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.
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 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
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.