Cold injuries

22.4 Cold injuries



Simon Chu, Nicholas Cheng




Essentials



1 Hypothermia is defined as a core body temperature less than 35°C.

2 Cold injury is uncommon in Australia, but when cold injuries do occur it is more significant in children than in adults.

3 The major complications of hypothermia are altered mentation and arrhythmias.

4 Hypothermia is treated with passive external, active external, active internal, and/or extracorporeal rewarming, depending on severity. In addition, hypothermia should be rigorously prevented in any patient with a severe illness.

5 The major complications of rewarming are an ‘after-drop’ in temperature and vasodilatory shock.

6 Severe local cold injuries are treated with rapid immersion rewarming, then care given as for burns.

7 In Australia patients are almost always dead, then cold. Situations where patients become cold enough to apply ‘warm and dead’ criteria are very rare.


Introduction


Cold injury is uncommon in Australia due to its warm climates. However, its significance is more important in children, as they have:


• a larger surface area:weight ratio;1

• underdeveloped behaviour responses to cope with extreme cold (young children can’t put on more clothes).

When cold injuries do occur, they can be subdivided into generalised injury (namely accidental or environmental hypothermia) and localised injury. Cold injury is also a common problem potentially complicating any other severe illness (especially trauma) and must be prevented.2



Normal physiology: a review


Heat production is derived from basal metabolism, digestion, and muscular activity, which may be voluntary (exercise) or involuntary (shivering). Emotional factors and hormonal fluctuations influence heat production. The main mechanisms by which the body compensates for low core body temperature are by increasing its metabolic rate, primarily through shivering, and by shunting blood away from non-essential organs to preserve vital organs. The capacity to shiver is dependent on local glycogen stores and the rate of change of core and external temperature.1,3


Neonates are the patients most prone to hypothermia. They are unable to shiver and have limited stores of energy. Because of this, newborn children utilise catabolism of brown fat to generate heat. This is an inefficient process that consumes oxygen, thus exacerbating hypoxia. In addition, the large surface area to weight ratio, due to a relatively large head, contributes to heat loss. At birth, neonates are covered in amniotic fluid, and evaporative losses are significant. An overhead radiant heater is not adequate to compensate for this evaporative loss.4,5


Heat loss from the human body is by four methods:


• radiation;

• conduction;

• convection;

• evaporation.1

Radiation occurs when heat energy leaves the skin at the speed of light. Patients with more fat become more hypothermic than thinner patients, due to the former’s larger surface area for radiation heat loss. In children, who have a higher surface area to weight ratio, it accounts for up to 50% of all heat loss; indeed, up to 75% in neonates. This higher number in neonates is due to a proportionally larger head increasing the surface area:weight ratio.1,4 Radiation losses decrease when a patient is clothed.


Conduction of heat is poor in air and therefore does not contribute much to hypothermia in normal circumstances. However, water-conductive heat loss is 24 times more than that of air. It is this method by which patients suffering from water immersion become profoundly hypothermic, and how patients in wet clothes become hypothermic quickly. The surface on which the patient is lying also contributes to conduction heat loss. For instance, a patient lying on snow is likely to become more hypothermic than a person lying on sand.


Convection occurs as warm air next to the skin is replaced by cool air. This can contribute to 25% of total body heat loss in still air. In a wind of 63 km hr–1, this increases by 14 times. This is described as the ‘wind chill factor’.


Evaporation from the skin accounts for only 7% of heat loss at rest. This may be increased in cold, dry conditions and by sweating. Evaporation from the respiratory tract removes another 7%. This can increase by breathing faster (such as at high altitude or during exercise).


Temperature is perceived through central and peripheral mechanisms. Heat sensors in the central hypothalamus receive input from the skin, central arteries, and viscera. It is this central thermostat that is reset, which causes fever. Skin receptors respond to a change in skin temperature but do not themselves indicate the patient’s core temperature. A result of all this input is that the body responds by those autonomic reflexes listed below to increase or decrease core body temperature.3



Hypothermia


This is defined as a core temperature of <35°C.1 Hypothermia is classified on the basis of severity. The reason for this classification is that it influences the rewarming mechanisms that are most often deployed. It is also related to the physiological ability of the patient to compensate for hypothermia. An easy way to remember these temperature ranges is:


• acceptable low temperature: 2°C below 37°C (>35°C);

• true mild hypothermia: 3°C below that (32–35°C);

• moderate hypothermia: 4°C below that (28–32°C);

• severe hypothermia: anything below that (<28°C).

Tables 22.4.1 and 22.4.2 show the main consequences of hypothermia at a given temperature.1,3,6 Much of our understanding of this pathophysiology comes from controlled hypothermia in cardiac surgery. Note that there is a huge variation of the onset of certain clinical signs based on temperature level. For instance, some patients may exhibit confusion at higher temperatures compared with others. Note that in children clinical manifestations of altered consciousness may be subtle.












Table 22.4.1 Compensatory mechanisms at different severities of hypothermia
Mild (32–35°C) Moderate (28–32°C) Severe (<28°C)

Increased basal metabolic rate by shivering

Vasoconstriction peripherally, leading to fluid shift

Mild tachycardia

Cold diuresis (see text)

Apathy, ataxia, amnesia, dysarthria
Limits of increasing basal metabolic rate reached

Shivering stops

Decreasing cerebral blood flow, causing delirium and gradual decreased level of consciousness

Decreased rate of neural impulse transmission, causing clumsiness and numbness

Muscle rigidity due to increasing acidosis
Complete loss of thermoregulation

Stupor, coma

Pulseless

Fixed dilated pupils

Absent reflexes

Dysrhythmias, initially slow atrial fibrillation then ventricular fibrillation

Appearance of death at < 25°C with asystole

Falling blood pressure

See text for more specific changes





















Table 22.4.2 Findings at low body temperatures
Temperature (°C) Findings
27 Reflexes absent, no response to pain, comatose
25 Cerebral blood flow one-third of normal, cardiac output one-half of normal
23 No corneal reflex, ventricular fibrillation risk is maximal
19 Asystole, flat EEG
15 Lowest temperature survived from accidental hypothermia

Note that only during severe hypothermia does protection from hypoxia occur, due to decreased demand for oxygen by tissues, and even then only at extremely low temperatures (patients <20°C can tolerate anoxia for up to 60 minutes). Metabolic processes slow by approximately 6% for each 1°C drop in body temperature.1 Thus at 28°C the basal metabolic rate is about 50% of normal. This leads to hypoventilation and hypoxia. However, at this temperature the decreased cellular metabolism affords some protection against hypoxia.


Cold diuresis is an initial brisk diuresis; this is due to decreased tubular reabsorption and also a decreased production of antidiuretic hormone. There is also an increased central blood circulating volume as blood is shunted away from the periphery, thus presenting the kidneys with an apparent increased blood volume for filtration.1



History


Important points in history are:


• approximate time of exposure, if known;

• environment in which patient was found;

• resuscitation at the scene, including duration of time with no cardiac output (‘downtime’);

• pre-existent illnesses (e.g. thyroid disease);

• drugs, medications, allergies, and immunisation status.


Examination



• Full primary and secondary assessment of patient.

• Accurate measurement of core body temperature.


Diagnosis


This requires only two essentials:1


• a thermometer able to record low core temperatures accurately;

• a high index of suspicion.

Core temperatures can be measured best with oesophageal or rectal probes. The most direct method of measurement is with a cardiac catheter such as a Swan–Ganz, but this is impractical in the emergency setting. Rectal probes are often used,7 but care must be taken when using these. The probe must be at least 10 cm into the rectum in older children (more than 8 years old) and 5 cm in younger children. Inaccuracies may occur due to the presence of faecal material,1 and the probe must be left in until the temperature equilibrates. Tympanic measurements are well known to be unreliable in the very young,7 but they are a good indicator of therapy progression in the older child. Oral and axillary temperature probes are unreliable and impractical in the setting of true hypothermia.



Treatment



Pre-hospital treatment


This is mainly the realm of passive external rewarming methods (see below). Patients should be carefully removed from the precipitant cold environment to a dry, sheltered area. If clothes are wet, they should be removed, and the patient dried and covered with a warm dry blanket. All patients should be gently handled, especially during transport, as there is evidence that sudden movements to a body in severe hypothermia can precipitate arrhythmias, particularly ventricular fibrillation.6 While this is occurring, one should attend to the patient’s airway, breathing, and circulation, as per any resuscitation.


Active rewarming should be avoided until the patient reaches the emergency department. This is because of the complications of rewarming, namely ‘after-drop’ and shock.1,6,8



Treatment in the emergency department


Once in the emergency department, the patient should be triaged to an appropriate area, which is warm. In the very young, a radiant warmer bed and heating lamps should be available when the patient presents.4,9 Patients should have their airway, breathing, and circulation reassessed and appropriate resuscitation commenced. Appropriate monitoring should be instituted, including electrocardiogram (ECG), and core temperature, either by rectal or oesophageal means. Oxygen saturation monitoring should be attempted, whilst understanding that initial vasoconstriction will give inadequate readings. Urine output should be monitored. Gentle handling should be continued to avoid precipitation of arrhythmias.6 Patients should continue 100% oxygen on arrival in emergency.


Blood tests taken should include arterial blood gas (ABG); full blood count; electrolyte, urea, creatinine (EUC); liver function tests; amylase; comprehensive metabolic panel (CMP); glucose; thyroid function tests (TFTs); coagulations; tests for infection; and, if suspected, a screen for sedative drugs and ethanol. Hypothermia causes measured pH to fall and pO2 and pCO2 to be higher. It is recommended that these ABG values should not be corrected for temperature to better reflect the physiological state of the patient.6


A 12-lead ECG should be taken. Classic changes include the presence of a J (Osborn) wave, interval (PR, QRS, QT) prolongation, atrial dysrhythmias, and ventricular dysrhythmias.10 Other less well-known changes include abnormalities similar to myocardial infarction. Hypothermia can also blunt the ECG changes with hyperkalaemia.6 Note that all these are not present in all hypothermic patients.


Once temperature is measured, the severity of hypothermia determines the methods of rewarming. Specific methods of rewarming are classically divided into four categories:6,8,11


• passive external rewarming;

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Sep 7, 2016 | Posted by in EMERGENCY MEDICINE | Comments Off on Cold injuries

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