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Disorders of Heat and Cold

Sarah Allen, DVM, DACVECC

Massachusetts Veterinary Referral Hospital, Woburn, MA, USA

Normal Thermoregulation the body, such as exam tables or cage floors. Convection
occurs when body heat is transferred to the air surrounding
Body temperature is closely maintained in mammals the body and radiation is the transfer of heat from the
around an optimal set point at which ideal cellular func- body to distant surfaces without direct bodily contact.
tion can occur. This is a complex process that ultimately In small animals, conduction and convection are the
results in a balance between heat production and heat primary means of heat dissipation. Individual patient
loss. The hypothalamus acts as the main regulator of this factors that contribute to increased heat loss may include
process with multiple sensors throughout the body in increased relative surface area in a neonate, decreased
the skin, thoracic and abdominal viscera, and spinal fat insulation in a thin patient, inability to prevent
cord. The body can be viewed as two thermal divisions: excessive heat loss to a conductive surface or decreased
the core and peripheral compartments. The core com- heat seeking due to weakness or altered mentation.
partment is well perfused with a mostly constant tem-
perature. The peripheral compartment is composed of
the extremities and peripheral temperature is dependent Hypothermia
on heat transmission from the core via blood flow, heat
conduction from adjacent tissues, and heat loss to the Hypothermia is defined as body temperature of less
environment. than 38 °C (100.4 °F) and is classified by severity as mild
Heat is generated by muscle activity and metabolism, (32–37 °C/89.6–98.6 °F), moderate (28–32 °C/82.4–89.6 °F),
with the brain, heart, and abdominal organs being the or severe (<28 °C/82.4 °F). With the onset of hypothermia,
main generators of metabolic heat. Changing blood compensatory responses including peripheral vasocon-
levels of thyroxine, epinephrine, and norepinephrine can striction and shivering are stimulated. Once body tem-
change the basal metabolic rate, and subsequently body perature reaches <34 °C (93.2 °F), vasodilation occurs and
heat. Increased muscular activity (i.e., shivering) is also metabolic rate is decreased. Below 31 °C (87.8 °F), ther-
used as a means of generating heat. Heat retention occurs moregulation ceases completely and if hypothermia is
by physiologic responses including piloerection and allowed to continue unabated, death shortly follows.
peripheral vasoconstriction, as well as behaviors such as Primary hypothermia is the result of prolonged
seeking shelter and warmth and curling up. exposure to low environmental temperatures, where the
In order to prevent hyperthermia, methods of heat processes of heat production and retention are insuffi-
dissipation are necessary to maintain an ideal body cient to maintain normal body temperature. Some
temperature. The four main mechanisms of heat loss patients may be more vulnerable to developing primary
from the body are evaporation, conduction, convection, hypothermia due to decreased body condition, increased
and radiation. Evaporation is loss of heat from moisture relative body surface area or a decreased ability to generate
on the body surface. In canine and feline patients, the heat. Neonates, geriatrics, and patients suffering from
main source of heat loss by the evaporative route is from chronic illnesses that result in decreased body fat and lean
the respiratory tract, with panting providing a means of mass may be more susceptible to primary hypothermia.
increasing heat loss from the body. Conduction is the Secondary hypothermia is a result of disease, anesthesia,
transfer of heat from body surfaces to objects contacting and/or surgery, or other processes that cause alterations

Clinical Small Animal Internal Medicine Volume I, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical