Drew Farmer, MD and Shariq Raza, MD Trauma Surgery, Surgical Critical Care & Emergency Surgery, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA The fundamental difference between hyperthermia and fever is that fever is the result of a functioning thermoregulatory system working at an elevated set point. Hyperthermia is a condition in which the thermal set point is normal, but the thermoregulatory system is unable to adequately regulate heat exchange to achieve the desired temperature. (answer e.) Fever has many causes and is not limited to infection (answer a). While the Society of Critical Care Medicine does define fever as a body temperature above 38.3 °C (or 101 °F), the fundamental difference between fever and hyperthermia is related to the mechanism by which the temperature rises (answer b). Hyperthermia is the result of an inability to mount an adequate adaptive response to increased body temperature. Fever is often a physiologic response to a pathologic condition (answer d). Answer: C Marino PL and Sutin KM . The ICU Book. 3rd ed. Philadelphia: Lippincott Williams & Wilkins, 2007. Society of Critical Care Medicine. Fundamental Critical Care Support. 5th ed., Society of Critical Care Medicine, 2012. In a healthy person in a physiologic state in a habitable environment, the majority (50–70%) of heat loss is via radiation of infrared rays from the skin. Conduction is the exchange of heat as kinetic energy by direct contact (i.e. the skin–air interface), and under normal conditions this accounts for only about 15% of heat loss. As the skin surface and the surrounding air temperatures equilibrate, heat transfer between the surfaces diminishes. Convection occurs by two processes – first, air movement over the skin replaces the warmed air with air that has not been warmed by conduction at the skin surface, and second, cutaneous vasodilation increases blood flow and surface area exposure to this interface. Each of these currents serves to maintain the energy gradient needed for heat exchange. At rest, evaporative heat loss occurs mainly through respiratory vapor and accounts for about 20% of total heat loss. Answer: D Rippe JM . Irwin and Rippe’s Intensive Care Medicine , edited by Richard S. Irwin and James M. Rippe, Wolters Kluwer Health, 2011. ProQuest Ebook Central, https://ebookcentral‐proquest‐com.proxy.library.upenn.edu/lib/upenn‐ebooks/detail.action?docID=2031844. When the body experiences thermal stress, evaporation surpasses radiation (answer d) in its capacity for dissipating excess body heat. In particular, evaporation is not dependent upon ambient temperature to yield heat loss. Heat energy is used up during the conversion of liquid sweat into vapor. This is called the “latent heat of vaporization.” Notably, for evaporative heat loss to occur, the sweat must evaporate. Therefore, significant evaporative heat exchange does not occur in conditions of high humidity (high vapor pressure) or when sweat is not allowed to evaporate from the skin surface (i.e. it is wiped away). Heat loss by conduction (answer a) and convection (answer b) rely upon ambient temperatures that are less than the body temperature. In the conditions described in the vignette, these would not be significant sources of heat loss. Answer: C Marino PL and Sutin KM . The ICU Book. 3rd ed. Philadelphia: Lippincott Williams & Wilkins, 2007. Rippe JM . Irwin and Rippe’s Intensive Care Medicine , edited by Richard S. Irwin and James M. Rippe, Wolters Kluwer Health, 2011. ProQuest Ebook Central, https://ebookcentral‐proquest‐com.proxy.library.upenn.edu/lib/upenn‐ebooks/detail.action?docID=2031844. The patient is suffering from heat exhaustion, a form of exertional heat illness. The recommended therapy for this illness is simply volume repletion and no cooling measures are usually needed as body temperature is typically ≤40 °C. The patient is not experiencing significant encephalopathy or other severe CNS dysfunction (which may include altered mental status, seizures, or persistent delirium) – hallmarks of exertional heat stroke (answers c and d). Classic heat stroke is characterized by hyperthermia with CNS derangements (not seen in this patient), whose pathology is non‐exertional, but rather commonly stems from another medical comorbidity that prevents physiologic or behavioral response to hyperthermia. Examples include physical impairment or entrapment that prevents mobility to a cooler environment, psychiatric disorders, or drug (pharmaceutical or illicit) use, which may exacerbate dehydration and/or hyperthermia, or which may result in impaired mental status. A diagnosis of “heat injury” could be made if hyperthermia and end‐organ damage are appreciated without severe neurologic dysfunction, but that was not seen in this patient. Answer: A Marino PL and Sutin KM. The ICU Book. 3rd ed. Philadelphia: Lippincott Williams & Wilkins, 2007. Rippe JM . Irwin and Rippe’s Intensive Care Medicine , edited by Richard S. Irwin and James M. Rippe, Wolters Kluwer Health, 2011. ProQuest Ebook Central, https://ebookcentral‐proquest‐com.proxy.library.upenn.edu/lib/upenn‐ebooks/detail.action?docID=2031844. For exertional heat stroke, the mainstay of therapy is rapid cooling (>0.1 °C/min) to a target temperature <39 °C prior to cessation of cooling measures. Conventional methods of external cooling include immersion in cold water (up to 0.35 °C/min cooling rate) or pouring large volumes of water over the body and fanning to maximize evaporative heat dissipation. Also available are commercially sold surface cooling systems that circulate cold fluid or cold air through blankets or pads that are wrapped around the patient. Invasive cooling systems use percutaneously placed central venous catheters in subclavian, internal jugular, or femoral veins, and temperature control is achieved by circulating cool saline in a closed loop through the catheter’s balloon. Invasive cooling systems thus have a higher risk of catheter‐related blood stream infections, however, can prove beneficial in difficult to cool patients such as those with high body mass index (BMI) (>30 kg/m2). Answer: D Epstein Y and Yanovich R . Heatstroke. The New England Journal of Medicine 2019; 380(25):2449–2459. doi: 10.1056/NEJMra1810762.
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Hypothermia and Hyperthermia
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