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INTRODUCTION

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The primary reason to maintain normothermia is to improve patient outcomes, both clinically and financially. With medical reimbursement in the balance, maintaining normothermia is becoming an important part of the surgical process. Patients have numerous disadvantages that contribute to the stress response, including preoperative anxiety, prolonged fasting, and arriving in a cold operating room in a thin backless gown.

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Perioperative hypothermia is associated with increased surgical site infection, increased intraoperative bleeding, prolonged stay in recovery room, increased cardiac morbidity and mortality, and increased requirements for postoperative mechanical ventilation.

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DEFINITIONS

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  • Normothermia—Core body temperature −36°C to 38°C (96.8°F-100.4°F)

  • Hypothermia—Core body temperature below 36°C (96.8°F)

  • Ideal thermic state—Near 37.0°C (98.6°F)

  • Conduction—Direct transfer of energy between two materials in contact with each other

  • Convection—Dispersion of heat via currents of air or fluid

  • Radiation—Infrared emission of heat

  • Evaporation—Phase change where heat is lost (liquid to gas)

  • Core temperature—The thermal compartment of the body composed of highly perfused tissues where the temperature is uniform

  • Ambient temperature—The temperature of the surrounding environment

  • Passive insulation—Containing body heat and insulating the body from heat loss via radiation

  • Active warming—Application of conductive, convective, or radiation to the skin

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PHYSIOLOGY

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Skin temperature fluctuates with patient’s surroundings, whereas core temperature remains relatively constant at 98.0°F to 98.6°F (37°C). In fact, core temperature normally remains between 97°F and 100°F even while environmental temperatures fluctuate from as low as 55°F to as high as 130°F. The “interthreshold range” is the narrow limit above and below the body’s normothermic state of 37.0°C (±0.2°C) and temperatures below the lower limit trigger the body’s cold responses of thermoregulation.

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Thermal-sensitive cells (cold receptors) are triggered by temperatures below a set threshold and generate impulses that travel mainly via A delta nerve fibers. Temperatures above threshold excite heat receptors that generate impulses along unmyelinated C fibers which also conduct pain sensation. Afferent information is integrated at several levels within the spinal cord and brain. Although some temperature regulation occurs in the spinal cord, the hypothalamus integrates most afferent input and produces efferent outputs to maintain normothermia. Additional factors known to alter temperature thresholds include circadian rhythm, food intake, infection, and drugs.

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The efferent response is modulated by neurotransmitters, including norepinephrine, dopamine, serotonin, and acetylcholine. Thermogenesis is accomplished by piloerection, shivering, vasoconstriction, decreased sweating, and increased metabolic rate.

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Development of intraoperative hypothermia occurs in distinct stages. During the first 40 minutes the body loses heat due to the lowered threshold vasodilation and redistribution via radiation. This results in a rapid decrease in core temperature of up to 1-2°C. Over the next 2-3 hours heat loss outpaces production in a linear fashion. After a 3-hour loss, it matches production as vasoconstrictive thermoregulation commences resulting in a stabilization of core temperature.

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