Page 465 - Clinical Small Animal Internal Medicine

44 Disorders of Heat and Cold 433

pleural lavage fluids should be warmed to 40–43 ° C and rewarming efforts are stopped once the patient has
VetBooks.ir are invaluable in rewarming and preventing further heat reached 37 °C (98.6 °F) to prevent hyperthermia.
Primary hypothermia is less common than secondary
loss in surgical patients.
When rewarming hypothermic patients, all these
temperature is generally more severe; however, the
measures can be employed based on the severity of the hypothermia. When it does occur, the decrease in body
hypothermia. When hypothermia is mild (>32 °C/89.6 °F), recovery may be less complicated due to the absence of
passive surface rewarming may be sufficient. With concurrent diseases, surgery or anesthetic drugs.
moderate hypothermia (>28 °C/82.4 °F), active external With rewarming, there is an increase in the metabolic
rewarming is required with the main efforts aimed rate and oxygen consumption that may not be appro-
toward the core region rather than the extremities. Once priately compensated for in patients that are hypotensive
hypothermia becomes severe (<28 °C/82.4 °F), active or hypoventilating. Supplemental oxygen may be benefi-
core rewarming is mandatory. Warm lavage allows heat cial in preventing hypoxia from this increase in oxygen
transfer from the fluids to the bowel and heart and can demand. When utilizing external warming techniques, a
help prevent further decrease in temperature. Heat condition known as afterdrop can occur in which the
transfer is able to occur as long as the lavage fluid tem- core temperature actually continues to decrease as
perature exceeds the core temperature, ideally 40–43 °C peripheral vasodilation allows movement of warm core
(104–109 °F). Warm peritoneal or pleural lavage can be blood to the periphery and return of cold peripheral
performed in nonsurgical patients but is technically blood to the core. Afterdrop may be prevented by insti-
more difficult than in animals with an open body cavity tuting core rewarming techniques concurrent with
in the operating room. If necessary, a peritoneal catheter external rewarming, and by directing rewarming tech-
or thoracostomy tube is placed using sterile technique niques at the trunk rather than the limbs. At the far end
and 10–20 mL/kg of warmed lavage fluid (0.9% NaCl) is of the afterdrop spectrum lies rewarming shock, which
infused. The amount of time that fluid is allowed to occurs when rapid vasodilation from external warming
remain within the body cavity (dwell time) will deter- results in significant venous blood pooling and circula-
mine how much heat is transferred. Dwell times should tory collapse. Thermal burns due to active rewarming
be of sufficient duration to allow for maximum heat efforts can occur if excessive heat is used (>42 °C/
transfer, with typical dwell times lasting 2–5 minutes. 107.6 °F), prolonged contact time between patient and
Transfers can be repeated as needed until the target body heat source is allowed, or there is a lack of insulation
temperature is reached. between the heat source and patient.
Similar to peritoneal or pleural lavage, warm lavage of
the urinary bladder and colon is an option but is of Prognosis
unknown benefit due to limited surface area available for
heat exchange. However, urinary bladder lavage and Rewarming complications can affect the prognosis of
warm water enema administration is less invasive and hypothermic patients.
does not require placement of catheters within the peri-
toneal or thoracic cavities. If used, the dwell times,
exchange volumes, and temperature of the infusate are Hyperthermia
the same as for the cavitary lavage techniques.
Hypothermic patients require cautious IV fluid resus- Hyperthermia is defined as an increase in body tempera-
citation to assist in restoring blood pressure and effective ture greater than 38 °C (100.4 °F) and can be due to an
circulating volume. As hypothermia progresses, the alteration of the set point of the hypothalamus to a higher
decreased affinity of catecholamines for receptors leads temperature with deliberate efforts of the body to raise
to reduction of cardiac contractility and vasodilation. itself to that temperature (true fever) or can result from
This may manifest as a minimal response to IV fluid loss of the thermoregulatory balance of the body in which
therapy and does not automatically indicate the need to heat production is increased or heat loss is inhibited but
continue volume expansion with fluids in the absence of the set point remains the same. Hyperthermia can be
ongoing hypovolemia. As the body temperature returns induced by extreme muscular activity. Sustained exercise,
to normal, rebound vasoconstriction and increased con- especially in an underconditioned or overweight patient,
tractility may result in fluid overload as the intravascular sustained seizure activity or frequent cluster seizures can
fluid volume is centralized. When IV fluid therapy is induce severe hyperthermia if not controlled.
needed, warm IV fluids are recommended so fluid resus- An uncommon cause of hyperthermia in dogs and
citation does not potentiate hypothermia. The ideal rate cats is malignant hyperthermia. This is a condition of
at which to rewarm a hypothermic patient is unknown disturbed calcium metabolism that is initiated by drugs,
but a general recommendation is 1–2 °C per hour. Active most commonly volatile anesthetic gases, that results in