Hypothermia and hyperthermia

Chapter 32 Hypothermia and hyperthermia



David Lewis-Driver


Although Canadians write more about hypothermia and Saudis write more about hyperthermia, in fact neither condition is rare in Australia. Heat waves and fun runs occur every year in every Australian city. At the opposite end of the spectrum hypothermia is a regular accompaniment to injury and disease throughout the year, and can occur in summer—for example when nursing home patients are left scantily clothed under the air conditioner to cool them. It is important too to remember that the average multiple trauma patient in any country will become hypothermic unless specific preventive steps are taken.



PHYSIOLOGY


Temperature control requires a functioning hypothalamic centre, adequate cardiovascular function to be able to dissipate or conserve heat, adequate muscle bulk to generate heat, intact skin and sweat glands, and sufficient commonsense and mobility to escape the heat or cold. All of these may be affected by disease, trauma or environmental factors.


In both hyperthermia and hypothermia, skin circulation is a problem. Overheating cold skin induces vasodilation which feels good but may overwhelm the pumping capacity of a cold heart, resulting in ‘rewarming shock’. Ice on hot skin induces vasoconstriction, which may limit heat transfer.



HYPOTHERMIA


This condition is defined as a core temperature < 35°C. It is classified in Table 32.1.


Table 32.1 Classification of hypothermia



















Temperature (°C) Grade Signs
35–32 Mild Shivering/apathy
32–28 Moderate Confusion ↓PR ↓BP
< 28 Severe CV failure


Physiology


The body responds to a fall in temperature by attempting to seek warmth and by shivering. If these efforts fail because of environmental factors or drugs, injury or disease and the temperature falls, an initial rise in respiration, pulse and blood pressure is followed by a gradual slowing of all body systems, with death due to cardiovascular failure. CNS slowing results in apathy and confusion, and cardiovascular system (CVS) slowing proceeds to death in asystole, unless an irritable heart is jolted into ventricular fibrillation (VF). In general, slow warming and gentle handling are appropriate. Vigorous attempts at external rewarming can result in skin burns or death from ‘rewarming shock’, an ill-understood phenomenon that can best be considered as shunting of needed blood to the surface while the core is still too cold to cope with the demand. Core temperature lags behind surface temperature during rewarming, and healthy volunteers start to feel better when rewarmed while their core temperature is still falling due to continued cold penetration. An ‘undressing phenomenon’ is also well described, where failure of skin vasoconstriction as a terminal event allows return of skin circulation and leads to a sensation of warmth so that victims are found to have undressed themselves as a last act before death.



Diagnosis and differential diagnosis


If temperature is not a routine observation on every patient, ensure it is taken in those patients who are potential hypothermia candidates, as outlined in Table 32.2.


Table 32.2 Conditions associated with hypothermia















































Conditions associated with accidental hypothermia
Trauma that limits protective mechanisms, e.g. neck of femur (NOF)
Overdose
Alcoholism
Conditions that may cause hypothermia
Sepsis
Myxoedema or adrenal insufficiency
Parkinsonism (failure to shiver)
Wernicke’s encephalopathy
Drugs, e.g. phenothiazines; beta-blockers; clozapine; sedatives
Hypoglycaemia; diabetic ketoacidosis (affect the thermostat)
Pancreatitis
Myocardial infarction or other cause of low CO
Malnutrition/anorexia
Burns; extensive skin rashes (excessive heat loss)
Conditions that hypothermia may be mistaken for
Cerebrovascular accident (CVA)
Dementia; confusion in the elderly; delirium
Hypoglycaemia
Myocardial ischaemia
Drunk and disorderly
Myxoedema

If the temperature is < 36°C on a standard instrument, use a low reading thermometer. The tympanic thermometers commonly used in emergency departments will generally read down to 26°C and are thus adequate to suggest the diagnosis. However, they are not accurate enough to guide treatment. Use an electronic probe—rectal in an awake patient and oesophageal in an intubated patient.



Investigations and management



Mild hypothermia (35–32°C)



1. Stop further heat loss. The most powerful way to prevent heat loss in any emergency department patient is to stem conduction and radiation loss—i.e. remove wet clothes and provide a blanket or space blanket. This is also called passive external rewarming, because prevention of heat loss allows the patient’s own metabolism to raise body temperature.

2. Treat the underlying cause (Table 32.2).

3. Monitor temperature. A rise of above 0.5°C per hour is acceptable.

4. Apply a Bair Hugger, if available. This method of active external warming using hot air is commonly available and unlikely to cause rewarming shock or burns. Active warming, however, is not vital unless the patient is incapable of generating heat because of systemic disease or a condition (e.g. CVA) that has reset the temperature centre. If a Bair Hugger is used, do not exceed a rise of 2°C/h. It is probably better to leave the arms exposed because the main risk is that sequestered cold blood will be restored to the circulating pool by skin vasodilation, causing an afterdrop in the core temperature. This risk is less from the trunk than the limbs.

5. Give O2 and monitor O2 saturation. If the saturation monitor will not read because of vasoconstriction, warm the finger or even do a digital block to overcome vasospasm.

6. Set up IV access and take blood for:
• Blood sugar level (BSL). Physiologically, hypothermia causes firstly a rise in BSL due to catecholamine-induced glycogenolysis, then reduced insulin secretion. Finally (below 30°C) insulin stops working. But there is a high association with hypoglycaemia in alcoholics, and elderly or malnourished patients may have exhausted glycogen stores by the increased metabolism of shivering. Treat hypoglycaemia but not hyperglycaemia. If rewarming does not correct hyperglycaemia, exclude haemorrhagic pancreatitis (a complication of hypothermia) or diabetic ketoacidosis (a cause of hypothermia).

• Urea and electrolytes. Fluid shifts and renal dysfunction mean that Na and K can go in either direction. Minor changes in the initial results do not require treatment and can simply be observed during rewarming. Significant changes should be treated and may be a clue to the underlying cause of the hypothermia.

• Lipase. Pancreatitis is both a cause and a result of hypothermia.

• Full blood examination (FBE). Expect the haematocrit to be high (due to cold diuresis—an attempt to compensate for central fluid overload due to peripheral vasoconstriction) and the WCC and platelets to be normal or low (due to sequestration). A normal Hb/Ht may indicate anaemia and a normal WCC does not rule out infection. Thus the main reason for doing the tests is that a high WCC increases suspicion of an underlying disease process.

• Troponin. Patients who become hypothermic during surgery have a higher incidence of myocardial events. Subendocardial infarctions have been found at autopsy in the absence of ECG changes. In elderly patients, repeat troponin after rewarming.

• CK. May be a clue to rhabdomyolysis in overdose or injury.

Related posts:

Default ThumbnailOrthopaedic principles: fractures and dislocations Ultrasound in emergency medicine Envenomation Infectious diseases Neurosurgical emergencies Dermatological presentations to emergency

Stay updated, free articles. Join our Telegram channel
Tags:
Jun 14, 2016 | Posted by in EMERGENCY MEDICINE | Comments Off on Hypothermia and hyperthermia

Full access? Get Clinical Tree
Get Clinical Tree app for offline access