Microsurgery: Transplantation and Replantation by Harry J. Buncke, MD, et al.
  Table of Contents / Chapter 39:
Anesthesia for Microsurgery


Hypothermia occurs readily in anesthetized patients. Intraoperative heat loss during prolonged general anesthesia may cause postoperative shivering and peripheral vasoconstriction, placing the flap at risk by decreasing inflow. The replanted vessels are denervated and therefore do not vasoconstrict in response to sympathetic efferent impulses.

Vasodilation occurs with induction of general anesthesia and may result in as much as a 1C core temperature reduction within the first 15 minutes of anesthesia.7 The anesthetized patient also has a reduced basal metabolic rate and disruption of normal thermoregulatory mechanisms. Intraoperative hypothermia can occur easily if its mechanisms are not understood and proper precautions taken to avoid it, or to reverse it once it has occurred. We aim to maintain a temperature of 36C or higher. The anesthetized patient loses heat from the following mechanisms: the heat of vaporization, conduction, convection, and radiation.

The heat of vaporization. The heat of vaporization is possibly the largest source of heat loss during anesthesia. In the nonanesthetized (i.e., nonintubated) person, the mouth, nose, and pharynx saturate normal inspired gases with water to a humidity of approximately 99% at the level of the carina. This natural humidification process is bypassed during endotracheal intubation. Dry, cool, inspired gases create an increased demand for heat and moisture from the respiratory tract. Moisture and heat (the heat of vaporization) are lost from the mucosa of the trachea and bronchi as water is vaporized to increase the inspired water vapor concentration. This heat loss can be prevented by providing warm, moist gas to the patient. Although they are somewhat cumbersome and must be carefully monitored to avoid too high a temperature, electric humidifiers are the most effective way to prevent heat loss from the respiratory tract and the most certain way of treating hypothermia. By increasing the temperature of the inspired gas slightly above that of the patient's, the inspired gas will have a greater water content. When this warm, moist gas mixture comes into contact with the respiratory mucosa, water will condense and heat will be added to the system.

  By combining two other methods for the prevention of respiratory tract heat loss, we have found that we usually do not need to use an electric humidifier. We administer the anesthetic in a closed or semiclosed circuit, which causes the exothermic and moisture-producing chemical reaction of carbon dioxide and soda lime. We also use a heat moisture exchanger or "artificial nose," a small, disposable device placed on the connector of the endotracheal tube. It is filled with cellulose baffles that trap the heat and moisture of the expired gas and then return it to the inspired gas. These two methods have proven effective and less cumbersome than an electric humidifier, although one should be available to treat hypothermia should it occur.

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