What is hypothermia?

Hypothermia is the medical term for a low core body temperature (below 35 degrees Celsius/95 degrees Farenheit) due to the failure of the body to maintain its normal temperature (thermoregulation). Mild hypothermia is a body temperature between 35 and 32 degrees Celsius. Severe hypothermia is when your body temperature drops below 32 degrees Celsius.

The body’s core temperature is monitored and maintained by the hypothalamus at around 37.5 degrees Celsius. In hypothermia, the body is either unable to restore its normal body temperature due to impaired thermoregulatory mechanisms or the environmental temperature is extremely low where even normal thermoregulation cannot cope. The patient may be unable to hold a thermometer in the mouth, so a rectal temperature should be taken.

Signs and Symptoms of Hypothermia

Stage One

The first signs and symptoms of hypothermia may be noticed when the body temperature drops below 36.5 degrees Celsius yet its above 35 degrees Celsius. At this stage the heart rate may increase above 100 beats per minute (tachycardia) and there are signs of reduced blood flow to the skin and periphery (pale skin/pallor or light blue tinge of the fingers and toes in anemic patients).

Stage Two – Mild Hypothermia

As the core body temperature drops further, below 35 degrees Celsius (mild hypothermia), other more serious igns and symptoms may be evident. This includes :

  • Skin feels colder than normal but may still be lukewarm.
  • Shivering.
  • Dehydration.
  • Mental confusion.
  • Poor coordination of voluntary movements (ataxia) – slurred speech, stumbling, clumsiness.

Stage Three – Severe Hypothermia

When the core body temperature drops below 32 degrees Celsius, there are more serious signs and symptoms and the hypothermia may be irreversible in some situations.

  • Skin cold to touch.
  • No shivering.
  • Reduced consciousness.
  • Low blood pressure (hypothermia).
  • Heart rate below 60 beats per minute (bradycardia).
  • Weak pulse.
  • Shallow breathing.
  • Muscle stiffness.
  • Abnormal ECG readings.

As the core body temperature drops below 28 degrees Celsius, a person may be mistaken for being dead.

  • Coma.
  • Absent pupil reflexes.
  • Faint pulse that may be difficult to detect.
  • Shallow breathing that may be barely detectable.

How is the Body Temperature Maintained?

A normal body temperature is maintained by the basal metabolic activity (temperature generated by metabolism within cells even at rest), muscular activity and insulation (skin and subcutaneous fat). However, heat is constantly dissipated into the environment and perspiration causes evaporation which also assists with cooling.

The hypothalamus maintains this balance between heat that is generated within the body and heat that is lost to the environment to maintain the body temperature within a range of approximately 37.5  to 36.5 degrees Celsius. Once the temperature drops below 36.5 degrees Celsius, the first signs and symptoms of hypothermia may become evident (stage one).

Causes of Hypothermia

In order to maintain a normal body temperature, many systems have to work together. Some of the essential processes to maintain the body’s temperature includes :

  • Oxygen and food supply is necessary for metabolism (heat production).
  • The thyroid gland regulates the metabolic rate  (heat production).
  • The heart pumps blood which circulates oxygen and nutrients to the cells for metabolism (heat production) and sends warm blood to the periphery.
  • The blood vessels relax and constrict to control the amount of heat lost to the environment (heat conduction).
  • The skin structure, subcutaneous fat and hair on the skin help to reduce loss of heat (heat retention).
  • Human cognitive abilities (cold awareness) allows a person to dress up warmly, seek an external source of heat and stay out of the cold (heat retention).
  • The hypothalamus coordinates many of the thermoregulating activities by sending nerve impulses to control specific organs and prevent hypothermia in the event of extreme cold (shivering to generate heat, reduced blood flow to the skin).

Any factor that interferes with the above activities may result in hypothermia. There are certain risk factors that may predispose a person to hypothermia which include pathogenic (disease), induced (drugs, alcohol, poisons), environmental (cold) and structural (skin, nerves). This includes :

  • Age (babies and elderly – explained below)
  • Alcohol.
  • Anemia.
  • Atherosclerosis.
  • Cold and/or windy environmental conditions/weather.
  • Dementia and other mental illness.
  • Diets – particularly starvation diets for weight loss.
  • Drugs – tranquilizers, hypnotics, diuretics, etc.
  • Head/neck trauma, particularly brainstem injury.
  • Heart attack (myocardial infarction).
  • Heart failure.
  • Hypoglycemia.
  • Hypothyroidism.
  • Glucocorticoid insufficiency (adrenal).
  • Falls – ice water or snow (sudden hypothermia which can lead to cardiac arrest).
  • Lack of insulation (warm clothing, proper housing).
  • Liver failure
  • Narcotic drug use or abuse.
  • Overdose.
  • Physical activity – lack of proper clothing after exertion or sports.
  • Pneumonia.
  • Poisoning.
  • Post-operative.
  • Skin diseases/injuries that affect large areas of the body (example – psoriasis, burns)
  • Stress.
  • Stroke.
  • Starvation.

Children and adults are able to take measures to source heat, dress appropriately and remove themselves from a cold or windy environment. In the event that there is no other risk factor and the person is unable to take the necessary measures to prevent hypothermia, abuse or captivity is a consideration. There is a link between poverty and the incidence of hypothermia during colder weather as there is difficulty to acquire proper clothing and shelter, coupled with low calorie diets.

Hypothermia in Babies

Infants are entirely dependent on their caregivers to ensure their needs are taken care of. Lack of an external source of  heat and improper attire in colder environments can quickly lead to hypothermia. In these cases, child neglect and/or abuse may be a factor.

In addition, the immature homeostatic mechanisms of the body may be unable to ensure adequate heat production and retention for proper thermoregulation.  The body surface area to body weight ration is also higher in infants which results in insufficient heat production compared to heat loss.

Hypothermia in the Elderly

The elderly are at the highest risk of developing hypothermia as age-related changes and diseases impair thermoregulation. The autonomic system may be delayed in its response to changes in the environmental temperature.

  • Activities like shivering to produce more body heat and peripheral vasoconstriction to reduce heat loss may be delayed.
  • Dementia with old age may impair the person’s ability to dress appropriately or take actions to seek external sources of heat.
  • Chronic medication and diseases may impair the body’s thermoregulatory mechanism.
  • Debilitating illnesses and incapacitated patients may be unable to seek proper heating sources and thermal clothing.
  • Structural changes in the skin and reduced subcutaneous fat (age-related) may impair heat retention.

Article reviewed by Dr. Greg. Last updated on September 9, 2012