Hypothermia

Essentials of Diagnosis

• Systemic hypothermia is a core body temperature (CBT) below 35°C

• Accurate CBT measurement must be obtained using a low-reading core temperature probe than measures as low as 25°C

• The CBT must be over 32°C before terminating resuscitation efforts

• Extracorporeal membrane oxygenation (ECMO) or cardiopulmonary bypass may be considered in hypothermic patients with hemodynamic instability or cardiac arrest

General Considerations

• Risk factors

  • Diabetes mellitus and other endocrine dysfunctions (hypothyroidism, adrenal insufficiency, hypopituitarism)

  • Kidney or liver dysfunction

  • Psychiatric conditions

  • Poor nutrition

  • Sedentary lifestyle

  • Homelessness, inadequately heated housing, inadequate or wet clothing

  • Occupational or recreational exposure

  • Prior cold weather injury

  • Pharmacologic effects

  • Sepsis, infection

  • Hypoglycemia

• Heat loss occurs more rapidly with

  • High wind velocity ("windchill factor")

  • Water immersion or wet clothing

  • Direct contact with a cold surface

• Accidental hypothermia may occur in the hospital setting due to

  • Prolonged postoperative hypothermia or administration of large amounts of refrigerated stored blood products (without rewarming)

  • Rapid infusion of intravenous fluids

  • Prolonged exposure of an undressed patient during resuscitation or surgical procedures

• Systemic hypothermia

  • Depresses physiologic function, resulting in decreased respiratory drive, oxygen consumption, central or peripheral nerve conduction, gastrointestinal motility, myocardial repolarization, metabolism of drugs

  • May decrease activity of coagulation cascade and the immune response

Symptoms and Signs

Systemic hypothermia

• In mild cases with core temperature between 35°C and 32°C (stage I):

  • Shivering

  • Impaired coordination

  • Poor judgment

  • Hemodynamic stability

  • Normal level of consciousness

• With core temperature between 32°C and 28°C (stage II):

  • Shivering stops

  • Bradycardia, dilated pupils, slowed reflexes, cold diuresis, and confusion and lethargy ensue

• With core temperature between 28°C and 24°C (stage III)

  • Loss of consciousness

  • Vital signs present

• Below 24°C (stage IV):

  • Vital signs lost

  • Coma, loss of reflexes, asystole or ventricular fibrillation, which may lead the clinician to assume that patient is dead despite reversible hypothermia

Hypothermia of the extremities

• Exposure of the extremities to cold produces immediate localized vasoconstriction followed by generalized vasoconstriction

• When the skin temperature falls to 25°C, the area becomes cyanotic

• At 15°C, there is a deceptively pink, well-oxygenated appearance to the skin. Tissue damage occurs at this temperature

Differential Diagnosis

• Infection

• Other cause of altered mental status (eg, hypoglycemia, drugs, stroke)

• Hypothyroidism

• Anorexia or malnutrition (poor fat stores)

• Adrenal insufficiency

• Burns

• Spinal cord injury

Laboratory Tests

• Must assess acid-base status; electrolytes (particularly potassium); glucose; kidney, liver, and pancreas function; and coagulation; and must exclude rhabdomyolysis

Diagnostic Procedures

• ECG may reveal J wave or Osborn wave (positive deflection in the terminal portion of the QRS complex, most notable in leads II, V₅, and V₆) (Figure 37–1)

• During the rewarming process, essential testing includes ECG, chest radiograph, arterial blood gas and bedside glucose monitoring

Therapeutic Procedures

• Mild hypothermia (Stage I hypothermia, rectal temperature 35°C to 32°C) in healthy patients: warm bed or rapid passive rewarming with warm bath or warm packs and blankets

• Moderate hypothermia (Stage II hypothermia, core temperature 32°C to 28°C; and Stage III hypothermia, core temperature 28°C to 24°C)

  • Establish cardiovascular support, acid-base balance, arterial oxygenation, and adequate intravascular volume before rewarming to minimize risk of organ infarction and "afterdrop" (recurrent hypothermia)

  • Active external and internal rewarming

  • Below 30°C, arrhythmias and asystole may be refractory to drug therapy until the patient has been rewarmed, therefore treatment should focus on excellent technique CPR done in conjunction with aggressive rewarming of the patient

• Severe hypothermia (Stage IV hypothermia, core temperature below 24°C)

  • For severely hypothermic patients in cardiac arrest, high-quality CPR must be initiated and continued until the patient's core body temperature is at least 32°C

  • Epinephrine or vasopressin may be given to a severely hypothermic patient in cardiac arrest

• International Commission for Mountain Emergency Medicine recommends extracorporeal life support as the treatment of choice for patients with high risk of hypothermic cardiac arrest

Active external rewarming

• Warm bedding, heated blankets, heat packs, and immersion into a 40°C bath

• Easier to monitor and perform diagnostic and therapeutic procedures using heated blankets

• Rewarming may cause marked peripheral dilation, predisposing to ventricular fibrillation and hypovolemic shock

• Afterdrop can be lessened by active external rewarming of the trunk but not the extremities and by avoiding any muscle movement by the patient

• There is some evidence to suggest that head warming is a viable alternative to increase core body temperature if torso warming in contraindicated (eg, when performing cardiopulmonary resuscitation)

Active internal (core) rewarming

• Warm humidified oxygen (43–46°C) is an easy, safe, and highly effective method

• Warmed intravenous saline infusions (43°C) should be used instead of lactated Ringer solution

• Volume resuscitation is needed to prevent shock as vasodilation occurs during rewarming

• Other methods include

  • Warm solution lavage of the stomach, colon, thoracic cavity, peritoneum, or bladder

  • Extracorporeal blood rewarming by cardiopulmonary, arteriovenous femorofemoral, or venovenous bypass

  • Hemodialysis

• Endovascular warming devices are a less invasive alternative than extracorporeal methods but are not widely available in hospitals

• For patients with core temperature < 30°C:

  • Active rewarming

  • Cardiopulmonary resuscitation (CPR)

  • One shock attempt for dysrhythmia

  • Withholding of intravenous medications

• Once the core temperature reaches 30°C:

  • Cardiac medications can be given but at longer than standard intervals because metabolism is slowed and there is a risk of toxic accumulation as circulation is restored

  • Defibrillation may be performed as needed

Follow-Up

• Monitor cardiac rhythm

• Monitor core temperature (esophageal preferred over rectal) often during and after initial rewarming to avoid recurrent hypothermia

Complications

• Metabolic acidosis

• Pulmonary edema

• Rhabdomyolysis

• Hyperkalemia

• Pneumonia

• Pancreatitis

• Ventricular fibrillation

• Hypoglycemia or hyperglycemia

• Coagulopathy

• Acute kidney injury

• Cardiac arrhythmias may occur, especially during rewarming

Prognosis

• Most otherwise healthy patients may survive moderate or severe systemic hypothermia

• Prognosis is poor with

  • Low pH (≤ 6.6)

  • Elevated potassium (≥ 4.0 mEq/L or ≥ 4.0 mmol/L)

  • Serious underlying condition (eg, chronic kidney disease)

  • Treatment delay

• Extracorporeal life support has been shown to substantially improve survival of patients with unstable circulation or cardiac arrest

• Comatose patients have a high risk of aspiration pneumonia

Prevention

• "Keep warm, keep moving, and keep dry"

• Arms, legs, fingers, and toes should be exercised to maintain circulation

• Wet clothing, socks, and shoes should be replaced with dry ones

• Caution must be taken to avoid

  • Cramped positions

  • Constrictive clothing

  • Prolonged dependency of the feet

  • Use of tobacco, alcohol, and sedative medications

  • Exposure to wet muddy ground and windy conditions

When to Admit

• Hypothermia patients must undergo close monitoring for potential complications. This is typically done during an inpatient admission or prolonged emergency department observation, depending on the comorbidities and home care situation

• Monitoring includes vital signs, temperature, cardiac rhythm, oximetry, serial examinations (including extremity cold-induced injuries), and serial laboratory studies (to exclude electrolyte abnormalities, kidney or liver dysfunction, cardiac ischemia, and infection)