pharmacologic trials with putative neuroprotective effects have failed to improve survival and quality of life of patients resuscitated from cardiac arrest. Mild TH has become a standard component of postcardiac arrest to prevent or mitigate various types of neurologic injury. The ultimate goal of resuscitation is to improve survival with good neurologic outcome. The evidence supporting this practice came from two randomized trials published consecutively in 2002—one small-sized Australian study done by Bernard et al that showed good neurologic outcome in 49% survivors in the hypothermia to 33°C group versus 26% of normothermic patients, among 77 patients with out-of-hospital cardiac arrest ...