Anesthetic and surgical effects

General anesthesia and surgery readily result in primary and secondary hypothermia. Intubated patients inspire cold, dry air delivered directly to the lungs. Routine aseptic preparation of surgical sites promotes evaporative heat loss, and cold table surfaces and open body cavities will exacerbate heat loss through conduction and radiation, respectively. Anesthetic agents affect the hypothalamic thermoregulatory center in such a way that thermogenic responses are not triggered until low temperatures are reached.⁶ Centrally mediated thermoregulatory vasoconstriction is directly inhibited to cause peripheral vasodilation. Anesthesia decreases the metabolic rate by 15% to 40% and inhibits muscular activity to cause decreased heat production.^4,6,54-56

Coagulopathy and platelet dysfunction logically represent serious complications in surgical or posttraumatic patients at risk for hemorrhage. Hypothermia delays anesthetic recovery and may lead to surgical complications such as dysrhythmias, hypotension, respiratory depression, bradycardia, coagulopathy, altered blood viscosity, and anesthetic drug overdose.⁵⁷ Minimizing the duration of anesthetic and surgical procedures may reduce the incidence of secondary hypothermia.

Hypothermia after trauma is common in people.⁵⁸ Although there is a correlation between hypothermia and mortality, there is no threshold below which mortality is assured.^58,59 A markedly hypothermic patient may initially appear deceased because of poor cardiac contractility, bradycardia, increased blood viscosity, and cold or stiff limbs. For this reason, you must thoroughly evaluate a patient with severe hypothermia, potentially using advanced diagnostic tests, such as electrocardiography, to ensure rapid identification and to afford rapid intervention.

Hypothermia in trauma patients is often proportional to the degree of shock and severity of tissue damage.^24,60 Previous laboratory studies suggest that patients with hemorrhagic shock have increased survival with concurrent mild hypothermia compared with those whose resuscitation protocols included rewarming interventions, thus underscoring the potential benefit of hypothermia during low perfusion states.^39,61

In Part 2 of this series, Cold critters: Assessing, preventing, and treating hypothermia, I describe three common rewarming methods, and I discuss when to apply these different methods and what rewarming complications you need to watch out for.

Christopher G. Byers, DVM, DACVECC, DACVIM (small animal internal medicine)
MidWest Veterinary Specialty Hospital
9706 Mockingbird Drive
Omaha, NE 68127