Page 464 - Clinical Small Animal Internal Medicine

432 Section 5 Critical Care Medicine

in heat production, heat retention or thermoregulation. extremities. During more severe hypothermia, there is
VetBooks.ir A common cause of secondary hypothermia in small decreased catecholamine responsiveness resulting in
bradycardia and hypotension. Arrhythmias can occur due
animal patients is general anesthesia. Perioperative
hypothermia consists of three phases, starting with an
with ventricular fibrillation as a commonly documented
initial rapid decrease in the core temperature of 1–1.5 ° C to myocardial ischemia as well as myocardial irritation,
during the first hour. Most anesthetics cause vasodila- rhythm.
tion, which results in venous pooling and redistribution Hypoxia is propagated by a decrease in respiratory rate
of heat from the core to the periphery. This is accompa- and tidal volume. While mild hypothermia may protect
nied by a decrease in the temperature threshold needed the brain from ischemia in cases of cardiopulmonary
to cause reflex vasoconstriction. This phase is followed by resuscitation and traumatic brain injury, in normal
a slower decrease in temperature over the next 2–3 hours patients the negative consequences of hypothermia are
as heat loss exceeds the rate of metabolic heat produc- likely greater than the benefits. Even mild hypothermia
tion that is decreased 20–30% by anesthetics. Heat loss can result in altered mentation due to decreased cerebral
during this period is more profound in patients undergo- blood flow. Prolonged recovery from anesthesia due
ing open cavitary surgical procedures due to increased to hypothermia and decreased drug metabolism can
heat loss by all means of heat transfer as a result of the potentiate altered mentation.
increased surface area available for heat exchange. The
plateau phase is the final phase where metabolic heat pro- Therapy
duction is equivalent to continued heat loss. At this point,
the core temperature has decreased to a point where The primary treatment goal of hypothermia, regardless
vasoconstriction occurs and heat loss is decreased. of cause, is the restoration of normal body temperature.
The consequences of primary and secondary hypo- The main difference between the treatment of primary
thermia are similar, although patients with secondary and secondary hypothermia is that since it can be antic-
hypothermia are generally more symptomatic at higher ipated, the best treatment for secondary hypothermia is
temperatures. In the anesthetic setting, postoperative prevention. Prevention of perianesthetic hypothermia
hypothermia has been associated with increased mor- requires close patient monitoring with rectal or esopha-
tality in humans. geal thermometers. Indwelling probes allow for continuous
Hypothermia has many physiologic effects and conse- temperature monitoring.
quences. One major effect of hypothermia is a decrease Prevention of hypothermia utilizes the same thera-
in the overall metabolic rate. This includes decreased pies and techniques as treatment of hypothermia. Heat
hepatic metabolism, which may prolong the effects of loss can be prevented with passive surface rewarming
anesthetic medications and consequently anesthesia‐ in the form of a blanket covering the patient and placed
associated hypothermia. In the kidneys, hypothermia between the patient and conducting surface. Extremities
initiates cold diuresis due to increased glomerular filtra- can be wrapped in insulating material. Active surface
tion rate (GFR) and decreased sensitivity to antidiuretic rewarming increases the temperature of the air and
hormone (ADH). During severe hypothermia, renal surfaces surrounding the patient to minimize heat loss.
effects can progress to decreased renal blood flow, Forced warm air blankets and circulating water blan-
decreased GFR and ischemic renal tubular damage, kets significantly decrease heat loss from anesthetized
which can lead to acute renal tubular necrosis. patients and should be used on all patients undergoing
Besides the effects on abdominal viscera, hypothermia general anesthesia of any significant duration (i.e.,
can lead to a hypocoagulable state due to thrombocyto- greater than 30 minutes).
penia, thrombocytopathia, alteration in coagulation fac- Active core rewarming increases the temperature of
tor function and disruption of fibrinolysis. Hypothermia has the core by introducing heat centrally and can be per-
also been documented to cause suppression of the immune formed by administration of warmed IV fluids, warm
system, resulting in increased surgical site infections and pleural or peritoneal lavage, warm water enemas, warm
altered wound healing in human and veterinary patients. saline urinary bladder lavage, and inhalation of warm,
Cardiovascular consequences of hypothermia include humidified air. Warmed IV fluids are helpful in preventing
an initial catecholamine‐stimulated increase in heart hypothermia but are of minimal assistance in rewarming
o
rate and blood pressure that increases myocardial oxy- hypothermic patients. IV fluids can be warmed to 40 C
gen demand and consumption. Hypothermia induces a (104 °F) in a microwave for 2.5 minutes (microwave
left shift in the oxygen–hemoglobin dissociation curve variation exists and each clinic should verify individual
resulting in decreased oxygen release to the tissues. microwave time) or stored in an incubator for immediate
Combined with reflex vasoconstriction, this results in use. In‐line fluid warmers help to maintain the tempera-
significant tissue hypoxia, especially to the skin and ture as the fluid is being administered. Peritoneal and