Thermoregulatory disorders and illness related to heat and cold stress

Abstract

Thermoregulation is a vital function of the autonomic nervous system in response to cold and heat stress. Thermoregulatory physiology sustains health by keeping body core temperature within a degree or two of 37°C, which enables normal cellular function. Heat production and dissipation are dependent on a coordinated set of autonomic responses. The clinical detection of thermoregulatory impairment provides important diagnostic and localizing information in the evaluation of disorders that impair thermoregulatory pathways, including autonomic neuropathies and ganglionopathies. Failure of neural thermoregulatory mechanisms or exposure to extreme or sustained temperatures that overwhelm the body's thermoregulatory capacity can also result in potentially life-threatening departures from normothermia. Hypothermia, defined as a core temperature of <35.0°C, may present with shivering, respiratory depression, cardiac dysrhythmias, impaired mental function, mydriasis, hypotension, and muscle dysfunction, which can progress to cardiac arrest or coma. Management includes warming measures, hydration, and cardiovascular support. Deaths from hypothermia are twice as frequent as deaths from hyperthermia. Hyperthermia, defined as a core temperature of >40.5°C, may present with sweating, flushing, tachycardia, fatigue, lightheadedness, headache, and paresthesia, progressing to weakness, muscle cramps, oliguria, nausea, agitation, hypotension, syncope, confusion, delirium, seizures, and coma. Mental status changes and core temperature distinguish potentially fatal heat stroke from heat exhaustion. Management requires the immediate reduction of core temperature. Ice water immersion has been shown to be superior to alternative cooling measures. Avoidance of thermal risk and early recognition of cold or heat stress are the cornerstones of preventive therapy.

Human Physiology in Extreme Heat and Cold

“…If neural thermoregulatory mechanisms fail or if exposure to extreme temperatures is sustained, the thermoregulatory capacity will fail sooner or later leading to body temperatures far from normothermia and finally, to life-threatening situations because of the loss of correct cellular function and their damage [1].…”

“…Some authors define it as an elevated core temperature of 40.5 °C or more because of a failure of thermoregulation due to an increase in heat production that overwhelms the heat dissipating mechanisms [1,10]. Other authors, such as Bouchama & Knochel [8], define hyperthermia as "a rise in body temperature above the hypothalamic set point when heat-dissipating mechanisms are impaired (by drugs or disease) or overwhelmed by external (environmental or induced) or internal (metabolic) heat" [8].…”

“…Although both definitions can be considered complementary, we will be using the former definition since it can be clinically demonstrated. According to Cheshire, it is clinically manifested by "sweating, flushing, tachycardia, fatigue, light-headedness, headache, paraesthesia, muscle cramps, oliguria, nausea, agitation, hypotension, syncope, confusion, delirium, seizures, and finally, coma" [1].…”

“…This internal organ temperature is referred as body core temperature (T c ). Although measurement of the pulmonary artery temperature through a thermistor seems to be the gold standard, rectal thermometry has evolved as the clinical standard method for determining core temperature [1]. From now on, body temperature and body core temperature will be used indifferently and when referring to temperature, we will be referring to ambient temperature, which is defined as the temperature of the surrounding environment, unaltered by direct human intervention [5].…”

“…limits [1][2][3]. Human beings can often tolerate extreme temperatures well above their thermal balance limits for abbreviated exposure times, which may vary depending on the individual's thermoregulatory response robustness and the environmental characteristics.…”