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614 S P E C I A LT Y P R A C T I C E I N C R I T I C A L C A R E

TABLE 22.13 Physiological effects of hypothermia 191-197

Degree of hypothermia Mild (32–35°C) Moderate (28–32°C) Severe (<28°C)
General metabolic Shivering Raised oxygen consumption Normal metabolic functions fail
Raised oxygen consumption Acidosis
Hyperkalaemia
Cardiac Vasoconstriction Atrial arrhythmias Ventricular arrhythmias
Tachycardia Bradycardia Decreased cardiac output
Increased cardiac output
Respiratory Tachypnoea Decreased respiratory drive Apnoea
Bronchospasm
Neurological Confusion Lowered level of consciousness Coma
Hyperreflexia Hyporeflexia Absent reflexes
Coagulation Platelet dysfunction Increased haematocrit Lower bleeding times due to
Impaired clotting enzyme function failure of clotting systems
Increased blood viscosity

persistent cardiovascular compromise may require ino- Ambient temperatures need not be particularly low, as
tropic support in conjunction with invasive haemody- other contributing factors such as wind may be signifi-
namic monitoring. 185-189 cant. A patient with a decreased LOC may present with
191
hypothermia after lying on a cool surface. As a person’s
Patients presenting with associated high-impact or
shallow-diving mechanisms should have cervical spine core temperature drops, progressive cardiac abnormali-
immobilisation instituted with the application of a rigid ties occur; normal sinus rhythm may progress to sinus
cervical collar, especially for complaints of neck pain or bradycardia, T wave inversion, prolonged P–R and Q–T
191
an altered level of consciousness (see Chapter 17). The intervals, atrial fibrillation and ventricular fibrillation.
management of hypothermia and re-warming methods A QRS abnormality, the Osborn wave (positive deflection
outlined below are appropriate for the management of at the junction of the QRS and ST segment), is frequently
194
near-drowning. described as being characteristic of cold injury.
Metabolic acidosis and blood-clotting abnormalities are
HYPOTHERMIA common, and hypoglycaemia (depletion of glycogen
stores caused by excessive shivering) or hyperglycaemia
DESCRIPTION AND INCIDENCE (inhibition of insulin action due to the lowered tempera-
Cold injury is a common problem in Australia and New ture) may occur. 191-197 The physiological alterations that
Zealand, despite the relatively warm weather zones in the accompany lowering of core temperature to below 30°C
former. The very young and very old are most susceptible are summarised in Table 22.13.
191
to injury. A normal core temperature of 37°C has a MANAGEMENT
variation of 1–2°C. Temperature maintenance is essential
for normal homeostatic functioning, and normal adap- A patient with severe hypothermia may appear dead:
tive mechanisms can respond to reductions in ambient cold, pale, stiff, with no response to external stimulation.
temperature. Hypothermia is a body temperature below Successful resuscitation of patients has occurred at tem-
35°C (measured centrally by oesophageal or rectal peratures as low as 17°C, due to the low body tempera-
probe), and occurs with exposure to low ambient tem- ture protecting vital organs from hypoxic injury. 192-196 This
peratures that are influenced by low environmental tem- is reflected in the anecdotal phrase, ‘patients are not dead
peratures, humidity, wind velocity, extended exposure until they are warm and dead’. 193 In most cases, therefore,
time or cold water immersion. 191-193 resuscitation should continue until the patient’s core
temperature reaches 30°C. 192-196
CLINICAL MANIFESTATIONS If a patient’s core temperature is below 32°C, ‘core
When skin temperature is reduced after exposure to the rewarming’ is indicated. This approach is favoured, as
cold, sympathetic stimulation occurs causing peripheral experimental evidence indicates that return to normal
vasoconstriction, decreased skin circulation and shunting cardiovascular function is more rapid with temperature
of blood centrally to vital organs. Blood pressure, heart rises of up to 7.5°C per hour. 191,192 A number of invasive
rate and respiratory rate rise, and shivering (involuntary internal warming options are available, including perito-
clonic movements of skeletal muscle) stimulates meta- neal dialysis and haemodialysis, although the most effec-
bolic activity to produce heat and blood flow to striated tive of all internal methods is cardiopulmonary bypass,
muscles 191,192 to maintain a normal core temperature. If as it transfers heat at a rate several times faster than
continued exposure to cold occurs these compensatory any other methods available (approximately 7.5°C per
functions fail, and hypothermia results. 191-193 hour). 195 While the technique is efficient, it is obviously