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.

For ventricular fibrillation (VF), experts have traditionally recommended avoiding pharmacotherapy or repeated attempts at defibrillation until rewarming has been achieved. Although durable cardioversion of VF has been reported at a temperature as low as 20° C, this was thought to be the exception rather than the rule, meriting perhaps a single attempt at cardioversion, with further attempts avoided until rewarming to a temperature > 30° C. Pharmacotherapy has traditionally been avoided on the basis of concerns that slowed metabolism of antiarrhythmics and increases in serum levels during rewarming could result in toxic levels. Although convincing human trials do not exist to support one strategy or another, on the basis of animal data and isolated clinical reports, the American Heart Association endorses the use of standard advanced cardiac life support (ACLS) protocols concurrent with rewarming, including cardioversion and administration of amiodarone for VF.

9 Aside from the life support measures described, what other initial management should be undertaken?

Volume resuscitation is necessary in most patients with severe hypothermia. Throughout the onset of hypothermia, patients are typically unable to maintain plasma volume both because of impairment of the normal thirst mechanism and because they may not be able to access water. Hypothermia also induces a cold diuresis, with loss of copious dilute urine even in the setting of dehydration. As rewarming reverses peripheral vasoconstriction, hypovolemia may become apparent. In neonates with hypothermia, adequate fluid resuscitation significantly reduces mortality. Such definitive mortality data are lacking in adults, though volume resuscitation does improve hemodynamics. Intravenous administration of 500 to 1000 mL of 5% dextrose in 0.9% sodium chloride is a reasonable starting point for adults, with ongoing resuscitation dictated by clinical response. The hypothermic liver may not be able to metabolize lactate, and therefore lactated Ringer solution is generally avoided.

10 What are the methods for rewarming patients?

Because no controlled studies of various rewarming methods have been rigorously carried out, no rigid protocol can be justifiably instituted. Clinicians should be aware of their own institutional capacities. A recommended algorithm for rewarming is presented in Figure 76-2.

Figure 76-2 Algorithm for initial treatment and method of rewarming in accidental hypothermia. T, Temperature.

The first decision for the clinician is whether to choose active or passive rewarming. Passive external rewarming (PER) is the treatment of choice for patients with mild hypothermia (temperature ≥ 32° C). PER involves covering the patient with insulating material in a warm environment (ambient temperature > 21° C). This minimizes the normal processes of heat loss while relying on the patient’s own metabolism to generate the heat necessary for rewarming. PER has the advantages of being simple and noninvasive and is usually effective in mild hypothermia. However, patients with primary defects that prevent adequate endogenous heat production such as glycogen depletion, endocrine deficiencies, untreated sepsis, central nervous system lesions, or hypovolemia need to have their primary abnormalities addressed and may require active rewarming.

Active rewarming can be divided into active external rewarming (AER) and active core rewarming (ACR).

11 Which patients with hypothermia should undergo CPB?

Any patient with severe hypothermia who is hemodynamically unstable should be considered for CPB. CPB is probably the best hope for patients with severe hypothermia without spontaneous circulation. Patients must have no contraindication to anticoagulation, or a CPB circuit that does not require heparinization must be available. Various centers have established different exclusion criteria aimed at identifying those in whom attempted resuscitation is fruitless including pH < 6.5, serum potassium level > 10 mmol/L, and temperature < 12° C, though none of these are extensively validated. One physician with a core temperature of 13.7° C survived after CPB and was able to return to work.

12 What are the therapeutic uses of hypothermia?

Therapeutic hypothermia is most widely accepted as a neuroprotective therapy for adults in cardiac arrest who remain comatose after return of spontaneous circulation. It is also routinely used during cardiovascular surgery requiring circulatory arrest. It is under investigation and used in some centers, for treatment after ischemic cerebral vascular accidents, traumatic brain injury, spinal cord injury, trauma resuscitation, and neonatal asphyxia. The appropriate indications for the use of therapeutic hypothermia in these various disease states are still being defined. Relative contraindications include severe coagulopathy and active bleeding.

13 What is the usual target temperature in therapeutic hypothermia?

Few studies directly compare different cooling mechanisms and protocols. Standard cooling regimens (as for cardiac arrest) target a core temperature of 30° to 34° C with cooling rates of 2° to 4° C per hour. This temperature range has demonstrated neuroprotective effects, with a low incidence of arrhythmias and other complications of more severe hypothermia.

14 How are patients cooled to the target temperature?

Cooling frequently must be initiated in an environment (such as the emergency department) where complex, self-regulating cooling equipment is not available. The cooling process can be begun by placing ice packs around the groin, axillae, head, and neck; administering refrigerated (4° C) intravenous fluids; and covering the patient in a mist of water or alcohol and applying a fan. Intravascular and surface cooling methods have recently been shown to be equivalent in a retrospective review. Intravascular cooling devices circulate cold water through a specially designed intravascular catheter. Surface cooling similarly circulates cold water through pads applied to the patient’s skin. In both systems the cooling device is attached to a patient thermometer that provides feedback control. Unless central blood temperature is measured directly, a lag time will exist between measured temperature (via the esophagus, rectum, tympanic membrane) and true core temperature. This can result in an overshoot below the desired core temperature during induction of hypothermia. The faster the cooling rate, the greater the potential to overshoot.

15 What should be done if a patient shivers during therapeutic hypothermia?

Shivering must be suppressed for effective cooling. Some centers use routine paralysis to prevent shivering, though suppression of shivering can be achieved without paralytics in most patients through the appropriate use of medications such as propofol, narcotics, and magnesium. Avoiding paralysis has some advantages. It:

16 How long are patients kept in hypothermia?

Hypothermia is usually discontinued after a specified period of time (24–48 hours) based on protocol. For certain disease processes, such as traumatic brain injury, alternative end points such as the normalization of intracranial pressure have been investigated. The cooling systems (intravascular or surface) described above can be used to regulate rewarming. A typical rate of rewarming is 0.2° to 0.5° C per hour.

17 What are the important side effects of therapeutic hypothermia?

Hypothermia affects nearly every organ system and so has myriad side effects of variable clinical significance. Electrolyte abnormalities and infectious complications are most likely to concern the clinician. Electrolyte abnormalities commonly require management and must be actively monitored. During cooling, hypomagnesemia and hyperglycemia are common, whereas, during rewarming, hypoglycemia and hyperkalemia are dangers. Most centers check electrolyte panels every 30 to 60 minutes during cooling and every 4 to 6 hours during the maintenance of hypothermia. Hypothermia may be immunosuppressive via a number of mechanisms and masks the normal febrile response to infection. Many centers draw routine daily blood cultures during induced hypothermia to monitor for infection. An aggressive stance toward investigation and empiric treatment of other potential infectious sources is appropriate, though routine antibiotic prophylaxis is not justified.