Hypothermia

Hypothermia is defined as a core body temperature below 35 degrees centigrade.

• The drop in core temperature may be rapid, as in immersion in near-freezing water, or slow, as in exposure to more temperate environments.
• The effects are proportional to the change in temperature, with metabolic rate reduced by about 10% for every 1 degree C fall in temperature.
• Trauma patients are susceptible to hypothermia and so hypothermia should be considered to be any core temperature below 36 degrees C, and severe hypothermia is any core temperature below 32 degrees centigrade.
• Perioperative hypothermia: During anaesthesia a temperature of <36 degrees C is defined as hypothermia. This may be deliberate (see below), or accidental.
• Therapeutic hypothermia: May be used in the post-resuscitation period, in traumatic brain injury with high intracranial pressure, and in the perioperative setting during various surgical procedures (eg vascular surgery for spinal cord protection and overall neuroprotection) and for various other indications (including ongoing trials with head cooling against whole body cooling, on neonatal encephalopathy).

People most likely to experience hypothermia include:

• Very old or very young
• Chronically ill, especially with cardiovascular disease
• Malnourished
• Exhausted
• Intoxicated with alcohol or drugs
• Mental impairment, e.g. Alzheimer's disease
• Underlying medical conditions, e.g. hypothyroidism, stroke, severe arthritis, Parkinson's disease, trauma, spinal cord injuries, burns

• Low-reading thermometers, preferably oesophageal, are required. Tympanic thermometers are unreliable in low temperature measurement. Check for localised cold injury.
• Hypothermia usually occurs gradually. Common signs include shivering, slurred speech, abnormally slow rate of breathing, cold and pale skin, fatigue, lethargy and apathy. A depressed level of consciousness is the most common feature of hypothermia.
• The patient is cold to touch and appears grey and cyanotic.
• Vital signs (pulse rate, respiratory rate and blood pressure) are variable. Severe depression of respiratory rate and heart rate may result in signs of respiratory and cardiac activity being easily missed.
• Mild hypothermia (32-35 degrees centigrade): lethargy, confusion, shivering, loss of fine motor coordination
• Moderate hypothermia (28-32 degrees centigrade): delirium, slowed reflexes
• Severe hypothermia (below 28 degrees centigrade): very cold skin, unresponsive, coma, difficulty breathing, abnormal heart rhythms

• Cerebrovascular accident
• Drug toxicity: barbiturate, benzodiazepine, cocaine

• Monitor for complications, e.g. blood gases, electrolytes, electrocardiogram monitoring.
• Coagulation studies: disseminated intervascular coagulation may occur.
• Chest x-ray: aspiration pneumonia and pulmonary oedema are common.
• Consider any underlying or associated problems, e.g. CT scan for possible head injury.

• Is directed at rewarming, careful patient monitoring and treatment of complications such as cardiac arrhythmias.
• The patient is given warmed, humidified oxygen, heated intravenous saline and surrounded by warmed blankets or heat lamps.
• Aggressive management of temperature with faster rather than slow rewarming has been shown to improve the outcome.¹

Initial management

• Immediate attention to airway, breathing and circulation. Initiation of cardiopulmonary resuscitation may be required.²
• Administer oxygen via a bag reservoir device.
• Establish intravenous access.
• Prevent heat loss by removing the patient from the cold environment and replacing wet, cold clothing with warm blankets.
• If the person is alert and can easily swallow, then give warm, sweetened, non-alcoholic fluids.

Management in hospital

• The patient should ideally be managed in a critical care setting. Attempts to actively rewarm the patient should not delay transfer to a critical care setting.
• Assess for and treat any associated disorders, e.g. diabetes, sepsis, drug or alcohol ingestion, or occult injuries.
• Blood investigations: full blood count, electrolytes, blood glucose, alcohol, toxins screen, creatinine, amylase and blood cultures.
• Cardiac monitoring: dysrhythmias, changes of hyperkalaemia; J-waves are pathognomonic of hypothermia.
  • Cardiac output falls proportionately to the degree of hypothermia and cardiac irritability begins at about 33 degrees centigrade. Ventricular fibrillation becomes increasingly more common as the temperature falls below 28 degrees centigrade, and at temperatures below 25 degrees centigrade, asystole can occur.
  • Cardiac drugs and defibrillation are not usually effective in the presence of acidosis, hypoxia and hypothermia. These treatments should be reserved until the patient is warmed until at least 28 degrees centigrade.
  • Bretylium tosylate is the only dysrhythmic agent known to be effective.
  • Dopamine is the only inotropic agent known to be effective in the hypothermic patient.
  • Cardiopulmonary bypass has been used in patients with severe hypothermia and for those with ventricular fibrillation.³
• Oxygen:
  • Administer 100% oxygen while the patient is being rewarmed.
  • Arterial blood gases are probably best interpreted uncorrected, i.e. the blood warmed to 37 degrees C, and those values used as guides to administering sodium bicarbonate and adjusting ventilator parameters during rewarming and resuscitation.
• Rewarming technique:
  • Depends on the patient's temperature, response to simple measures and the presence of any injuries.
  • Mild and moderate exposure: passive external rewarming in a warm room using warm blankets, clothing and warmed intravenous fluids.
  • Severe hypothermia: may require core rewarming methods that may include invasive surgical rewarming techniques, e.g. peritoneal lavage, A-V rewarming or cardiopulmonary bypass.
• Determination of death can be very difficult in the hypothermic patient. Patients who appear to have suffered a cardiac arrest or death as a result of hypothermia should not be pronounced dead until they are rewarmed.

• Cardiovascular: cardiac arrhythmias, hypotension (due to marked vasodilatation when rewarming), intravascular thrombosis
• Respiratory: pneumonia, pulmonary oedema
• Abdominal: pancreatitis, peritonitis, gastrointestinal bleeding, acute tubular necrosis
• Metabolic acidosis, hyperkalaemia
• Gangrene
• Severe hypothermia eventually leads to cardiac and respiratory failure, then death

• The prognosis depends on the severity and nature of the cause.
• Most people tolerate mild hypothermia, which is not associated with significant morbidity or mortality.
• Mortality is in the order of 20% in cases of moderate hypothermia.

• It will take more than simple education to reduce the estimated 30,000 deaths a year in the UK due to cold. Age concern estimates 8,000 more elderly will die for every time the temperature drops 1 degree centigrade below average.
• Carer and good neighbour surveillance is essential.
• Heating and insulation grants can make a difference if easy to apply for, but the extra heating allowance is only paid retrospectively.
• Hypothermia is not confined to the elderly, nearly every year a party of schoolchildren is rescued from the moors suffering from hypothermia. Even in summer, wet clothing (increases heat loss by 5-10 times) and wind can result in rapid loss of body heat, for the water to evaporate from the clothing it needs to absorb the latent heat of evaporation, 2.26 KJ/ml.
• To reduce the risk of hypothermia:
  • Avoid excessive alcohol consumption
  • Wear a hat or other protective covering to prevent body heat from escaping from your head, face and neck. Cover hands with mittens instead of gloves. Mittens are more effective than gloves because mittens keep the fingers in closer contact with one another.
  • Avoid activities that cause excessive sweating.
  • Wear loose fitting, layered, lightweight clothing. Outer clothing made of tightly woven, water-repellent material is best for wind protection. Wool, silk or polypropylene inner layers hold more body heat than cotton.
  • Stay as dry as possible