Advances in rewarming have improved the prognosis for patients with hypothermia, especially those with cardiac arrest treated with extracorporeal rewarming. The latest review in our Current Concepts series covers prehospital care, transport, resuscitation fluids, and extracorporeal membrane oxygenation.
Accidental hypothermia (i.e., an involuntary drop in core body temperature to <35 degreesC [95 degreesF]) is a condition associated with major morbidity and mortality. Each year, approximately 1500 patients in the United States have hypothermia noted on their death certificate; however, the incidence of primary and secondary hypothermia and the associated morbidity and mortality remain unknown.
• What is secondary hypothermia and what are the associated risks?
Primary hypothermia occurs when heat production in an otherwise healthy person is overcome by the stress of excessive cold. Secondary hypothermia can occur in ill persons with a wide variety of medical conditions, even in a warm environment. Death in patients with secondary hypothermia is often caused by the underlying condition rather than by hypothermia. In all types of hypothermia, consciousness, breathing, and circulation are initially intact but are impaired as the body cools. Some patients with deep hypothermia (core temperature <28 degreesC [82 degreesF]) engage in paradoxical undressing. Atrial fibrillation is common when the core temperature is less than 32 degreesC (90 degreesF) and is not worrisome in the absence of other signs of cardiac instability. The risk of cardiac arrest increases as the core temperature drops below 32 degreesC, and increases substantially if the temperature is less than 28 degreesC.
• What is the most accurate way of measuring the core temperature?
Properly calibrated, low-reading thermometers are required but are not always available in the prehospital setting. The recorded temperature can vary depending on the body site, perfusion, and environmental temperature. In an intubated patient, insertion of a thermistor probe in the lower third of the esophagus is the preferred method. Measurements obtained with the use of a proximally placed esophageal probe may be falsely elevated owing to ventilation with warmed gases. A thermistor probe in contact with the tympanic membrane accurately reflects brain temperature, provided that the ear canal is free of snow and cerumen and is well insulated against the environment. Measurements obtained with the use of infrared cutaneous, aural, and oral thermometers are often inaccurate in patients with hypothermia. Rectal probes should be inserted to a depth of 15 cm, but readings may lag behind core temperature during rewarming.
Morning Report Questions
Q: How should resuscitation fluids be administered to a hypothermic patient?
A: Intravenous fluids should be warmed (38 to 42 degreesC [100 to 108 degreesF]) to prevent further heat loss. In a cold prehospital environment, intravenous fluids cool rapidly, and cold fluids may aggravate hypothermia. A considerable volume of fluid is often required because of the volume loss with cold diuresis (polyuria due to hypothermia-induced vasoconstriction and diminished release of antidiuretic hormone) and vasodilatation during rewarming. Warm crystalloid-containing fluids should be administered on the basis of volume status and glucose, electrolyte, and pH measurements; resuscitation with a large volume of normal saline may aggravate acidosis, so alternative crystalloids should be considered.
Q: When should ECMO or cardiopulmonary bypass be considered in the setting of hypothermia?
A: Patients with prehospital cardiac instability (e.g., systolic blood pressure of <90 mm Hg or ventricular arrhythmias), those with a core temperature of less than 28 degreesC (82 degreesF), and those in cardiac arrest should be transported to a center capable of providing ECMO or cardiopulmonary bypass. ECMO or cardiopulmonary bypass should be considered for patients with hypothermia and cardiac instability who do not have a response to medical management. Support with ECMO has resulted in improved outcomes, as compared with cardiopulmonary bypass alone, probably owing to the high incidence of severe pulmonary failure after rewarming, which can be treated more efficiently with ECMO.