Hypothermia

GENERAL PRINCIPLES

Definition

Hypothermia is defined as a core temperature of lt;35°C (95°F).

Classification

Classification of severity by temperature is not universal.

Etiology

• The most common cause of hypothermia in the United States is cold exposure due to alcohol intoxication.

• Another common cause is cold water immersion.

DIAGNOSIS

Clinical Presentation

Presentation varies with the temperature of the patient on arrival. All organ systems can be involved.

• CNS effects

î At 32.2°C (90°F), the ability to shiver is lost and deep tendon reflexes are diminished.

î At temperatures below 32°C (89.6°F), mental processes are slowed and affect is flattened.

î At 28°C (82.4°F), coma often occurs.

O Below 18°C (64.4°F), the electroencephalogram is flat. On rewarming from severe hypothermia, central pontine myelinolysis may develop.

• Cardiovascular effects

î After an initial increased release of catecholamines, there is a decrease in cardiac output and heart rate with relatively preserved mean arterial pressure. ECG changes manifest initially as sinus bradycardia with T-wave inversion and QT interval prolongation and may manifest as atrial fibrillation at temperatures of lt;32°C (lt;89.6°F).

î Osborne waves (J-point elevation) may be visible, particularly in leads II and V₆.

î An increased susceptibility to ventricular arrhythmias occurs at temperatures below 32°C (89.6°F).

î At temperatures of 30°C (86°F), the susceptibility to ventricular fibrillation is increased significantly, and unnecessary manipulation or jostling of the patient should be avoided.

î A decrease in mean arterial pressure may also occur, and at temperatures of 28°C (82.4°F), progressive bradycardia supervenes.

• Respiratory effects

î After an initial increase in minute ventilation, respiratory rate and tidal volume decrease progressively with decreasing temperature.

° Arterial blood gases (ABGs) measured with the machine set at 37°C (98.6°F) should serve as the basis for therapy without correction of pH and carbon dioxide tension (PCO₂).¹⁰

• Renal manifestations: Cold-induced diuresis and tubular concentrating defects may be seen.

Differential Diagnosis

• Cerebrovascular accident

• Drug overdose

• Diabetic ketoacidosis

• Hypoglycemia

• Uremia

• Adrenal insufficiency

• Myxedema

Diagnostic Testing

LABORATORIES

• Basic laboratory studies should include CBC, coagulation studies, LFTs, BUN, electrolytes, creatinine, glucose, creatine kinase, calcium, magnesium, amylase levels, urinalysis, ABG, and ECG.

• Serum potassium is often increased.

• Elevated serum amylase may reflect underlying pancreatitis.

• Hyperglycemia may be noted but should not be treated because rebound hypoglycemia may occur with rewarming.

• Disseminated intravascular coagulation may also occur.

IMAGING

Obtain chest, abdominal, and cervical spine radiographs to evaluate all patients with a history of trauma or immersion injury.

TREATMENT

Medications

• Administer supplemental oxygen.

• Supplement thiamine to most patients with cold exposure because exposure due to alcohol intoxication is common.

• Administration of antibiotics is a controversial issue. Although some recommend antibiotic administration for 72 hours (pending cultures), antibiotics should be reserved for when an infection is suspected. In general, the patients with hypothermia due to exposure and alcohol intoxication are less likely to have a serious underlying infection than those who are elderly or who have an underlying medical illness.

Nonpharmacologic Therapies

• Rewarming: The patient should be rewarmed with the goal of increasing the temperature by 0.5- 2.0°C#8725;h (32.9-35.6°F#8725;h), although the rate of rewarming has not been shown to be associated with improved outcomes.

• Passive external rewarming

î This method depends on the patient's ability to shiver.

î It is effective only at core temperatures of 32°C (89.6°F) or higher. Patients cannot shiver below 32°C.

î Remove wet clothing, cover the patient with blankets in a warm environment, and monitor.

• Active external rewarming

î It is indicated for patients with hypothermia and stable circulation.¹¹

• Active core rewarming is preferred for treatment of severe hypothermia, although there are minimal data on outcomes.¹²

î Heated oxygen is the initial therapy of choice for the patient whose cardiovascular status is stable. This therapeutic maneuver can be expected to raise core temperatures by 0.5-1.2°C#8725;h (32.9- 34.2°F#8725;h).¹³ Administration through an ETT results in more rapid rewarming than delivery via face mask. Administer heated oxygen through a cascade humidifier at a temperature of 45°C (113°F) or lower.

î IV fluids can be warmed or delivered through a blood warmer.

î Heated nasogastric or bladder lavage is of limited efficacy because of low-exposed surface area and is reserved for the patient with cardiovascular instability.

î Heated peritoneal lavage with fluid warmed to 40-45°C (104-113°F) is more effective than heated aerosol inhalation, but it should be reserved for patients with cardiovascular instability. Only those who are experienced in its use should perform heated peritoneal lavage, in combination with other modes of rewarming.

î Closed thoracic lavage with heated fluid by thoracostomy tube has been recommended but is unproven.¹⁴ It can be considered in patients where extracorporeal circulation is not an option.

î Hemodialysis can be used for the severely hypothermic, particularly when due to an overdose that is amenable to treatment in this way.

î Extracorporeal circulation (cardiac bypass and extracorporeal membrane oxygenation [ECMO]) is used only in hypothermic individuals who are in cardiac arrest; in these cases, it may be dramatically effective.¹⁵ Extracorporeal circulation may raise the temperature as rapidly as 10- 25°C#8725;h (50-77°F#8725;h).

Resuscitation

• Maintain the airway and administer oxygen. Patients lt;32°C (89.6°F) should be moved gently owing to the risk of triggering ventricular fibrillation.

• If intubation is required, the most experienced operator should perform it.

• Conduct cardiopulmonary resuscitation (CPR). Perform simultaneous vigorous core rewarming; as long as the core temperature is severely decreased, it should be assumed that the patient can be resuscitated. Although reliable defibrillation requires a core temperature of 32°C (89.6°F) or higher, patients with temperatures below 32°C (89.6°F) can be defibrillated. Administration of vasopressors is controversial. Standard teaching was to withhold vasopressors in patients with temperatures lt;30°C (86°F), but some research indicates an increased rate of return of spontaneous circulation with vasopressors.¹⁶ However, patients may be resistant to most treatment modalities until their core temperature is gt;32°C (89.6°F). Prolonged efforts (to a core temperature gt;32°C [89.6°F]) may be justified because of the neuroprotective effects of hypothermia.

• If ventricular fibrillation occurs, begin CPR as per the advanced cardiac life support protocol.

• Monitor ECG rhythm, urine output, and if possible central venous pressure in all patients with an intact circulation.

Disposition

• Admit patients with an underlying disease, physiologic derangement, or core temperature lt;32°C (lt;89.6°F), preferably to an ICU.

• Consider discharge for individuals with mild hypothermia (32-35°C [89.6-95°F]) and no predisposing medical conditions or complications when they are normothermic, and an adequate home environment can be ensured.

Monitoring/Follow-Up

• Monitor core temperature.

• A standard oral thermometer registers only to a lower limit of 35°C (95°F). Monitor the patient continuously with a Foley catheter thermometer with a full range of 20-40°C (68-104°F).