Perspective

Accidental hypothermia is responsible for approximately 700 deaths per year in the United States.¹ It primarily affects those least able to ward off the effects of cold weather: the very young, the very old, and the poor, disabled, pharmacologically inquisitive, environmentally adventurous, and mentally ill. Urban people are common victims. Hypothermia can occur in many latitudes, with episodes reported even in Florida.² It occurs when a person’s ability to generate heat and remain warm is outstripped by the ambient temperature.

In the United States, frostbite is often a disease of the indigent, the intoxicated, the mentally ill, and winter outdoor recreation enthusiasts. For both frostbite and hypothermia the literature consists of primarily case reports, case series, and reviews.

Epidemiology

Accidental hypothermia is generally manifested as progression from an initial state of catecholamine release and stimulation (mild hypothermia) to one marked by a predictable slowing of metabolism and all critical body functions to the extent that patients may appear dead. The profound physiologic changes and metabolic slowing all reverse with rapid rewarming, and reports of remarkable neurologically intact survival despite hours of pulselessness and resuscitation are not infrequent. This has given rise to the adage “No one is dead until they are warm and dead.” Not surprisingly, there is little strong scientific evidence for treatment recommendations because trials cannot be conducted. The medical literature is composed mostly of animal experiments, case reports and series, and retrospective reviews. Still, rational approaches can be inferred from the extant literature for this uncommon but serious problem (Table 131.1).

Table 131.1 Definitions of Hypothermia

LEVEL OF HYPOTHERMIA	CORE TEMPERATURE (° C)
General	>35
Mild	32-35
Moderate	28-32
Severe	<28

Frostbite is a freezing injury to soft tissues secondary to cold exposure and results in loss of circulation to and therefore viability of the affected area. The extremities, nose, ears, and male genitalia are the most commonly affected areas.

Hypothermia

Pathophysiology

Mechanisms of Heat Loss

The mechanisms of heat loss are as follows:

• Radiation of heat occurs when the ambient temperature is less than body temperature and heat is lost directly to the environment via electromagnetic radiation.

• Conduction is heat transfer from one (warmer) solid to another (cooler) when they are in contact.

• Convection is loss of heat from a surface to a (usually moving) gas or fluid, typically air or water. It can be considered an adjunct to conduction.

• Evaporation causes heat loss through the energy required to vaporize water (i.e., sweat).

As a person cools, a fairly predictable procession of pathophysiologic changes occurs, as seen in Table 131.2 and Figure 131 .1.

Table 131.2 Pathologic Changes Seen with Hypothermia

Fig. 131.1 Osborn wave seen on the electrocardiogram of a patient with hypothermia.

The wave is usually seen at body temperatures lower than 30° C; however, it is neither diagnostic nor prognostic of hypothermia and is thus of academic interest only.

Presenting Signs and Symptoms

Patients with mild hypothermia are awake, occasionally drowsy, uncomfortable, and shivering. They simply need insulation (blanket), dry clothes, and food. They will recover completely and can be discharged when normothermic and feeling better.

Patients with moderate hypothermia are generally confused and lethargic, often have slurred speech, and are typically not shivering. They require more energetic rewarming measures, including heated blankets, resistive and hot air blankets (Bair Hugger), and close monitoring, including core temperature. Though strictly considered active internal rewarming, the use of heated, humidified oxygen and warmed intravenous (IV) fluids is reasonable in this situation. Patients whose hypothermia responds to these measures may be discharged when normothermic, awake, alert, and ambulatory. Patients with severe hypothermia require prompt intervention, close monitoring, and potentially aggressive, invasive rewarming therapies.

It is of the utmost importance to learn the circumstances that led to the patient becoming hypothermic. The possibility of a verify drug overdose, trauma, infection, drowning, or decompensated comorbid conditions—to name but a few examples—must be considered, sought, and treated along with the hypothermia.

The critical element of the diagnosis of hypothermia is accurate measurement of core temperature. Several methods exist, all of which have potential drawbacks (Table 131.3). Laboratory and physiologic changes are correlated with the temperature. For example, a normal hematocrit value in a severely hypothermic patient should prompt concern for hemorrhage because the hematocrit should rise in a predictable fashion with ever-lowering temperature. Alternatively, arterial blood gas values should be interpreted as though the patient is normothermic (the alpha-stat method) and not corrected for the actual core temperature (the pH-stat method). Evaluation for infection, metabolic derangement, and cardiac, neurologic, renal, and other organ system abnormalities is important because comorbid conditions are common as a cause, a consequence, or coincidence of hypothermia.

Table 131.3 Methods of Measuring Core Temperature

METHOD	COMMENTS
Esophageal probe	Easy to insert
Esophageal probe	Falsely high temperature readings possible with warmed oxygen via an endotracheal tube
Rectal probe	Insert to 15-20 cm
Rectal probe	If the probe is in or surrounded by cold stool, temperature recordings will lag behind true changes
Temperature-recording Foley catheter	Inflowing cold urine may falsely lower temperature recordings
Pulmonary artery catheter	Most accurate and most invasive method
Pulmonary artery catheter	Higher potential for iatrogenic injury, especially ventricular fibrillation in cold, irritable myocardium