Cold critters: Assessing, preventing, and treating hypothermia

The first part of this series (Cold critters: Understanding hypothermia) discussed how the body regulates heat and what happens when hypothermia occurs. Now, let's look at how to assess, prevent, and treat hypothermia once you've identified it.

MEASURING THE DEGREE OF HYPOTHERMIA

Documenting hypothermia in veterinary patients is relatively straightforward, simply requiring the use of a reliable mercury or digital thermometer. A previous investigation in dogs demonstrated that a rectal thermometer provides a more accurate assessment of core body temperature than an infrared auricular thermometer or a subcutaneous temperature-sensing microchip does.1 The superiority of rectal temperature to auricular temperature in dogs was confirmed in a subsequent investigation.2 When a patient is receiving general anesthesia, I recommend monitoring the patient's temperature continuously through rectal or esophageal probes available for most veterinary multiparameter monitors.

I strongly recommend obtaining a thorough history for and performing a complete physical examination on any patient presenting with hypothermia, evaluating specifically for concurrent injuries such as frostbite or evidence of a near-drowning accident or immersion hypothermia. Rapid identification and early intervention are essential to reduce morbidity and mortality associated with these concurrent injuries.

PREVENTION

Benjamin Franklin famously said, "An ounce of prevention is worth a pound of cure." Certainly, preventing medical complications is more desirable and efficient than treating them. For the astute clinician, timely recognition of heat-seeking and heat-conservation behaviors (curling up, burrowing under blankets, piloerection) is of paramount importance to prevent the development of hypothermia. Veterinarians frequently encounter hypothermia in anesthetized and surgical patients. Thus, I strongly encourage the appropriate use of warmed water blankets, forced-air blowers, warmed lavage fluids, and inline intravenous fluid warmers to help prevent hypothermia from developing in these patients.

A recent study documented that both conductive warming panels and warmed forced-air systems with and without the use of circulating water were effective in minimizing hypothermia during both celiotomy and nonceliotomy surgeries.3 This same study also showed that hypothermia developed during the early anesthetic period, so aggressively appropriate intervention to minimize hypothermia should be a priority during this time.

THERAPEUTIC REWARMING

Therapeutic efforts are aimed at rapidly rewarming patients during fluid resuscitation as well as at reducing additional heat loss. Resuscitative efforts should not contribute to the hypothermia, and you should strive to warm the animal by at least one or two degrees Celsius per hour.4 It is important to handle hypothermic patients with the utmost care so as not to induce or exacerbate dysrhythmias. Intravenous fluid therapy to support mean arterial pressure is recommended during active rewarming in hypovolemic shock and will resolve most cases of hypothermia-induced hypotension, bradycardia, hypoventilation, or coagulopathy. Acid-base and electrolyte derangements must be aggressively corrected, and extensive monitoring, including electrocardiography, blood pressure measurement, and capnography, should be performed continually as indicated.

The rewarming of hypothermic animals may be accomplished through several methods, including5,6

• Passive surface rewarming by using external covers such as blankets

• Active surface rewarming by using warm water bottles, heating pads, or forced-air blankets

• Active core rewarming by using warm water enemas, warmed or humidified oxygen, warmed intravenous fluids, warmed peritoneal or pleural lavage, warmed urinary bladder lavage, or warmed gastric lavage in an intubated patient.

The type of rewarming intervention selected varies depending on the cause and severity of the hypothermia, as well as on concurrent underlying processes. Some patients may be effectively treated with passive surface rewarming, but active surface and active core rewarming methods must be considered for more severely affected patients. In patients with mild hypothermia, I routinely apply blankets warmed in a clothes dryer. In patients with moderate or severe hypothermia, forced-air blankets and concurrent warmed urinary bladder lavage, warmed intravenous fluids, or warm water or saline enemas are commonly used. In our hospital, we keep our intravenous, lavage, and enema fluids warmed to 102 F [38.8 C] in a fluid incubator.

To reduce the likelihood of inducing thermal injury, place a barrier such as towel between the patient and an external heating device; electric heating blankets are not recommended. Hypotensive patients or those with significant peripheral vasoconstriction may be at increased risk for thermal injury, and external heat sources should be applied to the chest and abdomen to prevent peripheral vasodilation. Active core rewarming and passive surface rewarming should be prescribed concurrently to reduce the core-to-peripheral temperature gradient. Patients should not be immersed in a warm water bath as this intervention may lead to serious complications including peripheral vasodilation, dysrhythmias, potentiation of hypovolemia, and after-drop (see the section "Complications of rewarming").

Pleural and peritoneal lavage may be appropriate methods of active core rewarming, but a previous investigation documented no difference in efficacy between warming the thoracic cavity vs. the abdominal cavity.7 Therapeutic interventions should target the normalization of organ function, recognizing that rewarming aggressively or to traditionally normal temperatures may not be appropriate for all patient populations (e.g. patients with cardiac or renal disease). Active rewarming to temperatures > 98.6 F (37 C) is not recommended in any hypothermic patient because of the possibility of overwarming the patient with continued active rewarming. Continuous monitoring is required during and after rewarming until the patient is stable.

COMPLICATIONS OF REWARMING

Rewarming complications are common. After-drop describes the phenomenon characterized by a continued decrease in core body temperature after the onset of rewarming. Cold blood from the body's periphery returns to the core while warmed core blood moves to the periphery.5,8

Rewarming shock is a condition caused by rapid rewarming that induces vasodilation and places great metabolic demands on affected patients, both of which may readily overwhelm an already compromised circulatory system.5,9

Cats with hypovolemic shock will frequently present with three distinct clinical signs: hypothermia, hypotension, and bradycardia.5 Indeed, this constellation of signs is profound, is indicative of a life-threatening disease, and is anecdotally referred to as the triad of death. Previous research documented altered adrenergic reactivity in hypothermic cats.10 With effective treatment of hypothermia, adrenergic reactivity and vascular responsiveness are restored; thus, patients treated with large volumes of crystalloids are at risk for hypervolemia and pulmonary edema. Conservative fluid therapy with concurrent aggressive rewarming is an essential part of shock resuscitation therapy in hypothermic cats to help avoid the development of pulmonary edema.

CONCLUSION

Hypothermia is a lowering of the body's temperature. The body's ability to reestablish normothermia lessens and is ultimately abolished with progressive hypothermia. As body core temperature drops, multiple body systems suffer from the effects of the reduced temperature.

The primary treatment goal for hypothermic patients is to reestablish a normal core body temperature through passive surface, active surface, or active core rewarming interventions. To achieve a successful outcome, be prepared to monitor for and treat complications of both hypothermia and rewarming interventions. For a case example of how to treat hypothermia, see Rewarming a stray kitten.

Christopher G. Byers, DVM, DACVECC, DACVIM (small animal internal medicine)

MidWest Veterinary Specialty Hospital

9706 Mockingbird Drive

Omaha, NE 68127

REFERENCES

1. Greer RJ, Cohn LA, Dodam JR, et al. Comparison of three methods of temperature measurement in hypothermic, euthermic, and hyperthermic dogs. J Am Vet Med Assoc 2007;230:1841-1848.

2. Sousa MG, Carareto R, Pereira-Junior VA, et al. Comparison between auricular and standard rectal thermometers for the measurement of body temperature in dogs. Can Vet J 2011;52:403-406.

3. Franklin MA, Rochat MC, Payton ME, et al. Comparison of three intraoperative patient warming systems. J Am Anim Hosp Assoc 2012;48:18-24.

4. Frank SM, Fleisher LA, Breslow MJ, et al. Perioperative maintenance of normothermia reduces the incidence of morbid cardiac events. A randomized clinical trial. JAMA 1997;277:1127-1134.

5. Oncken A, Kirby R, Rudloff E. Hypothermia in critically ill dogs and cats. Compend Contin Educ Pract Vet 2001;23:506-521.

6. Armstrong SR, Roberts BK, Aronsohn M. Perioperative hypothermia. J Vet Emerg Crit Care 2005;15:32-37.

7. Gentilello LM, Moujaes S. Treatment of hypothermia in trauma victims: thermodynamic considerations. J Intensive Care Med 1995;10:5-14.

8. Reuler JB. Hypothermia: pathophysiology, clinical settings, and management. Ann Intern Med 1978;89:519-527.

9. Bessen H. Hypothermia. In: Tintinalli J, Kelen G, Stapczynski J, eds. Emergency medicine. 5th ed. New York City: McGraw-Hill, 2000;1231-1235.

10. Kitagawa H, Akiyama T, Yamazaki T. Effects of moderate hypothermia on in situ cardiac sympathetic nerve endings. Neurochem Int 2002;40:235-242.