Hypothermia is defined by a core body temperature lower than 35 degree C (95-degree F). Below this temperature, the body loses more heat than it generates. Hypothermia can be caused by metabolic dysfunction in association with decreased heat production (hypothyroidism, hypoglycemia, hypoperfusion, or hypoaldosteronism) or disturbed thermoregulation (intracranial disease, diffuse neurological disease or other). Accidental hypothermia is an unintentional decrease of core temperature caused by prolonged exposure to the cold. This could be a patient left outside for an extended time or seen during a surgical procedure. All of these mechanisms are seen commonly within our veterinary patients and can have detrimental effects.
Thermoregulation is a process that allows your body to maintain its core internal temperature achieved by both autonomic and somatic mechanisms, increasing thermogenesis and inhibiting thermolysis. Heat is produced as a result of normal metabolism with the primary contributors being brain and trunk organs along with skeletal muscle. The human body is composed of the peripheral compartment and the core compartment. The core compartment maintains a constant temperature where the peripheral compartment tends to be about 2-4 degrees C cooler. Animals and people thermoregulate by mechanisms of convection, conduction, radiation and evaporation. Heat loss in animals is primarily through convection or conduction, unlike people where radiation is the mainstay of thermoregulation.
There are many factors that can predispose our patient population to developing hypothermia. These include:
- Wet soaked fur as it cannot effectively insulate a patient, this could be patient that has been outside in the rain or a patient in surgery where they have gotten wet from the procedure.
- Low body condition score
- Lack of acclimatization
- Underlying disease processes
- Geriatric patients are at high risk for developing hypothermia. They not only have most of the predisposing factors but they can have impaired physiologic responses to the cold, this can be similar for pediatric patients. It is important to keep in mind that these patients may need extra attention and support to maintain normothermia.
One of the most emergent causes of hypothermia seen through our patient population is related to hypoperfusion and shock. A hypothermic patient presenting through the clinic should key you into important aspects that need to be addressed such as perfusion parameters. Hypothermia in relation to shock is part of the triad of death, this is a medical term describing the combination of hypothermia, acidosis, and coagulopathy. This combination is commonly seen in patients of both trauma and other forms of shock which can lead to increases in mortality rates.
Symptoms of Hypothermia
Hypothermia can be classified into mild (32-35), moderate (30-35) and severe (below 30) by the American College of Surgeons. At all classes of hypothermia there are dysfunctions in vitals organs such as the cardiovascular, neuromuscular, respiratory, coagulation, renal, along with hepatic and gastrointestinal (GI) systems.
Healthy animals respond to cold temperatures and mild hypothermia by findings shelter, shivering, or behaviors such as curling up into a ball. Shivering occurs when the core body temperature drops, the shivering reflex is triggered to maintain normothermia. Skeletal muscles begin to shake in small movements, creating warmth by expending energy. This mechanism can come with significant expenses such as increased basal metabolic rate and oxygen demand . As temperature drops these muscle movements decrease and muscle stiffness predominates and then progresses to areflexia. It is stated that for every 1 degree C drop in core temperature there is a 6-7% decline in cerebral blood flow, resulting in a concurrent deterioration in mentation.
However, altered mentation can lead to maladaptive behavior that only worsens the hypothermia. These patients could exhibit signs such as arrhythmias due to slowed myocardial conduction, resulting in abnormal depolarization and repolarization, along with tachypnea or respiratory depression as hypothermia progresses.
Another consequence of hypothermia is that splanchnic and hepatic perfusion decreases causing decreased hepatic metabolism and GI motility. An important phenomenon within the renal system relates to a term called “cold diuresis.” This can occur early on in hypothermia and refers to the increase in urine production with exposure to cold temperatures, due to increased renal blood flow from peripheral vasoconstriction. As hypothermia worsens there is also resistance to anti-diuretic hormone which only worsens the diuresis. This is an important clinical finding where these patients could be in severe hypovolemia secondary to diuresis. This can lead to severe acute kidney injury.
Another severe clinical finding that has been documented is the development of a coagulopathy on both primary and secondary hemostasis. Other biochemical abnormalities that can be seen are electrolyte changes, abnormal acid-base status, along with hyperglycemia or hypoglycemia.
Treating Hypothermia in Dogs and Cats
Every patient that presents to the hospital should have their temperature checked. Ideally, core temperature would be measured although in veterinary medicine we usually use rectal temperature as a surrogate. Infrared measurements of skin and aural are usually inaccurate, especially in hypothermic patients due to vasoconstriction.
The first line of defense for hypothermia is rewarming, this can be accomplished with warm blankets or “warmies” such as heated fluid bags or a Bair Hugger™. Active core rewarming involves directly supplying heat to the core compartment. Techniques include, warmed IV fluids, bladder/peritoneal lavage, rectal lavage with warm water along with extracorporeal warming. When core rewarming is implemented there should be concurrent surface warming as to prevent temperature gradients in these patients. Active core rewarming is usually only applied to severely hypothermic patients, most of the previous techniques along with perhaps only warmed IV fluids is sufficient to warm most of our patient population.
An important aspect to hypothermia during procedures and general anesthesia is prevention. These patients should have Bair Huggers™ and warmed fluids for peritoneal lavage to help prevent decreases in temperature and should have continued temperature monitoring during and post-operatively.
Approaches to the hypothermic patient starts with active rewarming, establishing venous access and obtaining full blood work or point-of-care blood work. These patients should also be resuscitated based on their perfusion parameters and constantly being monitored. All abnormalities such as hypoglycemia, hypotension, etc. need to be addressed simultaneously as rewarming. Once these patients are hemodynamically stable they need to be re-evaluated as they can be very dynamic depending on the disease process.
It is usually standard of care to maintain a body temperature of >98 degrees F. There are certain situations where groups of patients tend to maintain a lower body temperatures, in particular cats with chronic disease, which was concluded in a paper in 2016 that uremic hypothermia appears to be a clinical phenomenon that occurs in cats and dogs. Uremic patients are hypothermic compared to ill nonuremic patients and body temperatures increase when uremia is corrected. Antibiotics are recommended in patients that are immunocompromised due to decreases in neutrophil function in hypothermia.
Following the rewarming period with severe hypothermia, there can be a high risk for multiorgan dysfunction, these patients should be deemed critical and close monitoring within an ICU is warranted.
As you can see hypothermia has many detrimental effects, early recognition and treatment is pertinent. The most important aspect is prevention and helping our patients maintain normothermia through all aspects of disease processes and procedures can help decrease the mortality that hypothermia could bring.
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- Brodeur A, Wright A, Cortes Y. Hypothermia and targeted temperature management in cats and dogs. J Vet Emerg Crit Care (San Antonio). 2017 Mar;27(2):151-163. doi: 10.1111/vec.12572. Epub 2017 Jan 25
- Kabatchnick E, Langston C, Olson B, Lamb KE. Hypothermia in Uremic Dogs and Cats. J Vet Intern Med. 2016;30(5):1648–1654. doi:10.1111/jvim.14525.
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