Heat illness
Definition
"Heat Illness" means a serious medical condition resulting from the body's inability to cope
with a particular heat load, and includes heat cramps, heat exhaustion, heat syncope and heat
stroke. Heat illness is characterized by the loss of water and salt, the sweat gland dysfunction,
and the thermoregulatory center dysfunction. Heat illness can be divided into three categories
according to the severity of the clinical manifestation: 1.heat cramps, 2.heat exhaustion,
3.heat stroke. Heat stroke can be divided into exertional heat stroke and classis heat stroke.
Exertional heat stroke is mainly caused by the increase of heat production while the
non-exertional heat stroke is caused by the reduction of heat dissipation.
Etiology
The main cause of heat illness is the poor adaptation of patients to the hot and humid
environment. When the environment temperature is higher than 32℃, and the relative
humidity is more than 60%, previously healthy young person is easy to develop heat illness
when doing strenuous activities without taking proper measures of protection and adequate
adaptation. The causes of developing heat illness can be summarized as follows:
1. Adopting too much heat: high environment temperature ; direct sun or heat;
2. Iincrease of heat production: doing heavy exertion; long time of fever; epilepsy;
hyperthyreosis; some medicines( amfetamine, lysergic acid diethylamide)
3. Disorders of heat loss: high humidity and temperature, obsess, heavy clothing, chronic
heart failure, dehydration, CNS disorders, some medications (phenothiazine,
anti-cholinergic agents, diuretic and propanolol etc.)
4. Dysfunctions of sweat gland: scleroderma, previous burn wounds & scars, cystic fibrosis,
miliaria
5. Hypokalemia
Mechanism
The hypothalamus is the themonregulatory center in control of the heat production and loss.
The normal rectal temperature is fluctuated between 36.9-37.9℃.
1. the regulation of temperature
1.1 Heat production:
The body produces heat from food and from muscles during exercise. Normal metabolism
generates 209 to 251 kJ per hour and would raise the temperature of your body 1.5 degrees
every hour if it were not for the body’s cooling mechanisms. When you exercise heavily or in
hot temperatures, heat production by the body increases five to tenfold. Add to that hot and
humid environmental conditions and it is easy to overheat, a process called hyperthermia.
1.2 Heat loss
Normally, the body reduces heat by sending more blood to the skin where blood
vessels dilate to bring the blood closer to the surface where it can be cooled by the
lower air temperature. This only works when the air temperature is lower than the
body temperature. Sweating is another way the body uses to lower the temperature of
the blood. As the sweat evaporates, it cools the body down. At this rate, body
temperature can normally be regulated.
In the normal temperature(15-25℃) body loss its heat by the following 4 manners: a.
radiation: it’s the main way of heat loss, contribute to 60% of the whole amount of heat loss. b.
evaporation: consist 25% of the heat loss. In hot weather it’s the main manner of heat loss.
Each liter of sweat that is evaporated on the skin removes about 2427lJ of body heat. When
the humidity level is over 75%, evaporation reduced and the body’s ability to cool is
dramatically decreased. When the humidity achieve 90-95%, evaporation ceased absolutely. C.
convection: contribute 12% of heat loss. D. conduction: consist 3% of the heat loss.
2. Acclimatization
The body takes time to acclimatize to temperature. It takes 7-14 days to fully acclimatize to
hot weather. With acclimatization, sweating and cardiac output increase, the sweat glands
secrete less salt, and the heat production reduces. In hot weather, the elderly, mental sick
person, and the comatose person is more likely to develop heat illness. It’s because their
themonregulartory function is damaged.
3. the effect of hyperthermy on the body:
3.1 central nervous system: hyperthermy can lead to the cell death of the brain and spinal
cord, causing brain edeama, cerebral hemorrhage, increase of the intracranial pressure
and coma. The purkinye cells of the cerebellum is sensitive to the hyperthermy, their
damage will lead to ataxia, dysarthrosis, dysmetria.
3.2 Cardiovascular system: In the early stage of heat illness, dehydration and the dilation
of the vessel lead to hypotension. The cardiac output(CO) increase while the metabolism
elevates. CO increase 3L per minute if the rectal temperature elevates 1 ℃.
Hyperthermy will cause myocardial necrosis and myocardial ischemia, arrhythmia, heart
failure, then interfere the loss of body heat.
3.3 Respiratory system: hyperthermy can lead to ARDS.
3.4 Water and electrolyte metabolism: Too much sweating leads to loss of water and
electrolytes. Hypophosphatemia occurs in 20-80% non-exertional heat stroke. In
exertional heat illness patient , hyperphosphatemia and hyperkalemia are found due to
the severe impairment of the muscle. The impairment of cells caused by hyperthermy
leads to hypocalcemia at first. Hypercalcemia occurs 2-3 weeks later, the possible reason
is the activation of PTH.
3.5 Kidney: In the exertional and non-exertional heat stroke patient, the incidence of
developing acute renal failure is about 35% and 5% respectively. The development of
acute tubular necrosis is due to dehydration, hypoperfusion, rhabdomyolysis.
3.6 Digestive system: Hyperthermia and the secondary hypoperfution will lead to the
ischemic bowel ulcer, hence causing hemorrhage. Almost every patient develops hepatic
necrosis and cholestasis.
3.7 Hemotologic system: The secretion of catecholamine and the blood concentration will
cause leukocytosis, Thrombosis develop in 24% cases. In severely ill patient, DIC occurs
2-3 days due to the direct toxicity of the hyperthermy to the platelet, the reduction of the
synthesis of coagulation factors and the coagulation reaction activated by the endothelial
cell injury and the necrotic cells.
3.8 Endocrine system: In exertional cases, hypoglycemia happens due to increase of energy
exhaustion. While 90% non-exertional patients develop hyperglycemia and elevated
plasma cortisol. In seriously ill patient, the blood aldosterone level and growth hormone
level are elevated.
3.9 Muscle: In exertional stroke patients, muscle injuries and myolysis occurs and the blood
CK level rise dramatically, In non-exertional heat stroke patients, muscular injuries
seldom develop.
Pathology
Necrosis of neurocell in the cerebellum and brain is found especially the Purkinje cells.
Necrosis and bleeding of cardiomyocytes are also found. Necrosis of hepatic cells and
cholerostasis are seen. And degeneration and necrosis of myocytes are seen in exertional heat
stroke cases.
Clinical appearance
Heat cramps typically occur in hot weather when the exercising muscle (most often the legs)
contract painfully without reflex inhibition from the antagonist muscles. Heat cramps are
related to the excessive loss of the sodium, patients usually have a history of profuse sweating
without adequate fluid replacement. Cramps often begin when the individual is resting after
exercise and present as severe pain and spasms of the calf, thigh, abdomen, or hand muscles.
Heat exhaustion is also caused by severe dehydration and electrolyte loss. It’s the most
common form of heat illness. If not properly handled, it will deteriorate to the life-threatening
heat stroke. In the young, heat exhaustion usually occurs after strenuous activity in the hot
and humid weather without acclimatization. While in elderly the problem is related to
inadequate response of the heart and cardiovascular system to heat. Patients often complain of
headache, giddiness, anorexia, nausea, vomiting, malaise, thirst, muscle cramping. Physical
examination may reveal some evidences of dehydration: tachycardia, orthostatic hypotension,
the core body temperature may rise to 38-39℃. Mental status is usually normal, although
there may be minor confusion or agitation.
In laboratory examination, evidence of dehydration such as increased BUN, serum sodium,
hematocrit may also be noted, mild to modest rises in creatinine kinase and hepatic enzymes
may be found as well, hypoglycemia is occasionally revealed.
Heat Stroke: As heat exhaustion progresses, the body’s cooling system completely breaks
down and the blood and organs overheat. Known as heat stroke, this is a true medical
emergency that has an 80 percent chance of death if not treated. The difference between heat
exhaustion and heat stroke is technically the presence of tissue injury, something that is
difficult at best to tell in pre-hospital situation. For practical purposes, anyone who has
symptoms of heat exhaustion and abnormal changes in their mental state should be considered
to have heat stroke.
Heat stroke can be classified into classic heat stroke and exertional heat stroke. Classic heat
stroke occurs in infants and ill or elderly patients, usually develops over a periods of several
days, in hot and humid weather. The patient with exertional heat stroke usually is a healthy
young, who develops symptoms in a matter of hours after exertion heavily.
The symptoms of heat stroke can be manifested as the depression of central nervous system
such as headache, unsteadiness, bizarre behavior, piloerection, paresthesias, syncope, seizures,
coma, and other symptoms, i.e. fever, (T more than 40℃), nausea et al.
Physical examination may reveal the evidence of dehydration and some neurological signs.
Blood pressure is usually low (check for a weak pulse in the wrist), pulse high (greater than
100 beats per minute), and breathing rapid (over 20 breaths per minute). The skin may be red
and hot, although this does not occur with everyone. Sweating may have stopped or may be
present.
A variety of abnormal laboratory studies are noted in heat stroke patients. Arterial blood gas
analysis may reveal acidosis, due to elevated lactate levels. A leukocytosis is common and
may be as high as 30000-50000. The AST, ALT and LDH are markedly elevated. Creatinine
kinase levels are markedly elevated due to muscle fiber damage. Other abnormalities will also
be noted such as evidence of coagulopathy, abnormality in ECG et al.
Diagnosis and differential diagnosis
Typical heat illness is easy to diagnose according to the history and PE and the season. The
rectal temperature is higher than 41 in most cases. When the heat wave comes, the comatose
patient with high fever should be considered heat illness.
Severely ill patient should be differentiated with severe infection, acute cerebral accident,
thyroid crisis, delirium tremens etc.
Therapy
1. Cooling The core temperature should be cool down to 37.8-38.9 in one hour.
1.1 External cooling methods: The individual should stop all physical activity, rest in a cool,
shaded environment, and remove any heavy or restrictive clothing. Have them drink
plenty of fluids containing small amounts of salt, as you would for dehydration. Apply
steady, gentle massaging-type pressure to the cramped muscle. Cool water can be
splashed on the skin while fanning the individual. Better yet, ice or cold packs can be
placed along the side of the neck, armpits, and groin areas where large blood vessels are
relatively. If available, the individual can be carefully immersed in cool or lukewarm
water. Do not immerse in ice water, which can cause severe constriction of the skin blood
vessels and limit the ability to lose heat. It can also cause shivering, which is the body’s
way to generate more heat—not something you need at this time.
1.2 Internal cooling Gastric or rectal lavage can be done by icy normal saline. Peritoneal
dialysis or hematodialysis by the 9 ℃ or 20℃ of sterile normal saline can also be
applied to cool the body.
1.3 Drug cooling: chloropromazine can be given to halt shivering. It’s added in a dose of 2550mg to the solution of 500ml and infused intravenously in 1-2 hours. The blood
pressure should be monitored.
2. Therapy of the complications
2.1 Coma : Tracheal intubation should be done to avoid aspiration and keep the airway clean
in the comatose patients. Manicol should be given intravenously to treat the brain edema
and to decrease the intracranial pressure. Diazepam can be given to treat epilepsy.
2.2 Arrhythmia, heart failure and metabolic acidosis: Sodium bicarbonate should be given in
the hyperkalemia and metabolic acidosis patients.
2.3 Hypotension: Nomal saline or balanced solution should be given to restore the reduced
blood volume and blood pressure. If necessary, isoprenaline can be used to increase
blood pressure. Vasoconstrictor agents should be avoided for fear that it reduced the heat
loss.
2.4 Hepatic failure and renal failure: Dialyses can be done in renal failure patients. Liver
transplantation can be done in case of hepatic failure. Proton pump inhibitor or H2
receptor inhibitor can be given to prevent upper gastrointestinal hemorrhage.
3. Monitor
3.1 Temperature monitor
3.2 Urine monitor
3.3 Blood gas analysis
3.4 Monitor of coagulational function: PT, APTT, platelet count and fibrinogen should be
monitored.
Prognosis
The total mortality of heat illness is between 20-70%, and reaches 80% in cases older than
50 year old. The body temperature and duration of the fever are also related to the prognosis.
The mortality is 5% if the body temperature cool down to 38.5℃ in 1 hour, and it reaches
18% if the temperature come down more than 1 hour. Other factors in prognosis are the
severity of the injury of the nervous system, liver, kidney and muscles. The blood lactate level
is also of prognosis value. In most patients, the nervous system recovered totally soon after
the restoration of the temperature. Occasionally, residual nervous system disorders occur.
The mild and moderate liver and renal disfuntion can be totally recovered. In severe muscular
injured cases, amyosthenia will exist for months.
Prevention of heat illness
1. Avoiding direct sun shine and tough exertional activities in the hot and humid weather
without fully adaption.
2. Dress for the heat. Wear lightweight, light-colored clothing. Light colors will reflect away
some of the sun's energy. It is also a good idea to wear hats or to use an umbrella.
3. Drink water. Carry water or juice with you and drink continuously even if you do not feel
thirsty. Avoid alcohol and caffeine, which dehydrate the body.
4. Eat small meals and eat more often. Avoid foods that are high in protein which increase
metabolic heat.
5. Avoid using salt tablets unless directed to do so by a physician.
6. Slow down. Avoid strenuous activity. If you must do strenuous activity, do it during the
coolest part of the day, which is usually in the morning between 4:00 a.m. and 7:00 a.m.
7. Stay indoors when possible.
8. Take regular breaks when engaged in physical activity on warm days. Take time out to
find a cool place. If you recognize that you, or someone else, is showing the signals of a
heat-related illness, stop activity and find a cool place. Remember, have fun, but stay
cool.
Reference
1. 内科学 （七年制）人民卫生出版社
2. Cecil Textbook of Medicine，21st edition