The severity of hypothermia, clinical findings, and comorbid conditions of the patient are important factors for determining the aggressiveness of resuscitation techniques (Fig. 131-2). Once hypothermia is confirmed, assessment and treatment of the critically ill patient should take place simultaneously. Actions in all patients should include prompt removal of wet clothing, protection against continued heat loss, continuous monitoring of cardiac status, and avoidance of rough movement and excess activity, which can precipitate ventricular fibrillation.21,25-27 Patients received in the hospital with moderate or severe hypothermia should be resuscitated until adequate rewarming has occurred or efforts are deemed unsuccessful.14,25,26
A systematic approach to a patient with hypothermia.
Hypothermia should be confirmed by an accurate assessment of core temperature with a low reading thermometer. In patients with moderate to severe hypothermia, temperature should be assessed continuously. Other vital signs may be difficult to evaluate, but evidence of a spontaneous pulse or blood pressure should be aggressively sought. The use of a Doppler ultrasound device may be necessary. Pulse oximetry is unlikely to be accurate in the setting of hypothermia and poor perfusion.
Supplemental oxygen should be administered pending assessment of oxygenation. Endotracheal intubation is indicated unless the patient is alert and has intact airway reflexes. Orotracheal intubation is preferred due to the risk of traumatic bleeding with the nasal route. However, muscle rigidity of the jaw in moderate to severe hypothermia may preclude use of the oral route. Neuromuscular blockers are unlikely to be effective at temperatures below 30°C (86°F) and should be avoided. Topical vasoconstrictors and the use of a smaller endotracheal tube may facilitate blind nasotracheal intubation in the patient with some spontaneous respirations. Intubation is unlikely to induce dysrhythmias in hypothermic patients.10
A nasogastric or orogastric tube should be placed in patients with moderate or severe hypothermia to relieve gastric distention. A urinary catheter is also essential to monitor urine output and assess volume resuscitation efforts. Peripheral venous large gauge catheters are preferred over central venous access due to the potential to precipitate dysrhythmias. The femoral vein is the preferred site if a central venous catheter is needed. Intraarterial catheters for pressure monitoring should be used selectively. Pulmonary artery catheters are avoided due to potential dysrhythmias and the risk of vascular perforation. After rewarming, invasive monitoring may be warranted in complicated cases. Patients with moderate or severe hypothermia should be handled gently because movement and manipulation may precipitate arrhythmias. Associated conditions requiring urgent intervention, such as traumatic injuries, hypoglycemia, or endocrinologic insufficiency, should be sought.
Patients with moderate or severe hypothermia are volume depleted and require rapid assessment of volume status and administration of fluids. A reasonable approach is to administer a 250- to 500-mL fluid challenge of warmed 5% dextrose in normal saline until further evaluation can be completed. During rewarming, increased fluid requirements are often necessary to prevent or treat hypotension. It has been recommended that lactated Ringer solution be avoided due to decreased ability of the liver to metabolize lactate.
In moderate or severe hypothermia, intravenous fluids heated to 40°C to 42°C (104-107.6°F) should be administered. Heated intravenous fluids add little heat (unless very large volumes are infused) but do avoid further heat loss and the cooling effect of fluid therapy. Crystalloids can be heated in a microwave or commercial fluid warmer and should be mixed adequately before administration. Conductive heat loss can be minimized by using short segments of intravenous tubing and administering fluid as intermittent boluses.28 Alternatively, fluids can be heated to higher temperatures (60°C, 140°F) when long lengths of tubing cannot be avoided.
Hemoconcentration is usually present in the severely hypothermic patient. A low initial hematocrit suggests acute hemorrhage or preexisting anemia. Occasionally, transfusion of packed red blood cells may be necessary as part of acute resuscitation. Different blood warming devices are available to deliver warm red cell products. The patient should be monitored closely for clinical signs of fluid overload during rewarming.
Cardiopulmonary resuscitation is indicated in any patient with moderate to severe hypothermia in whom no pulse is present after appropriate evaluation or who has a nonperfusing rhythm such as ventricular fibrillation or asystole. The patient with a bradycardic pulse does not require pharmacologic manipulation of heart rate. Chest wall compressions are difficult and require more force in the hypothermic patient due to decreased chest wall elasticity. Frequent change of personnel performing chest compressions is necessary to prevent fatigue and ineffectiveness.
Initial defibrillation should be attempted for ventricular fibrillation or ventricular tachycardia without a pulse, although attempts may be unsuccessful at temperatures below 30°C to 32°C (86-90°F). If initial defibrillation is unsuccessful, rewarming is instituted and defibrillation is attempted again after every 1°C to 2°C increase in temperature or when core temperature increases above 30°C to 32°C. Antiarrhythmic and vasoactive drugs are usually ineffective at temperatures below 30°C (86°F). These drugs should generally be avoided until rewarming to above 30°C (86°F), and then the lowest effective dose should be used. Excessive administration of resuscitation drugs may result in toxicity with rewarming due to altered metabolism.
Most hypothermia-induced arrhythmias will convert spontaneously with rewarming. Atrial fibrillation is seen frequently and does not usually require specific treatment because the ventricular rate is usually slow. The best approach to ventricular arrhythmias has not been determined. Bretylium tosylate has been effective in animal studies but is not currently used in resuscitation.27,28 Lidocaine appears to be less effective, and procainamide may increase the incidence of ventricular fibrillation. The effects of amiodarone in hypothermic patients have not been evaluated. The optimal dose and infusion rate of bretylium and other vasoactive drugs are unknown in severe hypothermia.
Hypotension should be treated initially with volume replacement. Vasopressor agents have minimal effect on constricted vessels in moderate or severe hypothermia but may increase the risk of dysrhythmias.
Although rewarming is the primary treatment for moderate or severe hypothermia, controversy exists as to the optimal method, duration, and rate of rewarming. No controlled studies comparing rewarming methods exist, and rigid treatment protocols cannot be recommended. Rapid rates of rewarming have not been proven to improve outcome in patients with severe hypothermia. The clinical circumstances, availability of resources, and advantages and disadvantages of available methods should be taken into account when selecting specific interventions for the patient.3,7-9,14,18,21,25,31
PASSIVE EXTERNAL REWARMING
Passive external rewarming involves covering the patient with an insulating material to prevent further heat loss. This method is the treatment of choice for most patients with mild hypothermia and is used as an adjunct in patients with moderate or severe hypothermia. Patients must have adequate physiologic reserve to increase their metabolic rate and generate heat to rewarm themselves. Rewarming rates with passive external rewarming in mild hypothermia vary between 0.5°C and 2.0°C per hour.6
Active rewarming involves the transfer of exogenous heat to the patient by using external or internal techniques. Indications for active rewarming include cardiovascular instability, poikilothermia (<32°C), inadequate rewarming with other methods, endocrinologic insufficiency, and traumatic peripheral vasodilation (ie, spinal cord injury). Patients with endocrinologic diseases may have insufficient glycogen stores or insufficient endogenous thermogenesis.
ACTIVE EXTERNAL REWARMING
Different methods have been used for active external rewarming (AER), including immersion in a 40°C bath, warming blankets, heating pads, radiant heat, and forced air rewarming. Indications for use of these devices remain controversial. Concerns were raised in the past regarding AER because vasodilation in the extremities may facilitate transport of colder peripheral blood to the warmer core, thereby lowering the core temperature (“afterdrop”), but experience with AER has not found evidence of afterdrop. Peripheral vasodilation may also potentially worsen hypotension. Immersion in a warm water bath can impede monitoring and active resuscitation. Thermal injury can occur with heating pads, warming blankets, and radiant heat sources. The most practical technique for AER in hospitals is forced air rewarming, which transfers heat convectively and prevents heat loss. These devices are usually readily available from postoperative care units. They enable greater contact of warm air with the patient’s body than traditional warming blankets. Successful use of forced air as the primary rewarming method in patients with severe hypothermia with and without cardiac arrest has been reported.32 Newer resistive polymer blankets have been compared to forced air rewarming in volunteers and postoperative patients but there is no experience in hypothermia victims.33,34
Warming rates of 1°C to 2.5°C per hour have been reported with AER after accidental hypothermia.32,35 Circulatory problems may be minimized if AER is applied only to the trunk. Truncal AER may be combined successfully with other methods of active core rewarming such as warmed intravenous fluids and heated humidified oxygen. The advantages of AER are ease of institution, availability, low cost, and noninvasiveness.
Numerous alternatives are available for active core rewarming which is the application of heat to the body core. Airway rewarming using heated humidified oxygen is relatively simple to institute and should be a part of management of most patients with moderate or severe hypothermia.36 The delivery of heated oxygen is more effective through an endotracheal tube than by mask. Oxygen should be warmed to 40°C to 45°C (104-113°F) through modification of humidifier devices.37 A rewarming rate of 1°C to 2.5°C per hour can be expected.7 Although there are several proposed advantages of airway rewarming that include decreased respiratory heat loss, increased heat donation to the respiratory tract, and direct heat transfer to the hypothalamus, brain stem, and medulla, its efficacy remains equivocal.38
Heated irrigation has been used to transfer heat from fluids to internal body areas with a variety of techniques. Peritoneal dialysis or lavage is probably the most widely recognized method of heated irrigation. Peritoneal dialysis can deliver fluid heated to 40°C to 45°C (104-113°F) to the peritoneal cavity with flow rates of approximately 6 L/h. Potential advantages of this technique are hepatic rewarming, use during chest compressions, and the capability to simultaneously provide renal replacement when a dialysate is used. Rewarming rates average 1°C to 3°C per hour.6 This technique is not routinely advocated for stable patients but may be considered in combination with other rewarming methods for the patient with no evidence of perfusion.
Closed thoracic lavage also has been proposed for treatment of hypothermia.39-42 This technique uses a large thoracostomy tube inserted into the anterior second or third intercostal space in the midclavicular line. A second tube is inserted in the posterior axillary line in the fifth or sixth intercostal space. Sterile normal saline heated to 40°C to 42°C (104-107.6°F) is infused through the anterior tube and allowed to drain passively from the posterior tube. This technique may have the advantage of warming the heart and great vessels. However, clinical experience is limited. Mediastinal irrigation and myocardial lavage could be considered in patients with severe hypothermia and no spontaneous perfusion. These techniques require expertise for thoracotomy and clinical experience is limited.
Irrigation of the stomach, bladder, or colon has limited utility because the surface area available for conductive heat transfer is small. In addition, gastric lavage may predispose to aspiration and must be discontinued during chest compressions. Special double lumen esophageal tubes or modified Sengstaken tubes have had limited evaluation and use.43,44 These techniques are warranted only if no other methods are available for rewarming.
Several methods have been used for extracorporeal blood rewarming. These include hemodialysis, venovenous rewarming with continuous renal replacement techniques, venovenous extracorporeal membrane oxygenation (ECMO), and cardiopulmonary bypass (CPB). Hemodialysis uses a two-way flow catheter with percutaneous cannulation of a single vessel. The femoral vein is preferred over the subclavian vein to avoid myocardial irritation with the guidewire. This technique may be most appropriate in the patient without severe hemodynamic instability, although it has been used in unstable patients.45,46 Hemodialysis may be the preferred rewarming method when hypothermia is associated with severe renal dysfunction or intoxication with dialyzable substances. An alternative to hemodialysis is continuous venovenous rewarming.47 This technique uses countercurrent fluid warming in the dialysis cartridge with use of a roller pump.
More recently, venovenous ECMO has been utilized for treatment of severe hypothermia in patients with cardiovascular instability.48,49 Potential advantages include the availability of portable units, limited need for heparinization, percutaneous cannulation that does not interfere with resuscitation, support of pulmonary function and rewarming rates similar to CPB.
CPB using standard access through the femoral artery or femoral vein is the most invasive and labor-intensive technique for rewarming.50 It has the advantages of providing complete hemodynamic support during rewarming and rapid rewarming rates.18 Core temperature can increase 1°C to 2°C every 3 to 5 minutes with femoral flow rates of 2 to 3 L/min.6 Unfortunately, CPB may require considerable time to institute and is not available in all institutions. Systemic anticoagulation may be contraindicated in trauma victims or contribute to hypothermic coagulopathy. Heparin-bonded tubing, portable circuits, and methods using venovenous access may overcome some of these problems.51,52 These advances have allowed the institution of CPB in emergency departments and intensive care units.53 Long-term outcomes of patients with severe hypothermia treated with CPB have been favorable.54
Another technique for active core rewarming is intravascular warming with an endovascular temperature control device. These systems are used most commonly to induce mild hypothermia in patients suffering cardiac arrest. Experience is very limited but potential advantages may include percutaneous femoral insertion and no use of anticoagulation.55,56
Techniques such as the use of very high-temperature intravenous fluids and diathermy are being explored for the treatment of moderate or severe hypothermia. Intravenous fluids heated to 65°C (149°F) have been used in animal studies and resulted in rewarming rates of 2.9°C to 3.7°C per hour with minimal intimal injuries.57,58
Diathermy involves the conversion of energy waves into heat. Ultrasound or low-frequency microwave radiation can deliver large amounts of heat to deep tissues. Although animal studies are promising, further investigation is needed to determine optimum clinical use.6,38
Numerous complications are associated with moderate and severe hypothermia and should be anticipated by the clinician. Continuous monitoring and frequent reassessment of metabolic and hemodynamic parameters are essential to successful outcome. Rhabdomyolysis is frequent and may result in electrolyte disturbances and renal dysfunction. Compartment syndromes may become apparent several days after initial treatment. Acute respiratory distress syndrome, acute tubular necrosis, and disseminated intravascular coagulation may require intensive interventions.