Hypothermia Treatment & Management

Prehospital management focuses on preventing further heat loss, rewarming the body core temperature, and avoiding precipitating ventricular fibrillation or another malignant cardiac rhythm. This should be the preeminent concern. Conscious patients can develop ventricular fibrillation suddenly; prehospital workers, particularly those operating in remote search-and-rescue operations, should avoid inadvertent jerky movement of severely hypothermic patients. Patients who develop hypothermia-induced dysrhythmia in the field may be beyond resuscitation. How the hypothermic heart deteriorates into the rhythm of ventricular fibrillation remains under debate.

Patients developing hypothermia from cold-water immersion appear to be at high risk of fibrillation; rescuers probably are justified in instructing such patients to minimize motion and to await careful extrication.

Anecdotal reports of sudden cardiac death associated with tracheal intubation appear to be exaggerated, particularly if a patient is adequately preoxygenated.

Both cardiac pacing and atropine are generally ineffective for bradyarrhythmia.

Lidocaine is ineffective in preventing hypothermia-induced ventricular dysrhythmias.

Many authors have advocated prophylactic bretylium in cases of severe hypothermia when spontaneous conversion to ventricular fibrillation is possible. This recommendation is due to the success of such therapy both in controlled animal studies and in anecdotal human reports.

• Cardiac dysrhythmias begin to develop at a core temperature of 30°C.
• Ventricular fibrillation susceptibility is greatest below the core temperature of 22°C.
• Bretylium (5 mg/kg initially) is recommended for any hypothermic patient manifesting significant new ventricular ectopy or frank dysrhythmia. However, bretylium has been discontinued by all manufacturers resulting in a worldwide shortage and has been unavailable to many centers since 1999.
• Although the optimal dosage and ideal infusion rate for bretylium are unknown, consider prophylactic bretylium for patients with core temperatures below 30°C.

To prevent cardiac dysrhythmia with continued hypothermia, rescuers or paramedics should attempt rewarming in the field. (A notable exception would be isolated frostbite injury in which limb rewarming would preclude self-rescue because of pain.)

• Gently place patients in an environment most favorable to reducing further heat loss from evaporation, radiation, conduction, or convection.
• Remove wet clothing, and replace it with dry blankets or sleeping bags.
• Initiate active external rewarming with heat packs (eg, hot water bottles, chemical packs) placed in the axillae, on the groin, and on the abdomen.
• Be aware of the risk of causing body surface burns from exuberant active external rewarming.
• In dire circumstances, when heat packs are unavailable, rescuers can provide skin-to-skin contact with patients.

Ventricular fibrillation in a cold patient is a desperate event. Generally, defibrillation is ineffective at hypothermic core temperatures and when equipment for heroic attempts at resuscitation is unavailable. In such circumstances, attempt a round of chemical conversion with intravenous bretylium (if available), followed by extended cardiopulmonary resuscitation (CPR) until rescuers can begin active rewarming and perform successful defibrillation.

Patients with respiratory failure should be endotracheally intubated and placed on a mechanical ventilator. Intubation and insertion of vascular catheters should not be delayed but performed gently while closely monitoring cardiac rhythm for ventricular fibrillation.

Measure core temperatures using a low-reading esophageal, rectal, or bladder thermometer. Tympanic thermometers are unreliable in a setting of profound hypothermia and should not be used. If using a rectal probe, be careful not to insert it into stool.

Determine whether a cold patient is profoundly or mildly hypothermic. Profoundly hypothermic patients present with stupor or cardiac dysrhythmia (regardless of the recorded temperature) and a core temperature of 30°C or lower. Mildly hypothermic patients may be rewarmed in any available manner (eg, warm blankets, removal of cold, wet clothing) since their risk for cardiac dysrhythmia is low. Surface rewarming is adequate in these cases, but it is ineffective in very low body temperatures and carries an additional risk of temperature after drops and shock secondary to peripheral vasodilation.

Remove any wet clothing, and replace it with warm, dry materials.

Profound hypothermia is a true emergency, warranting the same resource-intensive resuscitation as myocardial infarction. Direct treatment at maintaining or restoring cardiac perfusion; maximizing oxygenation is indicated for a prolonged period of time until the core temperature is at least 32°C.

Do not attempt resuscitation on the patient with a frozen chest where compressions are not possible.

Gingerly handle patients identified with profound hypothermia, and take immediate measures to prevent degeneration of cardiac activity into malignant dysrhythmia.

Profoundly hypothermic patients who demonstrate cardiac ectopy may be ideal candidates for bretylium, if available. Administer an initial dose of 5 mg/kg IV (repeated at 10 mg/kg, as needed) to prevent ventricular fibrillation. Lidocaine is ineffective for treatment of hypothermia-induced dysrhythmias. While no randomized human trials have been reported, at least 4 animal trials and 2 human case reports support using bretylium for any patient with profound hypothermia. Based on such evidence, the US Wilderness Emergency Medical Services Institute recommends using empiric bretylium for profound hypothermia.

Initiate warmed, humidified oxygen; provide heated intravenous saline; and place warmed blankets or heat lamps around a hypothermic patient.

Although many texts suggest that intravenous fluids be heated to 45°C, this temperature choice is based on convenience of previous study designs rather than any hard evidence. A trial using fluids heated to 65°C demonstrated more efficacy in treating severe hypothermia. Emergency departments that routinely treat hypothermia can keep blankets and intravenous fluid bags in a shared heater. In urgent situations, intravenous fluids that contain no dextrose or blood can be heated in a microwave oven. Once these simple measures have been applied, consider more difficult rewarming therapies.

Optimal rewarming techniques depend upon a patient's condition. Treatment is often limited by hospital availability and physician inexperience with rewarming approaches. If core body temperature does not respond to warming efforts, underlying infection or endocrine derangements must be considered.

A patient who is not becoming progressively colder, is conscious, and has a perfusing cardiac rhythm may not require intensive intervention beyond the methods already discussed.

Debate centers on interventions for patients who are worsening, are comatose, have nonperfusing rhythms, or appear dead. Most texts advocate aggressive therapy for severely hypothermic patients, basing the recommendation on anecdotal reports of success.

Researchers recently confirmed justification for aggressive treatment in a 16-year longitudinal review of profound hypothermia. In this series of 32 Swiss patients presenting with hypothermia and cardiac arrest, 15 patients were resuscitated with aggressive techniques, and all 15 patients showed full neurologic recovery.

In an older review, rewarming at rates faster than 2°C/h was noted to reduce mortality when compared with slower rates.

An optimal warming strategy is elusive. Some have postulated that rapidly warming a patient to 33°C and maintaining him or her at that temperature, using hypothermia therapeutically as though he or she was a cardiac arrest patient might be beneficial.

For simplicity, aggressive rewarming methods can be categorized as slow, moderate, or rapid. Slow rewarming provides heat from 17-30 kcal/h, corresponding to increasing temperature by 0.3-1.2°C/h. (Comparisons are somewhat difficult since different study groups used different measurements of heat gain.) Slow rewarming methods include IV solutions heated to 45°C (17 kcal/h); heated, humidified oxygen by mask (30 kcal/h or 0.7°C/h); warmed blankets (0.9°C/h); and heated, humidified oxygen via endotracheal tube (1.2°C/h). If intact, a patient's endogenous physiologic mechanisms (other than shivering) provide similar rates of rewarming (30 kcal/h).

Moderate rewarming methods provide heat at approximately 3°C/h. Methods include warmed gastric lavage (2.8°C/h), intravenous solutions heated to 65°C (2.9°C/h), and peritoneal lavage with 45°C fluid at 4 L/h (70 kcal/h or 3°C/h).

Rapid rewarming methods provide heat at levels higher than 100 kcal/h. Methods include thoracic lavage at 500 mL/min (6.1°C/h), cardiopulmonary bypass (400 kcal/h or 18°C/h), thoracic lavage at 2 L/min (19.7°C/h), ECMO, and AV dialysis (1-4 degrees per hour, and warm-water immersion [1500 kcal/h]).

In comparison, endogenous shivering provides rewarming at a rate of 300 kcal/h. No noninvasive technique rewarms as rapidly as full-body immersion in warm water. Known as the Hubbard tank technique, immersion has successfully rewarmed humans with severe hypothermia. Unfortunately, patients who require rapid rewarming in the emergency department also need cardiac monitoring and intravenous therapy, both of which are difficult to manage under water.

Defibrillation also is difficult; however, defibrillation is likely futile once a patient's core temperature falls below 30°C.

Initiate CPR for hypothermic patients who deteriorate into ventricular fibrillation. These patients also warrant immediate weight-based defibrillation (2 J/kg), along with prompt administration of high-dose bretylium (10 mg/kg).

Consider initiating cardiopulmonary bypass for any case of ventricular fibrillation or profound hypothermia with deterioration. Patients with this degree of hypothermia have optimized outcomes with procedures such as cardiopulmonary bypass and pleural lavage. However, these methods are invasive, often unavailable, and infrequently used and as such are subject to user-inexperience.

Ventricular fibrillation should be treated immediately with defibrillation, despite the fact that most other dysrhythmias will correct with warming alone. If initial attempts at defibrillation are unsuccessful, further attempts at defibrillation and antiarrhythmic intravenous medications should be held until the patient is warmed to above 30°C. During this interval, basic life support is continued. If ventricular fibrillation persists despite rewarming, current AHA guidelines recommend administration of amiodarone.

Although studies in emergency medicine are lacking, cardiothoracic surgeons who induce hypothermia to perform open-heart procedures rewarm patients on a daily basis using open cardiac massage with warmed saline solution. Therefore, a desperate case of severe hypothermia may warrant consideration of direct cardiac rewarming via open emergency department thoracotomy with open cardiac massage.

Cardiothoracic bypass has been successful for treating cases of accidental hypothermia with prolonged cardiac arrest. To be successful, bypass must be performed rapidly. If a delay is expected, the physician can expedite bypass during an interim period by placing cordis catheters in the patient's femoral vein and artery. Groin cutdowns may be necessary to facilitate such placement; if cutdowns are needed, perform them without hesitation. If bypass is unavailable or delayed, 2 previously described methods of internal rewarming are available: heated thoracic lavage and arteriovenous (AV) heated countercurrent exchange.

The literature describes 2 methods of thoracic lavage; the simplest method uses available equipment and provides rewarming rates equivalent to cardiopulmonary bypass.

The technique involves placing 2 left-sided, 38 French chest tubes (third intercostal space midclavicular line and sixth intercostal space midaxillary line). Isotonic saline, in 3-liter bags heated to at least 41°C, is infused through the anterior tube at 2 L/min, then drained by gravity via the posterior tube. When warmed saline was not available, physicians successfully infused warmed tap water.

The AV heating method, developed at the University of Washington, uses a modified bypass technique for rapid blood rewarming using a level one fluid warmer that is familiar to physicians experienced in trauma resuscitation. The treatment is preferred for patients with profound hypothermia and markedly depressed hemodynamic status or cardiac arrest. AV heating requires a spontaneous pulse, since the patient's intrinsic blood pressure drives flow through the countercurrent module. (In true cardiothoracic bypass, an external pump is built into the machine.) Catheters are placed into the femoral artery and venous cordis.

Once catheters are placed, the arterial output is connected to the inflow port of a level one countercurrent warmer, where intravenous fluids are connected. The outflow port is connected to the femoral venous catheter. Water is circulated, at a temperature preset on the level one device, around the blood-containing tubing; the blood warms as it flows through the countercurrent module. The AV method has rewarmed profoundly hypothermic patients 5 times more rapidly (39 min vs 199 min) than standard methods and was demonstrated to decrease the mortality rate.

In an alternative endovascular warming technique,^[5] a catheter is advanced into the inferior vena cava and circulates warmed fluids. The catheter acts as indwelling radiator as it is connected to an esophageal temperature probe and uses a feedback loop to attain and maintain programmed patient temperature. By this method, the core body temperature may be elevated at a rate of 3 degrees an hour. Additionally, it is an invasive technique to raise core temperature that utilizes skills that emergency physicians are already well trained and comfortable with.

Vasodilation increases the vascular space; consequently, patients that have been hypothermic for more than 45-60 minutes often require fluid administration. Hypotension should be addressed with volume resuscitation; inotropic agents, such as dopamine, should be avoided unless the hypotension is refractory to intravenous fluids due to the possible cardiac stimulation/ectopy that pressors may induce.

Probes for pulse oximetry placed on the ears or the forehead appear to be less influenced by the peripheral vasoconstriction of the digits associated with decreased body temperature.

Assessment should include a total body survey to exclude local cold-induced injuries.