Aetiology

The underlying mechanism is heat generation exceeded by heat loss. Although environmental cold exposure is a major cause (such as drowning - see our topic Drowning), anyone with reduced physiological reserve will be prone to hypothermia.[10] Other scenarios include inadequate insulation of newborn babies and patients during surgery.

Pathophysiology

Hypothermia occurs because of a lowering of the core temperature. Core temperature is a reflection of the balance between heat production and heat loss. Heat is produced during the breakdown of high-energy phosphate bonds, and heat is lost through the lungs and skin. Radiation heat loss from the body occurs primarily from infrared emission.[11]

In the initial stages of hypothermia, thermoreceptors situated in the skin and subcutaneous tissues sense the low ambient temperature and cause a regional vasoconstriction. This causes the hypothalamus to stimulate the release of thyroid-stimulating hormone and adrenocorticotrophic hormone, leading to stimulation of the thyroid and adrenal glands. The hypothalamus also stimulates heat production by promoting shivering, typically occurring between 34°C and 36°C (93.2°F to 96.8°F). Owing to the effects of prolonged vasoconstriction, acidosis may occur, which may blunt the response to catecholamine production.

Continuous ECG monitoring demonstrates progressive bradycardia. J waves (Osborn waves) may occur in most, but not all, patients and are usually best seen in the lateral precordial leads.[12] Broad QRS complexes, ST elevation or depression, and T wave inversion may also occur as myocardial conduction slows.[5] PR, QT, and QTc intervals are prolonged. Some patients develop atrial fibrillation or junctional rhythms.[13]

Early on, the respiratory centre is stimulated, but as time passes, the respiratory rate and tidal volume become depressed. Anatomical and physiological dead space increases, as does bronchiolar and alveolar oedema.[14] The body begins to limit energy-producing functions.

The renal blood flow and glomerular filtration rate decrease as well. Tubular reabsorption decreases as this is an energy-requiring process. As a result, cold-induced natriuresis and diuresis occur.

Hypothermia is also associated with insulin resistance and hyperglycaemia. Platelet dysfunction commonly occurs and may lead to a bleeding disorder. Vasoconstriction may lead to tissue hypoxia.[15]

Classification

Stages of hypothermia and clinical features[3]

  1. Mild hypothermia (stage 1)

    1. Core temperature of 32°C to 35°C (90°F to 95°F).

    2. Characterised by tachycardia (although this may progress to bradycardia, even in mild hypothermia), shivering, vasoconstriction, and tachypnoea.

    3. With time, patients develop fatigue, ataxia, and cold-induced diuresis, and become apathetic.

    4. Patients become hypovolaemic, secondary to diuresis.

    5. Impaired judgement will also occur.

  2. Moderate hypothermia (stage 2)

    1. Core temperature of 28°C to 32°C (82°F to 90°F).

    2. Bradycardia

    3. Hypotension

    4. Respiratory depression

    5. Altered consciousness

    6. Pupil dilation

    7. Reduced gag reflex

    8. Hyporeflexia

    9. Loss of shivering.

  3. Severe hypothermia (stage 3)

    1. Core temperature of <28°C (<82°F)

    2. Coma

    3. Decreased, and ultimately no, activity on EEG

    4. Apnoea

    5. Fixed dilated non-reactive pupils

    6. Pulmonary oedema

    7. Oliguria.

  4. Severe hypothermia (stage 4)

    1. Variable core temperature

    2. Apparent death

    3. Vital signs absent.

Some experts have suggested a further (more severe) category of profound hypothermia, at a core temperature <24°C (75.2°F) according to some and <20°C (68°F) according to others.[4]

Note that arrhythmias can occur at any stage of hypothermia, and also during re-warming. Initially, the ECG may show tachycardia. In more severe cases of hypothermia, the ECG may show sinus bradycardia, atrial or ventricular fibrillation, junctional rhythms, ST segment changes, T-wave inversion, prolongation of the QT interval, and eventually asystole.[5]

Weak signs of life can be present when core temperature is below 28°C (82°F). Clinical findings are influenced by multiple patient factors that affect the body's response to cold and do not consistently correlate with core temperatures. As a result, cardiac arrest may occur in one patient with a core temperature just below 32°C (90°F), while another patient has vital signs despite a core temperature <20°C (<68°F).[6][7] Other causes for cardiac arrest may need to be excluded; cardiac arrest is unlikely to be solely due to hypothermia unless the core temperature is less than 28°C (82°F).[3]

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