Passive rewarming allows patients to rewarm on their own, using endogenous heat produced by metabolism. Since patients often become hypothermic over a period of hours to days, slow, passive rewarming is physiologically sound, avoiding rapid changes in cardiovascular status and the complications associated with active rewarming methods.
Patients must have intact thermoregulatory mechanisms and be capable of metabolic heat production for successful passive rewarming. With severe hypothermia or hypothermia secondary to an underlying illness (see Table186-1), patients may fail to rewarm passively; active rewarming is then indicated. In addition, since temperature rises slowly with passive rewarming, it is inappropriate for patients with cardiovascular compromise.
Active external rewarming (application of exogenous heat to the body) is often very effective in raising body temperature. Most of the methods listed in Table 186-3
are easily instituted, although some, especially warm water immersion, make resuscitation and monitoring difficult. Rewarming with heated air forced through slits in commercially available plastic or paper blankets appears very promising; this method has been used successfully in moderately to severely hypothermic patients. 1 I5
External rewarming does have disadvantages. It may be ineffective with poor perfusion of the periphery, especially in patients in cardiac arrest. Application of external heat may cause peripheral vasodilation and venous pooling, leading to relative hypovolemia and hypotension (rewarming shock). Washout of lactic acid from the peripheral tissues may lead to "rewarming acidosis," and an increase in metabolic demands of the periphery before the hypothermic heart can provide adequate tissue perfusion may lead to further tissue hypoxia and acidosis.
The core temperature may continue to decline after rewarming has begun. This "core temperature afterdrop" was previously ascribed to the return of cold blood to the core induced by external warming and peripheral vasodilation. This mechanism is unlikely; afterdrop can be explained by the continued conduction of heat from the relatively warmer core to the colder peripheral tissues. The incidence and magnitude of afterdrop are unclear, and it is probably of little clinical significance. 6
Active core rewarming has several theoretical advantages. Internal organs including the heart are preferentially rewarmed, decreasing myocardial irritability and returning cardiac function. Peripheral vasodilation is avoided, decreasing the incidence and magnitude of rewarming shock and acidosis. However, some internal rewarming techniques are invasive and may be difficult to institute.
Inhalation rewarming—administration of warmed, humidified oxygen via mask or endotracheal tube—provides a fairly small heat gain and is not effective for rapid rewarming. This is an important modality, however, because it minimizes heat loss from the lungs, a potential loss of up to 30 percent of the total metabolic heat production. Similarly, intravenous fluids should be warmed to avoid further cooling by the administration of fluids at room temperature. Heat gain is usually fairly small, although warming of infused fluid and blood can contribute significant amounts of heat to patients receiving massive fluid resuscitation. Commercial fluid warmers allow the temperature of infused fluids to be precisely controlled. Inhalation rewarming and warm intravenous fluids should be used in all but mild cases of hypothermia, since these are simple, noninvasive techniques with minimal risk of complications.
Gastrointestinal tract (gastric or colonic) lavage with warmed saline is technically simple, and patients can be lavaged with large volumes of fluid in a short time period. The obtunded hypothermic patient may develop pulmonary aspiration if lavaged with an unprotected airway. In a manner similar to gastrointestinal tract lavage, the bladder can be lavaged with warm saline solution using a Foley catheter.
Peritoneal lavage affords relatively rapid rewarming.16 It is widely available, may be instituted rapidly and with little technical difficulty, and has been shown to be effective in both animal studies and human applications. Potassium-free dialysis solution is warmed to 40 to 45°C (104-113°F), instilled, and then removed; the use of two catheters (one for fluid instillation and one for removal) may increase the rewarming rate.
Pleural lavage using thoracostomy tubes has provided effective rewarming in animal studies and a few human cases.17 Lavaging the left thoracic cavity delivers heated fluid in close proximity to the heart, potentially allowing rapid cardiac warming. Two thoracostomy tubes (for fluid inflow and outflow) have generally been employed. If this technique is chosen, care must be taken to monitor the net fluid infusion, as increased intrathoracic pressure and tension hydrothorax may complicate the procedure. The risk of precipitating dysrhythmias during chest tube insertion is unknown.
Rapid internal rewarming also can be accomplished through an extracorporeal circuit.1 l9 This consists of an arteriovenous shunt in which blood is routed to a warming device and then returned to the patient. Pump-assisted cardiopulmonary bypass using the femoral vessels for access is the most commonly used extracorporeal technique; right atrial-aortic bypass using a median sternotomy and heated hemodialysis also have been employed. Continuous arteriovenous rewarming using a countercurrent heat exchanger (a modified commercial fluid warmer) interposed between catheters placed in the femoral vessels, with flow driven by the patient's blood pressure, also has been reported.20 This technique obviates the need for pump support and systemic heparinization but is ineffective in hypotensive patients.
Profoundly hypothermic patients may be rewarmed in a very short time period with these methods. —I9 In addition to allowing rapid rewarming, pump-driven partial (femoral-femoral) or complete cardiopulmonary bypass provides circulatory support and oxygenation of blood, a great advantage in the management of patients in cardiac arrest or with severe cardiovascular compromise. Specialized equipment and personnel are required, however, and lack of immediate availability often precludes the use of this technique. In addition, the heparinization required for some extracorporeal techniques may cause complications in hypothermic trauma patients.
Various diathermy and radiowave techniques, although promising, have had limited use in hypothermic humans.
Finally, mediastinal irrigation using open thoracotomy has been used successfully as a rewarming technique in a few patients. It is possible that these patients could have been resuscitated using less invasive modalities. Thoracotomy has many potential complications and should only be considered in arrested patients. Even then, indications for this procedure are unclear.
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