Normal body temperature changes during the course of the day and is regulated by the thermoregulatory center in the anterior hypothalamus. The normal temperature at 6 AM is 37.2°C and at 4 PM is 37.7°C. Rectal temperatures are normally higher than oral temperatures because of mouth breathing. To obtain core temperature, readings should be obtained in the esophagus or tympanic membrane. During a fever, the setpoint in the hypothalamus is shifted upward. During hyperthermia, the setpoint is unchanged, but the rest of the body overcompensates to remove heat. At the other end of the spectrum, hypothermia is defined as core temperature less than 35°C. This chapter discusses the therapeutic and pathologic implications of hypothermia and hyperthermia.
THERAPEUTIC AND PATHOLOGIC HYPOTHERMIA
Therapeutic hypothermia (TH) is part of targeted temperature management (TTM). It is currently advocated as part of the postresuscitation care that includes optimization of oxygen supplementation and blood pressure and treatment of acute coronary syndromes. The consequences of anoxia are secondary to loss of adenosine triphosphate (ATP) and glucose, loss of cellular integrity, mitochondrial damage, and loss of calcium homeostasis.1 Increased calcium and glutamate perpetuates necrosis or apoptosis. In addition, restoration of perfusion leads to reperfusion injury caused by reactive oxygen species that further exacerbates endothelial dysfunction, vasomotor dysregulation, and edema.2 Hypothermia reduces release of excitatory amino acids and free radicals and improves oxygen supply and demand mismatch with reduction of cerebral metabolic rate of oxygen, blood volume, and pressure.2
Therapeutic hypothermia is indicated for patients who received return of spontaneous circulation (ROSC) after cardiac arrest from ventricular tachycardia or ventricular fibrillation (Class I) or ROSC after cardiac arrest from nonshockable rhythm (Class IIb).2 These patients must be comatose, which was initially defined by the Glasgow Coma Scale but has been broadened to include patients who are not answering verbal commands.2 Contraindications include sepsis, surgery within 14 days, and bleeding diathesis.
The temperature goals have evolved but every hour of delay of initiation increases the mortality rate by 20%.3 Therefore, it is advocated that TH is initiated within 6 hours. Initially, the temperature goal is 32°C to 34°C for 24 hours as per the 2010 guidelines.3-5 By 2015, the guidelines suggest 32° to 36°C because one randomized control trial suggested no survival or neurologic benefit between 32° and 36°C. (Class I; Level of Evidence: B).3-5 In 2017, The American Academy of Neurology (AAN) further stratified the recommendations. AAN recommendations are based on the following levels of evidence. Level A Recommendation is established effectiveness, ineffectiveness, or harmful and requires two Class I studies.6 Level B Recommendation is probably effective, ineffective, or harmful and requires one or more than one Class I study or two Class II studies.6 Level C Recommendation is possibly effective, ineffective, or harmful with at ≥ 1 Class II study or more than two Class III studies....