oPatientPlus articles are written by UK doctors and are based on research evidence, UK and European Guidelines. They are designed for health professionals to use, so you may find the language more technical than the condition leaflets.
Hypothermia is defined as a core body temperature below 35°C. 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 by about 10% for every 1°C fall in 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 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 anaesthesia until 24 hours after entry into the recovery area) should be warmed using a forced air warming device.
- Secondary hypothermia is low body temperature resulting from a medical illness lowering the temperature set-point:
- Decreased heat production, eg hypopituitarism, hypoadrenalism, hypothyroidism, severe malnutrition, hypoglycaemia and neuromuscular disorders.
- Increased heat loss, eg vasodilatation (pharmacologic or toxicologic causes), erythrodermas, burns, psoriasis, or iatrogenic (eg cold infusions, overenthusiastic treatment of heatstroke or emergency deliveries).
- Impaired thermoregulation, eg trauma affecting the central nervous system, strokes, toxicologic and metabolic derangements, intracranial bleeding, Parkinson's disease, brain tumours, Wernicke's disease, multiple sclerosis, sepsis, multiple trauma, pancreatitis, prolonged cardiac arrest, and uraemia.
- Drug administration; such medications include betablockers, 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:
- May be used in the post-resuscitation period, in traumatic brain injury with high intracranial pressure, in the perioperative setting during various surgical procedures (eg vascular surgery for spinal cord protection and overall neuroprotection) and for various other indications.
People most likely to experience hypothermia include:
- Very old or very young
- Chronically ill, especially with cardiovascular disease
- Intoxicated with alcohol or drugs
- Mental impairment, eg Alzheimer's disease
- Underlying medical conditions, eg 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.
- 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
- Monitor for complications, eg blood gases, full blood count, electrolytes, electrocardiogram (ECG) monitoring.
- Coagulation studies: disseminated intravascular coagulation may occur.
- May show prolonged PR, QRS and QT intervals, and atrial or ventricular arrhythmias.
- The length and height of the respective QT-interval prolongation and characteristic J waves are often proportional to the degree of hypothermia.
- Chest X-ray: aspiration pneumonia and pulmonary oedema are common.
- Consider any underlying or associated problems, eg 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 is surrounded by warmed blankets or heat lamps.
- Aggressive management of temperature with faster rather than slow rewarming has been shown to improve the outcome.
- 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, nonalcoholic fluids.
Management in hospital
- The patient should ideally be managed in a critical care setting. Attempts to rewarm the patient actively should not delay transfer to a critical care setting.
- Assess for and treat any associated disorders, eg 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°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 be reserved until the patient is warmed until at least 28°C.
- 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.
- Administer 100% oxygen while the patient is being rewarmed.
- Arterial blood gases are probably best interpreted uncorrected, ie 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 rewarming methods that may include invasive surgical rewarming techniques, eg 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
- 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°C 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.
Further reading & references
- Hypothermia; Circulation Journal of the American Heart Association, 2005
- Hypothermia; Joint Royal Colleges Ambulance Liaison Committee, October 2006
- Edelstein JA, Li J; Hypothermia; eMedicine, October 2009.
- Perioperative hypothermia (inadvertent); NICE Clinical Guideline, April 2008
- McCullough L, Arora S; Diagnosis and treatment of hypothermia. Am Fam Physician. 2004 Dec 15;70(12):2325-32.
- Gentilello LM, Cobean RA, Offner PJ, et al; Continuous arteriovenous rewarming: rapid reversal of hypothermia in critically ill patients. J Trauma. 1992 Mar;32(3):316-25; discussion 325-7.
- Resuscitation Council UK; Adult advanced life support guidelines, (2005).
- Walpoth BH, Walpoth-Aslan BN, Mattle HP, et al; Outcome of survivors of accidental deep hypothermia and circulatory arrest treated with extracorporeal blood warming. N Engl J Med. 1997 Nov 20;337(21):1500-5.
Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.
Dr Colin Tidy
Dr Colin Tidy