Hypothermia
Decreases in body temperature may occur when environmental stresses (cold, wet, wind) overwhelm the body’s capability of heat production (especially when the body is weakened from disease), when CNS disease has resulted in damage to the regulatory centers within the hypothalamus, or when adrenergic or sympathetic effector systems have been damaged.
Newborns, cachexic, and aged animals are most susceptible to heat loss caused by cold exposure (see Neonatal Sepsis and Weak Calf Syndrome, Chapter 20). Concurrent signs of septic disease in hypothermic animals signify a guarded prognosis, because the body’s defense mechanisms are often overwhelmed when core body temperature declines. Severely hypothermic animals (core body temperature >30° C) are profoundly depressed and have marked reduction in ventilation, absence of muscle activity, and decreased reflexes. Decreased intravascular volume and depressed cardiac function lead to hypoxia, acidemia, and cardiac dysrhythmias. Newborns are often hypoglycemic and have potassium imbalances. These animals should be warmed by protecting them from wind or drafts, drying them, and providing a microenvironment of high ambient temperature. Applying thermal blankets and housing them in an insulated stall, with or without supplemental heat, is superior to using direct external heat from heat lamps or other sources. Direct external heat without environmental control causes cutaneous vasodilation, often exacerbates central hypothermia, and contributes to cardiovascular compromise.Animals with severe hypothermia should be warmed gradually over 24 hours, with careful monitoring of body temperature and the cardiovascular system. Maintenance of adequate systemic perfusion is the most important means of preventing cardiac failure.67-68 Acidosis and potassium imbalances are common and may fluctuate rapidly.
Consequently, repeated measurements, especially when a patient’s clinical condition worsens in the process of warming, are often necessary. Appropriate crystalloid fluids, warmed to body temperature, are usually necessary throughout the warming process. Evaluation of blood glucose and concurrent dextrose therapy, especially in neonates, should also be performed.Warmed humidified oxygen therapy both as an aid in treatment of hypoxia and as a means of warming is helpful. Gastric (rumen) or rectal lavage with warmed fluids may also be used. However, care should be taken in rapid rewarming, because an imbalance in the basal metabolic rate (which is temperature dependent) and systemic perfusion may result in life-threatening cardiac dysrhythmias and worsening of metabolic acidosis and hypoxia. Hypothermia attenuates the inflammatory response by a multiplicity of effects on cytokines and other key signaling mechanisms.69 Thus the adverse metabolic effects of disease are slowed at low body temperatures, and as body temperature elevates, signs of systemic disease become apparent.70 In hypothermic animals in shock, particularly neonates, severe anoxic changes in the bowel wall may result in severe diarrhea, sloughing of mucosa, or clostridial growth in the bowel.