Hypothermia

Hypothermia is a decrease in internal body temperature below 35 °C. Symptoms progress from shivering and drowsiness to confusion, coma and death.

In moderate hypothermia, it may be sufficient to stay in a warm environment and warm up with blankets (passive rewarming). Severe hypothermia requires active warming of the body surface (in particular, with systems with a flow of warm air, radiant heaters, electric heating pads) or the internal environment of the body (for example, lavage of body cavities, extracorporeal blood rewarming).

Hypothermia occurs when heat loss exceeds heat production. Hypothermia is most common in cold weather or when immersed in cold water, but it is also possible in warm weather, after a person has been lying motionless on a cool surface for a very long time (for example, when intoxicated) or after being in water at a temperature normal for swimming (for example, 20-24 °C) for a very long time.

Primary hypothermia causes about 600 deaths in the United States each year. Hypothermia also has a significant and not always understood impact on the risk of death in cardiovascular and neurological diseases.

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Causes of hypothermia

Immobility, damp clothing, windy conditions, and lying on a cold surface increase the risk of hypothermia. Conditions that cause loss of consciousness, immobility, or both (e.g., trauma, hypoglycemia, seizures, stroke, drug or alcohol intoxication) are the most common predisposing factors.

Hypothermia slows all physiological functions, including cardiovascular and respiratory function, nerve conduction, mental activity, neuromuscular reaction time, and metabolic rate. Thermoregulation ceases at body temperature below about 30°C; beyond this point, rewarming is possible only from an external source. Renal cell dysfunction and decreased antidiuretic hormone levels result in the production of large volumes of dilute urine (cold diuresis). Diuresis plus fluid leakage into the interstitial space causes hypovolemia. The vasoconstriction that occurs with hypothermia may mask hypovolemia, which may then manifest as sudden shock or cardiac arrest during rewarming (rewarming collapse) when peripheral vessels dilate.

Immersion in cold water may induce a "diving" reflex, with vasoconstriction in visceral muscles; blood is shunted to vital organs (e.g., heart, brain). The reflex is particularly pronounced in young children and may have a protective effect. In addition, total immersion in water at near-freezing temperatures may protect the brain from hypoxia by reducing metabolic demands. This phenomenon probably underlies cases of survival after prolonged cardiac arrest due to critical hypothermia.

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Symptoms of hypothermia

At first, intense shivering occurs, but it stops when the body temperature drops below 31 °C, which contributes to an even more rapid decrease in body temperature. As the body temperature decreases, CNS dysfunction progresses; people do not feel the cold. Drowsiness and numbness are followed by confusion, irritability, sometimes hallucinations and, ultimately, coma. The pupils stop responding to light. Breathing and heart contractions slow down and eventually stop. Sinus bradycardia and slow atrial fibrillation develop first, the terminal rhythm is ventricular fibrillation and asystole. However, such rhythm disturbances are potentially less dangerous than with normothermia.

Diagnosis of hypothermia

The diagnosis is established by rectal thermometry. Electronic thermometers are preferable, since standard mercury thermometers have a lower measurement limit of 34 °C, even special low-temperature ones. Esophageal sensors and thermistor sensors for pulmonary artery catheters provide the most accurate information, but are not always available.

It is necessary to identify the causes. Laboratory tests include a complete blood count, determination of plasma glucose concentrations, electrolytes, urea nitrogen, creatinine, and blood gas composition. Blood gas composition at low temperatures is not corrected. The ECG is characterized by the appearance of a J wave (Osborne wave), and prolongation of the PR, QT, and QRS intervals, although this does not always happen. If the cause of hypothermia is unclear, the content of alcohol and drugs in the blood is determined, and thyroid function is tested. Sepsis, hidden skeletal or craniocerebral trauma should be considered.

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Prognosis and treatment of hypothermia

Patients immersed in ice water for an hour or more (rarely) have been successfully rewarmed without residual brain injury (see relevant section), even when their core temperature was 13.7°C and their pupils were unresponsive to light. Predicting outcome is difficult and should not be done using the Glasgow Coma Scale. Strong prognostic markers include evidence of cell lysis (hyperkalemia >10 mEq/L) and intravascular thrombosis (fibrinogen <50 mg/dL). Children are more likely to recover than adults for a given degree and duration of hypothermia.

The first step is to stop further heat loss, remove damp clothing, wrap the patient in blankets, and insulate the head. Subsequent measures depend on the severity of hypothermia, the presence of hemodynamic instability, or cardiac arrest. Returning the patient to normal body temperature after hypothermia is not as urgent as after severe hyperthermia. For stable patients, an increase in core body temperature of 1 °C/hour is acceptable.

If hypothermia is moderate and thermoregulation is not impaired (this is indicated by shivering and body temperature within 31-35 °C), warming with blankets and hot drinks is sufficient.

Fluid replenishment in hypovolemia is essential. Patients are given 1-2 L of 0.9% sodium chloride solution intravenously (20 mL/kg body weight for children); heated to 45 °C if possible. More may be needed to maintain normal organ blood flow.

Active rewarming is required if patients have hemodynamic instability, body temperature <32.2°C, endocrine insufficiency, or hypothermia secondary to trauma, poisoning, or illness. If body temperature is closer to the upper limit of the critical range, heating pads or hot air blowing can be used for external rewarming. Patients with lower temperatures, especially those with low blood pressure or cardiac arrest, require internal rewarming. The method of choice is lavage of the abdominal and thoracic cavities with hot 0.9% sodium chloride solution. Warming of blood in an arteriovenous or venovenous circuit (as in hemodialysis) is more effective but much more difficult to perform. The most effective is a heart-lung machine. These extracorporeal measures require a pre-prepared treatment protocol and trained medical personnel.

Cardiopulmonary resuscitation is not performed if the heart rate is sufficient to supply organs with blood, even in the absence of a pulse; fluid administration and rewarming are continued as described above. Arterial hypotension and bradycardia are expected at low core body temperature and do not require aggressive treatment in isolated hypothermia. Patients with ventricular fibrillation or asystole are treated with cardiopulmonary resuscitation, closed cardiac massage, and tracheal intubation. At low body temperature, defibrillation is difficult. If the 1st or 2nd attempts are ineffective, defibrillation should be postponed until the temperature limits increase to >28 °C. Intensive care is continued until the body temperature reaches 32 °C, in the absence of injuries or diseases incompatible with life. However, cardiotropic drugs (such as antiarrhythmics, vasopressors, inotropic agents) are usually not used. Small doses of dopamine (1-5 mcg/kg x min) or infusion of other catecholamines are administered to patients with disproportionately severe arterial hypotension or who do not respond to crystalloids and warming. Severe hyperkalemia (>10 mEq/L) during resuscitation usually indicates a fatal outcome and may serve as one of the criteria for cessation of resuscitation measures.