Cold critters: Understanding hypothermia - Veterinary Medicine
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Cold critters: Understanding hypothermia
Likely, you've had some experience with patients with hypothermia, but how well do you understand the condition? This specialist lays out the mechanics of hypothermia and the many potential complications associated with it.


VETERINARY MEDICINE


Clinical pathology effects

Hypoglycemia and hypothermia frequently occur concurrently, and low blood sugar may exacerbate a decrease in metabolic activity that may perpetuate hypothermia.34,35 Hypokalemia is commonly documented in patients with hypothermia and is a result of intracellular shifting (rather than true loss) that is thought to be due to a temporary depression in the function of the potassium pump mechanism in the cell membrane.36,37 Potassium derangements secondary to hypothermia must be monitored for and carefully corrected to avoid the development of dysrhythmias or perfusion abnormalities, particularly during rewarming.36-39

Hypothermia-induced coagulation abnormalities include reversible platelet sequestration and decreased platelet thromboxane production, granule secretion, and von Willebrand factor expression leading to decreased platelet aggregation, as well as enhanced fibrinolytic activity and slowing of enzymatic activity required for clotting.40 One study showed surface cooling to 89.6 F (32 C) induced reversible platelet dysfunction, and another study showed bleeding time in pigs doubled at 86 F (30 C).41,42 Coagulation abnormalities may be easily missed in the clinical setting because most coagulation tests are conducted at 98.6 F (37 C), potentially preventing identification of a coagulopathy present at hypothermic temperatures. Hypothermia-induced coagulation disorders rapidly reverse once normothermia is reestablished.41

Hypothermia induces diuresis and a reduction in glomerular filtration rate, which is secondary to both a reduced release of vasopressin and a reduction in renal medullary hypertonicity.43 With progressive hypothermia, hypovolemia and subsequent mild increases in hematocrit and blood viscosity develop.44 Hemoconcentration and low microcirculatory flow increase blood viscosity by 4% to 6% for each one degree Celsius that body temperature declines.44 Suppression of antidiuretic hormone and core-directed shunting of peripheral blood induce diuresis in hypothermic states that may contribute to hypovolemia.45,46

Neurologic effects

Cerebral blood flow and cerebral autoregulation are adversely affected by declining body temperature, most frequently resulting in mentation changes.31 Cerebral metabolic rate and cerebral blood flow decrease about 5% for each one degree Celsius drop in body temperature.47 Severe hypothermia is associated with abnormal neurologic signs ranging from depression to coma. Hypothermia also results in decreased metabolism of anesthetic agents, potentially prolonging recovery and affecting mentation in postoperative patients.31 A previous study showed mild hypothermia conveyed a cerebral protective benefit against ischemia during resuscitation without inducing cardiovascular consequences; interestingly, moderate hypothermia resulted in cardiovascular decompensation despite conferring neurologic benefits.48

Immune system effects

Hypothermia causes vasoconstriction and a left shift of the oxygen-hemoglobin curve that leads to impaired oxygen delivery to tissues and may be associated with a diminished resistance to infection.49,50 Tissue hypoxia is associated with impaired oxidative killing by neutrophils, and decreases in body temperature cause a reduction in phagocytosis, impaired chemotaxis, and pancytopenia, as well as a depression of the production of cytokines and antibodies.51

People with postoperative hypothermia have poor wound healing and increased incidence of infection.52 Wound healing is further impaired by tissue hypoxia because hydroxylases required for granulation tissue depend on adequate oxygen tension.51 A previous study clearly showed wound cultures were significantly more often positive in patients with mild perioperative hypothermia compared with normothermic patients.52 However, a retrospective veterinary study showed no difference in infection rates in patients experiencing perioperative hypothermia.53


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Source: VETERINARY MEDICINE,
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