Hypothermia – Risk Factors and Prevention

Hypothermia is defined as a core body temperature below 35°C. Along with coagulopathy and acidosis, hypothermia belongs to the lethal triad of trauma victims requiring critical care.

The drop in core temperature may be rapid as in immersion in near-freezing water, or slow as in prolonged exposure to more temperate environments. The effects of hypothermia are proportional to the change in temperature, with metabolic rate reduced in proportion to the fall in core body temperature.

Causes and classification

Hypothermia is usually caused by accidental exposure but may be caused or aggravated by underlying medical conditions or may be deliberate as part of patient therapy.

Primary hypothermia

This is due to environmental exposure, with no underlying medical condition causing disruption of temperature regulation:

• Trauma patients are particularly susceptible to hypothermia.
• Perioperative hypothermia:
  • Hypothermia may be deliberate (see below), or accidental.
  • Any patient whose core temperature drops accidentally below 36°C at any stage of the perioperative pathway (from the hour before induction of anesthesia until 24 hours after entry into the recovery area) should be warmed using a forced air warming device.^[2]

Secondary hypothermia

This is low body temperature resulting from a medical illness lowering the temperature set-point:

• Decreased heat production – e.g., hypopituitarism, hypoadrenalism, hypothyroidism, severe malnutrition, hypoglycaemia and neuromuscular disorders.
• Increased heat loss – e.g., vasodilatation (pharmacologic or toxicologic causes), erythrodermas, burns, psoriasis; or iatrogenic – e.g., cold infusions, over-enthusiastic treatment of heatstroke or emergency deliveries.
• Impaired thermoregulation – e.g., trauma affecting the central nervous system, strokes, toxicologic and metabolic derangements, intracranial bleeding, Parkinson’s disease, brain tumors, Wernicke’s disease, multiple sclerosis, sepsis, multiple trauma, pancreatitis, prolonged cardiac arrest, and uraemia.
• Drug administration; such medications include beta-blockers, clonidine, meperidine, neuroleptics, and general anaesthetic agents.
• Ethanol, phenothiazines, and sedative-hypnotics also reduce the body’s ability to respond to low ambient temperatures.

Therapeutic hypothermia

• Hypothermia with intracorporeal temperature monitoring may be used for hypoxic perinatal brain injury.
• May be used in the post-resuscitation period, in traumatic brain injury with high intracranial pressure, in the perioperative setting during various surgical procedures (e.g., vascular surgery for spinal cord protection and overall neuroprotection) and for various other indications.

Risk factors

People most likely to experience hypothermia include:

• The very elderly or the very young.
• Those who are chronically ill, especially with cardiovascular disease.
• People who are malnourished.
• People who are exhausted.
• Those intoxicated with alcohol or drugs.
• People with cognitive impairment – e.g., in Alzheimer’s disease.
• Those with underlying medical conditions – e.g. hypothyroidism, stroke, severe arthritis, Parkinson’s disease, trauma, spinal cord injuries, and burns.

Presentation

• Low-reading thermometers, preferably esophageal, are required. Tympanic thermometers are unreliable in low temperature measurement.
• Hypothermia usually occurs gradually. Common signs include shivering, slurred speech, an 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.
• Hypothermia can be classified as mild, moderate or severe:
  • Mild hypothermia (32-35°C): lethargy, confusion, shivering, loss of fine motor co-ordination.
  • Moderate hypothermia (28-32°C): delirium, slowed reflexes.
  • Severe hypothermia (below 28°C): very cold skin, unresponsive, coma, difficulty breathing, abnormal heart rhythms.

Differential diagnosis

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

Investigations

• Monitor for complications – e.g., blood pressure, FBC, electrolytes, electrocardiogram (ECG) monitoring.
• Coagulation studies: disseminated intravascular coagulation may occur.
• ECG:
  • May show prolonged PR, QRS and QT intervals, and atrial or ventricular arrhythmias. As the body core temperature decreases, sinus bradycardia tends to give way to atrial fibrillation followed by ventricular fibrillation and finally asystole.
  • The length and height of the respective QT-interval prolongation and characteristic heat waves are often proportional to the degree of hypothermia.
• CXR: aspiration pneumonia and pulmonary edema are common.
• Consider any underlying or associated problems – e.g., CT scan for possible head injury.

Management

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

Initial management

• Immediate attention to airway, breathing and circulation. Initiation of cardiopulmonary resuscitation may be required.
• 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 re-warm the patient actively 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: FBC, electrolytes, blood glucose, alcohol, toxin screen, creatinine, amylase and blood cultures.
• Cardiac monitoring: dysrhythmias, changes of hyperkalaemia; heat waves are pathognomonic of hypothermia:
  • Cardiac output falls proportionately to the degree of hypothermia and cardiac irritability begins at about 33°C. Ventricular fibrillation becomes increasingly more common as the temperature falls below 28°C, and at temperatures below 25°C, asystole can occur.
  • Cardiac drugs and defibrillation are not usually effective in the presence of acidosis, hypoxia and hypothermia. These treatments should usually be reserved until the patient is warmed until at least 28°C.
  • Cardiopulmonary bypass has been used in patients with severe hypothermia.^[8]Patients presenting in cardiac arrest from accidental hypothermia may also be re-warmed effectively using thoracic lavage.^[9]
• Oxygen:
  • Administer 100% oxygen while the patient is being re-warmed.
  • Arterial blood gases are probably best interpreted uncorrected, i.e. the blood warmed to 37°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 re-warming methods that may include invasive surgical re-warming techniques – e.g., peritoneal lavage, A-V re-warming 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 re-warmed (e.g., to 35°C; don’t have to reach 37°C).

Complications

• Cardiovascular: cardiac arrhythmias, hypotension(due to marked vasodilatation when re-warming), intravascular thrombosis.
• Respiratory: pneumonia, pulmonary edema.
• Abdominal: pancreatitis, peritonitis, gastrointestinal bleeding, acute tubular necrosis.
• Metabolic acidosis, hyperkalaemia.
• Severe hypothermia eventually leads to cardiac failureand respiratory failure, and then

Prognosis

• 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.
• Overall mortality then increases with the degree of hypothermia.

Prevention

• The elderly are mostly at risk; surveillance and good neighbor watch is essential.
• Heating and insulation plays a major part in stabilizing desirable temperature.
• Even in summer, wet clothing (increases heat loss by 5-10 times) and wind can result in rapid loss of body heat.
• To reduce the risk of hypothermia:
  • Dress warm.
  • Wear a hat or other protective covering to prevent body heat from escaping from your head.
  • Cover hands with mittens instead of gloves. Mittens are more effective than gloves because mittens keep the fingers in closer contact with one another.
  • 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 warm as possible.