Rewarming methods

Rewarming may be passive (endogenous heat production by the patient) or active (application of heat externally or internally). In active external rewarming, a heat source is applied to the skin surface, whereas in active internal rewarming heat is delivered to the core of the patient.

In passive rewarming the patient is first removed from the cold wet environment. The success of this method depends on the integrity of the thermoregulatory mechanisms. The temperature increases slowly (0.5-1 °C/h), depending on the physical conditions, comorbidities, and the degree of hypothermia.

External active rewarming involves applying a heat source on or over the body surface, which induces vasodilatation and improves circulation to superficial areas so that cold blood returns to the heart. The central core temperature may actually fall. However, the fall in temperature observed after removal from the cold environment may simply be due to the temperature gradient between the surface and internal tissue masses. This further drop in temperature may induce arrhythmias, further depress myocardial function, and lead to hemodynamic instability, shock, and death.

Heating pads, hot water bottles, circulating-water mattresses, and heating blankets should be used cautiously as they may cause burns. Immersion in a hot bath places the patient in an uncomfortable position which is inaccessible for physicians and nurses. Cardiopulmonary resuscitation and defibrillation are impracticable in water.

Warm air (heated to 43 °C) can be pumped through slits in a blanket. This method has been shown to increase body temperature by more than 2 to 3 °C/h. It should not be used for patients with cardiac arrest, hemodynamic instability, or hypothalamic dysfunction. Because of the vasodilatation that occurs when this method is used, rigorous and precise fluid resuscitation must be performed using warmed intravenous solutions. The volume of fluid administered is governed by hypotension and oliguria provided that no pulmonary edema has developed. The therapy should be guided by continuous monitoring.

Techniques for internal active rewarming include airway rewarming, gastrointestinal, peritoneal, or pleural cavity lavage, extracorporeal rewarming, and cardiopulmonary bypass.

A heated and humidified oxygen-air mixture may be administered through a face mask or endotracheal tube; the latter is more efficient because less heat is dissipated into the nasopharynx. The temperature increases by only 0.2 to 1.5 °C/h, although heat loss by respiration is prevented. Other advantages of this simple and readily available method include rewarming the heart and lungs first, restoring ciliary motility, and improving mobilization of bronchial secretions. Inspired air should be warmed to 40 °C in conscious patients and to 46 °C in intubated unconscious hypothermic victims. This procedure is relatively risk free.

Fluids (glucose 5 per cent, NaCl 0.9 per cent, or blood) should be warmed prior to intravenous infusion. Glucose or saline in plastic bags can be heated to 40 °C by microwave; however, this method usually cannot be used alone for rewarming, as the volume required is too large. A rapid fluid warmer-infuser can be used sucessfully.

Warmed solutions can also be administered through a nasogastric tube and aspirated after about 15 min. Bronchoaspiration of gastric contents is prevented by performing an endotracheal intubation in unconscious patients. Awake patients may partake of hot drinks. This will help to deliver heat to the heart and liver, thus promoting their functional recovery. Because of the small exchange surface of the stomach, this has a limited efficacy (0.5-1 °C/h).

Colonic lavage has been proposed for rapid rewarming of the liver and heart. However, this technique is not commonly used. Complications of colonic or gastric tube placement include perforation. Recent gastrointestinal tract trauma or surgery is a relative contraindication to these procedures.

Two peritoneal dialysis catheters can be placed and used to infuse warm dialysate (40-42 °C) which is then recovered from the peritoneal cavity. Because of the large surface exchange area, the temperature can rise by up to 5 °C/h. However, insertion of the catheters may cause intra-abdominal injuries. Recent abdominal trauma and intra-abdominal free air are contraindications and usually indicate laparotomy, during which warmed irrigation can be used.

Pleural lavage can be performed using two large-bore thoracostomy tubes. One tube is placed in the second or third anterior intercostal space at the mid-clavicular line, and the other is placed in the fifth intercostal space in the posterior axillary line. Warmed saline (39-42 °C) can be infused through the anterior tube and recovered in the posterior tube. Peritoneal or pleural lavages appear equally effective in raising temperature (5 °C/h). Complications include the risks of thoracostomy (lung perforation, vascular or cardiac injuries, infection) and the risk of increased intrathoracic pressure and mediastinal deviation due to accumulation of fluids, leading to pulmonary and myocardial dysfunction.

Hemodialysis and continuous arteriovenous or venovenous rewarming techniques are useful. Blood flows from a catheterized femoral artery or vein through a warming device back to a central vein. In the warming device, hot water (40 °C) separated from the blood by highly conductive layers is pumped in a countercurrent direction. These methods are effective, raising the temperature by 5 to 8 °C/h, and are technically feasible in most intensive care units. If anticoagulation is contraindicated, heparin-bonded tubing can be used.

Cardiopulmonary bypass provides the highest rewarming rate (up to 10 °C/h) under strict control. Fluid resuscitation and oxygen delivery are guaranteed, with optimal hemodynamic support being provided. All these advantages make this technique the best choice in the presence of cardiac arrest. Risks include hemolysis, hemorrhage, vascular perforation, and embolism. Heparin-bonded tubing can be used in trauma patients at high risk of bleeding, thus rendering systemic heparinization unnecessary.

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