Overheating

Heat exposure disrupts many physiological functions and can lead to dehydration. Most people in this situation experience mild but unpleasant symptoms, although in some cases they can range from swelling and cramps to fainting and heat stroke. In some types of heat illness, body temperature increases. With dehydration, tachycardia, tachypnea, and orthostatic hypotension are possible. Dysfunction of the central nervous system indicates the most serious pathology - heat stroke, in which disorientation and drowsiness further reduce the ability to leave the area that has become the source of overheating and begin rehydration.

Cause of overheating

Thermal disorders develop as a result of increased heat intake and decreased heat loss. Clinical manifestations are aggravated by the inability to tolerate increased load on the cardiovascular system, dehydration, electrolyte disturbances, and the use of certain medications. The high-risk group includes children and the elderly, as well as patients with cardiovascular pathology or electrolyte imbalance (for example, when using diuretics).

Excessive heat intake by the body occurs during high loads and/or when the ambient temperature increases. Increased body temperature can also be caused by certain disease states (e.g. hyperthyroidism, neuroleptic malignant syndrome) or the use of stimulant drugs such as amphetamines, cocaine, ecstasy (an amphetamine derivative).

Cooling is impeded by thick clothing (especially protective clothing for workers and athletes), high humidity, obesity, and anything that interferes with the production and evaporation of sweat. Sweat production may be impaired by skin lesions (e.g., prickly heat, extensive psoriasis or eczema, scleroderma) or by the use of anticholinergic drugs (phenothiazines, H2-receptor blockers _, and antiparkinsonian drugs).

Pathophysiology of overheating

The human body receives heat from the external environment and heat generated by metabolism. Heat is released through the skin by radiation, evaporation (e.g., sweating), and convection; the contribution of each of these mechanisms depends on the temperature and humidity of the environment. At room temperature, radiation predominates, but as the ambient temperature approaches body temperature, the importance of convection increases, providing cooling of almost 100% at >35 °C. However, high humidity significantly limits the possibility of convective cooling.

Heat transfer depends on changes in skin blood flow and sweating. The skin blood flow rate, at a normal ambient temperature of 200-250 ml/min, increases to 7-8 l/min under stressful heat exposure, which requires a significant increase in cardiac output. In addition, with an increase in the ambient temperature, sweating increases from insignificant to 2 l/h or more, which can quickly lead to dehydration. Since sweat contains electrolytes, significant losses are possible during hyperthermia. However, with prolonged exposure to high temperatures, adaptive physiological changes (acclimatization) occur in the body, for example, sweat contains Na ⁺ in a concentration of 40 to 100 mEq/l in unadapted people, and after acclimatization its content decreases to 10-70 mEq/l.

The body can maintain normothermia under significant heat loads, but severe or prolonged exposure to high temperatures leads to an increase in core temperature. Moderate hyperthermia of short duration is tolerated, but a marked increase in core temperature (usually >41 °C), especially during heavy work in the heat, leads to protein denaturation and release of inflammatory cytokines (such as tumor necrosis factor-a, IL-1β). This leads to cellular dysfunction, activating a chain of inflammatory reactions leading to functional impairment of most organs and triggering the coagulation cascade. These pathophysiological processes are similar to those in multiple organ failure syndrome that follows prolonged shock.

Compensatory mechanisms include an acute-phase response involving other cytokines that inhibit the inflammatory response (e.g., by stimulating the production of proteins that reduce free radical production and suppress the release of proteolytic enzymes). In addition, elevated body temperature triggers the expression of heat shock proteins. These substances regulate cardiovascular reactions and temporarily increase the body's temperature tolerance, but the mechanism of this process has been poorly studied to date (it is possible that the inhibition of protein denaturation plays a role). With prolonged or sudden increases in body temperature, compensatory mechanisms are disrupted or do not function at all, which leads to inflammation and the development of multiple organ failure.

Preventing overheating

The best prevention is common sense. In hot weather, children and the elderly should not stay in unventilated and unair-conditioned rooms. Children should not be left in a car in the sun. If possible, avoid increased physical activity in high temperatures and unventilated rooms; it is not recommended to wear heavy, heat-insulating clothing.

To monitor dehydration after exercise or heavy work, use the body weight loss indicator. If body weight is reduced by 2-3%, it is necessary to drink an increased amount of fluid so that before the start of the next day's exercise the difference in body weight is within 1 kg of the initial value. If the loss is more than 4% of body weight, physical activity should be limited for 1 day.

If physical exertion in hot weather is unavoidable, fluid (the loss of which is usually imperceptible in very hot and very dry air) should be replaced by frequent drinking, and evaporation should be facilitated by wearing open clothing and using fans. Thirst is a poor indicator of dehydration during intense physical exertion, so regardless of its occurrence, it is necessary to drink every few hours. However, hyperhydration should be avoided: athletes who drink too much fluid during training have significant hyponatremia. Plain water is sufficient to replace fluid losses during maximal physical activity; cool water is better absorbed. Special rehydration solutions (such as sports drinks) are not necessary, but their taste helps to increase the volume of fluid consumed, and moderate salt content is useful when the body's need for fluid is increased. It is recommended to drink water in combination with a richly salted meal. Laborers and other heavily sweating individuals can lose more than 20 g of salt per day through sweat, which increases the likelihood of heat cramps. In this case, the loss of sodium must be compensated for with liquids and food. A pleasant-tasting drink containing about 20 mmol of salt per liter can be prepared by adding a heaping spoon of table salt to 20 liters of water or any soft drink. People on a low-salt diet should increase their salt intake.

With a gradual increase in the duration and severity of loads in the heat, acclimatization eventually occurs, which allows people to work in conditions that were previously unbearable or life-threatening. Increasing work in the hot season from 15 minutes of daily moderate physical activity (sufficient to stimulate sweating) to 1.5 hours of intense load for 10-14 days is usually well tolerated. With adaptation, the volume of sweating (and, therefore, cooling) for a certain period of work increases significantly, and the electrolyte content in sweat decreases noticeably. Acclimatization significantly reduces the risk of developing heat illness.