Key Clinical Questions
What is the difference between hyperthermia and fever?
What are the underlying mechanisms of hyperthermia and fever?
What are the implications of treatment for hyperthermia and fever?
Who is at greatest risk of developing hyperthermia?
What are the lasting effects of prolonged hyperthermia?
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, pulse rate was 120 beats/min, respiratory rate was 18 breaths/min, blood pressure was 90/60 mm Hg, and pulse oximetry was 98% on room air. He responds to 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, with multiple contributing factors. The ambient temperature is extremely hot, significantly increasing the risk of heat stroke. Older patients do not sense changes in temperature as well as young adults. He also takes oxybutynin and diphenhydramine, medications with anticholinergic properties that make him susceptible to anticholinergic poisoning and will 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.
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. Body temperature typically varies no more than 0.5°C (0.9°F) throughout the day. The hypothalamic thermoregulatory center maintains a normal temperature in the face of heat dissipation from the skin and lungs and heat generation from metabolic activity in 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 92-1).
TABLE 92-1Body Temperature Measurements |Favorite Table|Download (.pdf) TABLE 92-1 Body Temperature Measurements
|Type of reading ||Normal 36.0-38.0°C ||Core body temperature is tightly regulated between a normal diurnal range of 36.0 and 37.5°C |
|Rectal ||0.4°C (0.7°F) higher than oral readings ||May be 0.2-0.3°C higher than actual core body temperature |
|Oral ||Lower readings due to mouth breathing ||Influenced by eating, drinking, breathing devices, tachypnea, and mouth breathing |
|Tympanic membrane (TM) ||Measures radiant heat from TM and nearby ear canal; ...|