Chapter 30

### INTRODUCTION

Perioperative hypothermia occurs to some degree in all patients undergoing general or regional anesthesia for more than 30 minutes. Hypothermia occurs through several mechanisms:

• Redistribution—The initial intraoperative temperature drop is secondary to redistribution of heat from the core to peripheral tissues and is proportional to the gradient between these two compartments. This gradient depends on the room temperature, vasomotor status of the patient, adiposity, and anesthetic drug effects.

• Radiation—Radiation is the transfer of heat between two objects that are not in contact. An example of this is the sun warming the earth. The emitted radiation carries the warmth from the warmer object to the cooler object and occurs in the infrared light spectrum. Most heat lost in the perioperative setting occurs through radiation.

• Convection—Convection contributes a great deal to perioperative heat loss as well. Convective heat loss is the transfer of heat to moving molecules, such as air or liquid. This depends on the rate of air movement (wind speed), the surface area exposed, and the temperature difference between the object and ambient temperature.

• Conduction—Conduction is the transfer of heat between two surfaces in direct contact. It depends on the temperature difference between the two objects and the surface area of the objects in contact.

• Evaporation— Evaporative heat loss occurs through the skin and respiratory system and consists of three main components: sweat (sensible water loss); insensible water loss from the skin; respiratory tract and wounds; and evaporation of liquids (ie, skin preparation solution) from the skin. Factors affecting evaporative heat loss include the vapor pressure difference between the body surface and the environment, the relative humidity of the ambient air, the velocity of airflow, and lung minute ventilation.

### WARMING STRATEGIES

One of the easiest ways to reduce intraoperative radiant heat loss is to maintain operating room temperature at a sufficiently high level. In adults, 21°C has been reported as the critical ambient temperature to maintain normal esophageal temperatures between 36°C and 37.5°C. However, operating rooms are often kept cooler than this for operator comfort. Several strategies exist, therefore, to achieve and maintain perioperative normothermia. By instituting a multimodal approach, drops in temperature can be minimized. These approaches are divided into three broad strategies.

#### Passive Insulation

Passive insulation minimizes thermal dispersion by insulating the air layer between covers placed on the patient and the patient’s skin surface. Examples of these insulating covers include: surgical draping, cotton blankets, and metalized plastic covers. These devices reduce radiant, convective, and evaporative heat losses, minimizing thermal dispersion by about 30%. Their efficacy is not dependent on the material they are made of, but rather seems to be directly proportional to the covered surface area.

#### Active Cutaneous Warming Devices—

Forced air warmers are the most commonly used active warming systems in the perioperative period. These consist of an ...

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