Chapter 20
Troubleshooting Hypothermia and Hyperthermia
Brrrr is bad too!
Lysa Pam Posner
North Carolina State University, USA
- Q. What is normal body temperature in dogs and cats?
- A. Normal rectal temperature in dogs is 39 ± 0.5 °C (102 ± 1 °F) and in cats is 38.5 ± 0.5 °C (101.5 ± 1 °F) [1]. Unlike, humans, body temperature can vary (∼0.5 °C (1 °F)) and is known as the inter-threshold range. Body temperatures within this range do not trigger any thermoregulatory responses.
- Q. What is the difference between core temperature and peripheral body temperature?
- A. Core body temperature is the temperature at which vital organs (e.g., brain, heart) are maintained. Peripheral body temperature is measured from structures located in the periphery (e.g., skin, mouth, rectum). Core and peripheral body temperatures can vary by up to 4 °C [2]. Core body temperature can be accurately measured at the tympanic membrane, nasopharynx, esophagus, and the pulmonary artery. Although rectal temperature can more easily be measured, it can lag behind changes in core body temperature. Skin is not a reliable indicator of core temperature as it is often 2–4 °C lower than core temperature [2].
Normally, there is a heat gradient between the periphery and the core. In general, core temperature is more constant and peripheral body temperature varies (because it buffers core temperature changes). This gradient from core to periphery allows the skin to act as a gate for heat dissipation and conservation. Arteriovenous anastomoses present in the skin are major contributors to thermoregulation. Dilation of these shunts allows for large amounts of heat to be lost to the environment from the core and conversely, vasoconstriction of these shunts prevents heat loss from the core.
- Q. How is body temperature regulated?
- A. Thermoregulation is a complex interaction between thermal sensing, central processing in the hypothalamus, and behavioral and physiologic responses. The hypothalamus regulates temperature by “comparing” current body temperature with the threshold temperatures that trigger thermoregulatory responses (i.e., those temperatures that are outside the inter-threshold range).
If the body temperature has exceeded the threshold temperatures (either above or below) a series of behavioral and physiologic responses will be triggered. For example, behavioral responses to decreasing body temperature can include a more compact posture or heat seeking behavior (e.g., sun seeking). Autonomic responses to declining body temperature include vasoconstriction in arteriovenous anastomoses and activation of the sympathetic nervous system (SNS) which increases thermogenesis. The ability to tightly control thermoregulation declines with age.
- Q. How is body heat generated?
- A. The production of heat (thermogenesis) is primarily accomplished by increases in metabolism (e.g., muscle movement or by non-shivering thermogenesis). Shivering is a major source of heat production and occurs when muscle groups involuntarily contract in small movements, creating warmth by expending energy. Shivering can increase metabolic rate 2–3-fold from normal. Similarly, vigorous exercise can increase heat production 30–40-fold compared with resting muscle [3].
Non-shivering thermogenesis includes increased metabolic rate via increased sympathetic tone or increased thyroid hormone or chemical thermogenesis. Chemical thermogenesis occurs by increased metabolism of brown fat and enhances heat production. Chemical thermogenesis has been shown to occur in infants, but the contribution in adults is less clear [3].
Regardless of pathway, thermogenesis is metabolically expensive as it increases consumption of oxygen, ATP, and glucose.
- Q. What are the types of heat loss?
- A. Heat loss is generally divided into five types; radiation, convection, evaporation, respiration, and conduction. Radiation and convection are the most important causes of heat loss and can account for 80% of the total heat losses in a patient [4].
- Radiation: Heat loss from radiation occurs when a hot object emits radiation waves. These waves carry energy (heat) away from the body and cause it to cool. Fifty percent of body heat loss can be from radiation [4]. Keeping the air temperature around your patient warmer will decrease radiative losses.
- Convection: Losses from convection occur when the warmed air surrounding the body (from radiation) rise and is carried away from the body (how fans cool patients). Keeping your patient covered (with a towel or blanket) prevents convection losses.
- Evaporation: Evaporative heat loss occurs due to the loss of the latent heat of vaporization (the heat created by changing from liquid to vapor), and can increase with large surgical exposures, as in an abdominal exploratory or thoracotomy. Evaporative losses are difficult to prevent.
- Respiration: Respiratory heat loss occurs when gases are warmed and humidified by the body. Normally respiratory heat loss is less than 10% of total heat loss; however, during anesthesia the inspiration of dry cooled gases can increase heat loss through respiration. Respiratory losses are also hard to prevent, but there are some advanced techniques that can decrease heat loss via respiration, such as the use of a heat and moisture exchange device (HME).
- Conduction: Conduction losses occur by way of heat energy being transferred through a substance and normally accounts for minimal losses. However, metal surgical tables can increase heat loss through conduction. Limiting contact with conductive surfaces, such as placing a towel or blanket over a metal surgical table, will help limit conductive losses.
- Evaporation: Evaporative heat loss occurs due to the loss of the latent heat of vaporization (the heat created by changing from liquid to vapor), and can increase with large surgical exposures, as in an abdominal exploratory or thoracotomy. Evaporative losses are difficult to prevent.
It should be noted that most types of heat loss are dependent on the amount of exposed skin. Thus if skin exposure is minimized, heat loss can be minimized too.
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- Radiation: Heat loss from radiation occurs when a hot object emits radiation waves. These waves carry energy (heat) away from the body and cause it to cool. Fifty percent of body heat loss can be from radiation [4]. Keeping the air temperature around your patient warmer will decrease radiative losses.
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