Hypothermia

1 How is hypothermia defined?

Hypothermia is defined as a core temperature < 35° C (95° F) and is further classified as follows: mild 35° C to 32° C, moderate 32° C to 28° C, severe 28° C to 20° C, and profound < 20° C. In suspected hypothermia, the temperature is best measured via the rectum or esophagus. A rectal probe should be inserted 10 to 15 cm and not placed into cold feces. Care should be taken to use a thermometer without a minimum temperature; many commonly used thermometers have a minimum temperature of 35° C.

2 What are the five modes of heat loss?

Heat is lost through the following:

Radiation normally accounts for approximately 60% of heat loss and is dependent on the ambient temperature and the amount of body surface exposed. Conduction normally accounts for a small percentage of heat loss, but in patients lying uninsulated on the ground, wearing wet clothing, or, in the most extreme example, immersed in cold water it may account for the majority of heat loss. Convection normally accounts for 10% to 15% of heat loss, though this increases in minimally clothed people and in windy conditions. Inspiration of cold air can account for 2% to 9% of heat loss. Evaporation from the skin and respiratory tract accounts for approximately 25%.

Accidental hypothermia usually results from increases in heat loss through one of the mechanisms listed previously. It may also be caused by decreased heat production (as can occur in hypothyroidism or adrenal insufficiency) or impaired thermoregulation (as sometimes occurs with central nervous system injury or certain toxic ingestions).

3 What is the significance of shivering?

Shivering is an intrinsic mechanism for augmenting heat production in response to hypothermia. It occurs at core temperatures between 32° C and 37° C. Shivering can result in a fivefold increase in basal metabolic rate but cannot continue when muscles fatigue, their glycogen stores are depleted, or body temperature falls below 32° C. The cessation of shivering in a patient still exposed to the cold should be considered an ominous sign of progressive hypothermia.

4 What is the J wave?

Also known as the Osborn wave, or hypothermic hump, the J wave is a hypothermia-related elevation of the J point at the junction of the QRS complex and ST segment (Fig. 76-1). J waves appear at temperatures at or below 32° C and are usually first seen in leads II and V₆. As the temperature decreases further, they increase in size and may appear in the precordial leads. The J wave is neither specific, nor sensitive, nor prognostic in hypothermia. Automated electrocardiographic interpretation software may misinterpret the J wave as ischemic injury (ST elevation), and it is important not to fall into this trap.

Figure 76-1 J waves in lateral leads during moderate hypothermia.

5 What is core temperature afterdrop?

Core temperature afterdrop is the phenomenon of continued decrease in core temperature when a patient with hypothermia is removed from the cold. It likely results from a combination of thermal equilibration between the relatively warmer core and colder extremities and an increase in convective heat loss due to peripheral vasodilation. It can be avoided by confining active external rewarming to the torso and stabilizing core temperature before thawing frozen extremities. Limiting activity, which stimulates peripheral blood flow, may also be helpful.

6 Which patients with hypothermia should receive cardiopulmonary resuscitation (CPR)?

CPR should be instituted in the patient with hypothermia with no signs of life and no verifiable spontaneous circulation. Tissue destruction, dependent lividity, rigor mortis, and fixed dilated pupils are not reliable indicators of death in hypothermia.

Even extremely limited perfusion and circulation, which can be difficult to detect, may be sufficient to meet the reduced metabolic needs in hypothermia. Because mechanical agitation is a possible source of arrhythmias in hypothermia, unnecessary CPR (as can occur if a hypothermic bradyarrhythmia is not appreciated) may be especially detrimental to the patient with hypothermia. A Doppler device to detect blood flow or an ultrasound machine to identify cardiac activity may be useful to identify otherwise difficult-to-detect perfusion. At least 1 minute should be spent seeking evidence of a pulse in the patient with hypothermia before instituting CPR.

Once CPR is initiated, it should be continued until the patient has return of spontaneous circulation, cardiopulmonary bypass (CPB) is initiated, or the patient remains unresuscitated despite reaching a core temperature > 35° C. Many cases of prolonged CPR resuscitating patients with hypothermia have been documented. Guidance is provided by the aphorism that “you’re not dead until you’re warm and dead.”

7 You said mechanical agitation can cause arrhythmias. Is it acceptable to endotracheally intubate? What about central venous or pulmonary artery catheterization?

Endotracheal intubation is safe in the patient with hypothermia, and the indications are the same as in normothermia. It will often be necessary to secure the airway because patients with severe hypothermia are obtunded and have depressed airway reflexes, bronchorrhea, and ileus.

The hypothermic heart is extremely prone to arrhythmia. Even central venous line placement may induce arrhythmias if the guidewire or tip of the catheter enters the atrium. It is best to avoid internal jugular or subclavian placement or to be extremely cautious about the depth to which the guidewire and catheter are advanced. If central access is indicated, it is safest to establish via the femoral vein. Pulmonary artery catheters should be avoided because of the potential for induced arrhythmias, as well as a possible increased risk for pulmonary artery rupture.

8 How should arrhythmias be treated in the patient with hypothermia?

Atrial fibrillation and other supraventricular arrhythmias are common and almost universally benign. They will nearly always resolve with rewarming and should not be treated pharmacologically. Asystole in the patient with severe hypothermia is not as ominous a rhythm as in normothermia and may resolve with rewarming.