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Chapter 90. Hyperthermia/Fever

Chad S. Miller, MD, FACP, FHM; Jeffrey G. Wiese, MD, FACP, FSM ,SFHM

  1. What is the difference between hyperthermia and fever?

  2. What are the underlying mechanisms of hyperthermia and fever?

  3. What are the implications of treatment for hyperthermia and fever?

  4. Who is at greatest risk of developing hyperthermia?

  5. What are the lasting effects of prolonged hyperthermia?

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Case 90-1

An 82-year-old man was brought to the emergency department with altered mental status. His neighbor found him unresponsive in his apartment on an extremely hot, humid summer day. He has a history of poorly controlled type 2 diabetes, hypertension, benign prostatic hypertrophy, and urinary urgency. He was currently taking glipizide, lisinopril, hydrochlorothiazide (HCTZ), doxazosin, oxybutynin, and diphenhydramine.

His temperature was 40°C, a pulse of 120 beats per minute, a respiratory rate of 18 breaths per minute, a blood pressure of 90/60 mm Hg, and pulse oximetry of 98% on room air. He responds to a sternal rub, but is otherwise nonresponsive and does not follow commands. His skin is flushed, warm, and dry. His pupils are 4 mm and minimally responsive to light. Bowel sounds are present.

What is the most likely cause of this patient's altered mental status and hyperthermia?

This man most likely has heat stroke, but there are multiple contributing factors in this case. The ambient temperature is extremely hot significantly increasing the risk of heat stroke. Patients his age do not sense changes in temperature as well as younger adults. This man also takes oxybutynin and diphenhydramine, two medications with anticholinergic properties that make him susceptible to anticholinergic poisoning as well as lower his threshold for heat stroke. Although anticholinergic toxicity is possible, his lack of mydriasis and present bowel sounds suggest that this is not the primary contributing factor. Uncontrolled diabetes and HCTZ have also likely contributed to this man being chronically volume depleted, further lowering his threshold for heat stroke.

Vital signs are routinely measured for all hospitalized patients on admission, during nursing shifts, and when infusions are being administered. Clinicians should be able to recognize when abnormal temperatures require immediate action to avoid adverse consequences that may be potentially life threatening.

The 99th percentile for healthy individuals defines the maximum oral temperature as 37.2°C (98.9°F) at 6 am and 37.7°C (99.9°F) at 4 pm. The normal daily temperature typically varies no more than 0.5°C (0.9°F). The hypothalamus thermoregulatory center maintains a normal temperature despite variations in environment causing heat dissipation from the skin and lungs balanced by metabolic activity from muscle and liver. The postprandial state, pregnancy, and endocrine disorders may affect body temperature. The morning temperature tends to be lower in the 2 weeks prior to ovulation in menstruating women and then rises by 0.6°C (1.6°F) with ovulation until the next period (Table 90-1).

Table 90-1 Body Temperature Measurements
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Table 90-1 Body Temperature Measurements
Type of readingNormal 36.0°–38.0°CCore body temperature ...

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