Pathophysiology: The role of the hypothalamus
Humans maintain body temperature by balancing heat production and heat loss to the environment. Heat is lost through the skin and lungs by 5 different mechanisms: radiation, conduction, convection, evaporation, and respiration. Convective heat loss to cold air and conductive heat loss to water are the most common mechanisms of accidental hypothermia.7
To maintain temperature homeostasis at 37°C (98.6°F) (±0.5°C [±0.9°F]), the hypothalamus receives input from central and peripheral thermal receptors and stimulates heat production through shivering, increasing the basal metabolic rate 2-fold to 5-fold.1 The hypothalamus also increases thyroid, catecholamine, and adrenal activity to increase the body’s production of heat and raise core temperature.
Heat conservation occurs by activation of sympathetically mediated vasoconstriction, reducing conduction to the skin, where cooling is greatest. After time, temperature regulation in the body becomes overwhelmed and catecholamine levels return to a pre-hypothermic state.
At 35°C (95°F), neurologic function begins to decline; at 32°C (89.6°F), metabolism, ventilation, and cardiac output decrease until shivering ceases. Changes in peripheral blood flow can create a false warming sensation, causing a person to remove clothing, a phenomenon referred to as paradoxical undressing. As hypothermia progresses, the neurologic, respiratory, and cardiac systems continue to slow until there is eventual cardiorespiratory failure.
Assessment and diagnosis
History and physical examination. A high index of suspicion for the diagnosis of hypothermia is essential, especially when caring for the elderly or patients presenting with unexplained illness. Often, symptoms of a primary condition may overshadow those reflecting hypothermia. In a multicenter survey that reviewed 428 cases of accidental hypothermia in the United States, 44% of patients had an underlying illness; 18%, coexisting infection; 19%, trauma; and 6%, overdose.3
There are no strict diagnostic criteria for hypothermia other than a core body temperature <35°C (<95°F). Standard thermometers often do not read below 34.4°C (93.2°F), so it is recommended that a rectal thermometer capable of reading low body temperatures be used for accurate measurement.
Hypothermic patients can exhibit a variety of symptoms, depending on the degree of decrease in core body temperature1:
- A mildly hypothermic patient might present with any combination of tachypnea, tachycardia, ataxia, impaired judgment, shivering, and vasoconstriction.
- Moderate hypothermia typically manifests as a decreased heart rate, decreased blood pressure, decreased level of consciousness, decreased respiratory effort, dilated pupils, extinction of shivering, and hyporeflexia. Cardiac abnormalities, such as atrial fibrillation and junctional bradycardia, may be seen in moderate hypothermia.
- Severe hypothermia presents with apnea, coma, nonreactive pupils, oliguria, areflexia, hypotension, bradycardia, and continued cardiac abnormalities, such as ventricular arrhythmias and asystole.