Temperature Regulation During Anesthesia: Anesthetic-Associated Hypothermia and Hyperthermia



Temperature Regulation During Anesthesia


Anesthetic-Associated Hypothermia and Hyperthermia





Thermoregulation




I Thermoregulation is a complex function of both central and peripheral mechanisms and is easily influenced by environmental and therapeutic manipulations. Abnormal body temperature in conscious animals is an indicator or a cause of metabolic dysfunction. Abnormal body temperature in anesthetized animals is a direct consequence of anesthetic–drug-related effects, environmental factors, and the temperature, volume, and rate of intravenous (IV) fluid administration.



II Thermoregulatory mechanisms



A Cold and warm sensors are located in or beneath epidermal skin


B Temperature signals are transmitted through A-δ and C-fibers in peripheral nerves: cold signaling is through A-δ fibers, warm signaling is through C-fibers


C Thermal afferent nerves transmit information to the lateral spinothalamic tract



D Central regulation is in the hypothalamus



E The initial response of the body to temperature variation occurs in blood vessels and is regulated by α1– and α2-receptors; α2-receptors are important regulators of arterial-venous shunt vessels in peripheral tissues



III Heat loss mechanisms (Fig. 17-1)


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FIG. 17-1 Heat loss occurs through four mechanisms: conduction, evaporation, convection, and radiation. Evaporative and conductive heat loss can be significant in animals prepared for surgery.


A Radiation



B Conduction



C Convection (facilitated conduction)



D Evaporation



IV Hypothermia



A Can be considered any temperature below a core body temperature of 37.8° C (100° F)



B Rate of heat loss is related to species, animal’s weight, animal’s current temperature, and animal’s percentage of body fat



1. Hypothermia is common in small animals due to their increased body surface area compared with their smaller body mass (Fig. 17-2)


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FIG. 17-2 As weight-to-surface area ratio decreases, heat loss increases. Smaller animals have more surface area compared with their body mass, so heat is lost at a more rapid rate.

2. Increased body fat is associated with a decreased rate of heat loss (Fig. 17-3)


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FIG. 17-3 As the percentage of body fat increases, the rate of heat loss decreases. Fat acts as insulator. Heat is lost at a more rapid rate in leaner animals.

3. Three phases are typically associated with intraoperative hypothermia (Fig. 17-4)


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FIG. 17-4 The three phases of intraoperative hypothermia: Body heat is lost rapidly during phase I as reflected by a decrease in body temperature; phase II is a slow linear reduction in body temperature; and phase III is a thermal plateau.


C Three categories of hypothermia have been defined (Table 17-1)



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Sep 6, 2016 | Posted by in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off on Temperature Regulation During Anesthesia: Anesthetic-Associated Hypothermia and Hyperthermia

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