The hypothalamus controls thermoregulation. As afferent receptors from the skin and body core reach the hypothalamus, heat-sensitive neurons from the hypothalamus transmit signals to the autonomic nervous system. Sympathetic vasoconstrictor tone is reduced, increasing blood flow to the skin. Sweating soon follows and is mediated via parasympathetic fibers. These physiologic changes can be marked; cutaneous blood flow increases from 0.2 to 0.5 L per minute in a cool environment to 7 to 8 L per minute in a hot environment.
Cardiac output also rises sharply, approximately 3 L per minute for each 1 [degrees] C rise in core body temperature. When cardiac output cannot rise enough to maintain blood pressure (because of dilated cutaneous blood vessels), the body responds by increasing peripheral vascular resistance. This vasoconstriction restores blood pressure but severely impairs heat loss.
When cooling mechanisms fail and core temperature rises, metabolic demand is at a maximum and energy stores are quickly depleted. Cellular enzyme systems fail above 42 [degrees] C (107.6 [degrees] F). Cell membrane permeability progressively increases. Further heat insult denatures proteins, leading to organ and system failure.
Types of Heat Conditions
Heat cramps are painful muscle cramps that can occur on their own or with other heat-related illness such as heat exhaustion. Lay the person in the shade, remove outer clothing, provide cool water and fan vigorously to increase evaporation.
Heat exhaustion is a serious condition that can develop into heat stroke. It is sometimes suffered by people new to Queensland’s hot climate. A person with heat exhaustion may complain of weakness, nausea and/or giddiness, may look pale and be breathless. The skin is usually wet from sweating. Lay the person in the shade, remove outer clothing, provide cool water and fan vigorously to increase evaporation.
Heat stroke is a medical emergency, caused by a rise in core body temperature. A person suffering heat stroke becomes confused, and may stagger or collapse. The skin may be either dry or wet. Call an ambulance and apply urgent first aid. Remove outer clothing, wet the skin and fan vigorously to increase evaporation.
Prickly heat is an intense, itchy red skin rash. It is caused by a blockage of the sweat ducts from prolonged wetting of the skin. Treat by keeping the skin cool and dry, wearing suitable clothing and avoiding hot work.
Heat fainting occurs when blood vessels (particularly in the legs) dilate in order to increase heat transfer to the skin and cause reduced return blood flow to the heart. This response temporarily reduces blood flow to the brain, which can cause a person to faint. If a person faints, lay him/her in the shade, remove outer clothing, provide cool water and fan vigorously to increase evaporation.
Risks for Heat Emergencies
Several factors often coexist to produce a heat emergency. Increases in body temperature associated with dehydration are most evident when the level of dehydration exceeds 3 percent of body weight. Under extremely hot conditions, vigorous physical activity can result in sweat loss of more than 2.5 L per hour. Unreplaced daily fluid losses are cumulative. Workers often consume diuretic beverages, such as coffee or tea, while wearing hot protective suits. They may not become thirsty until they are 2 to 5 percent dehydrated and already prone to heat injury.
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