by Lee Burstiner, D.V.M.
Hypothermia is a common clinical syndrome in the veterinary setting which is often induced during prolonged anesthetic procedures.
Conditions Associated with Various Stages of Hypothermia
- > 96 F. Leads to increased post anesthetic shivering, which in turn increases O2 consumption and decreases ventilatory capacity.
- 90 -94 F. Hypometabolism occurs and the minimum alveolar concentration necessary to maintain anesthesia of many inhalants is reduced, leading to prolonged anesthetic recovery if the dose of delivered anesthesia is not lowered. Peripheral vasoconstriction may be profound.
- < 90 F. Atrial fibrillation may be noted and ventricular fibrillation may be induced very easily by mild myocardial trauma (such as that from cardiac compressions during CPR). At this temperature, the myocardium may be unresponsive to electrical defibrillation and to many cardiac medications such as the anticholinergic agents. Artificial ventilation may be required. Anesthetic recovery will be prolonged. Intestinal ileus develops and ulcerations occur in the stomach and intestines due to impaired splanchnic and GI blood flow. Hemorrhagic pancreatitis may also occur. At this temperature, metabolic acidosis increases during rewarming and external heat sources are required for rewarming.
In addition, hypothermia may:
- predispose many species to higher rates of postoperative infection
- effect renal compromise
- dramatically increase ATP and O2 requirements during rewarming to correct the hypothermia
- increase pain due shivering and to tensing of muscles
- cause occurrence of DIC due to the disruption of the microvasculature, hemoconcentration, platelet deformation and activation of the coagulation cascade
Methods of Treatment and Prevention of Hypothermia.
- Decrease anesthetic time.
- Limit the surgical prep to the surgical field if possible. Use warmed prep. solutions/materials. Avoid the use of alcohol in patients at high risk for hypothermia.
- Carefully and judiciously use peripheral skin warming on the extremities. This has been shown to be more effective than trunk warming, especially the hindlimbs of dogs. The primary problem with peripheral skin warming is that there tends to be poor perfusion to the skin in the hypothermic patients. Since the heat is not being distributed well into the deeper tissues by the altered peripheral blood flow, this is ineffective. The shunting of blood also leads to localized concentration of the heat and thermal necrosis at contact points.
- Warm air convection units such as the Bair Hugger ?rand of forced air warming blanket are safe and effective in most cases. These devices circulate warm air through fenestrated disposable sterile blankets of various sizes and shapes. Since they diffusely apply the warm air over a large region and there is no direct heat contact with the skin, there is less chance of localized thermal necrosis. An additional advantage of these units is that they tend to dry a moist environment, thereby decreasing the chance of thermal injury which can occur when there is moisture between the tissues and the heat source acts as a conductor.
- Circulating water pads are the next safest, peripheral skin-warming-type device. Gaymar?roduces a popular unit with a range of different sized and shaped pads. Although these are less likely to cause burns when used correctly, direct contact with moist hypoperfused tissue can still result in thermal injury.
- Hot water bottles “warmies” can be dangerous if they are too warm, if applied directly to the skin or if there is prolonged moist contact between the heat source and the skin.
- Electric heating pads are among the most dangerous sources of heat for peripheral skin rewarming since they have focal areas of severe heat along their heating element and can cause necrosis to hypoperfused tissues even on the lowest settings. They should not be used without a heat buffer of dry material between the pad and the element. They can also short and cause thermal burns, fires and/or electrocution.
- Electric, low voltage, thermal barriers such as the Vetko?hermal Barrier are designed specifically to maintain a 95-100 degree environment underneath the patient and to prevent radiant heat loss but do not add any heat to the patient.
- Other insulating devices such as the Olympic Vac Hold?r materials such as foam rubber and bubble wrap are safe, but only prevent radiant heat loss between their contact surface and the patient as well as any potential heat sinks such as the stainless steel surgery table. These can be used in combination with warming devices like the circulating water pad or Bair Hugger.
- Core heating can be achieved most dramatically by the administration of thermal energy to the interior of the patient. This circumvents the problems associated with peripheral warming, but also has several drawbacks. In general, any substance or fluid administered to a patient should not exceed 420C to prevent local or systemic thermal injury.
- Intravenous warming can be safely utilized if precautions are taken not to affect the integrity of the fluid and the administration unit and if the temperature arriving to the patient is not allowed to exceed 420C. This can be accomplished by the use of prewarmed intravenous fluids. Using IV solutions warmed in baths, heaters or even in microwaves are cheap and effective methods of decreasing hypothermia. . Running intravenous fluids through a warm water bath or commercial line heaters, such as the Hotline?r the Ranger?deliver 420C fluids directly to the patient.
- The use of prewarmed irrigation fluids is advisable, again with care not to cause localized hyperthermia to any tissues, especially hypoperfused and ischemic tissues. Peritoneal lavage with warmed fluids can be used to raise core body temperature, but careful introduction/technique and strict asepsis is required.
- Warm enemas and warm gastric lavage can be effective but may also rob the compromised patient of minerals and electrolytes. Therefore, these techniques should be used only in patients who require rewarming alone. Care should be taken to avoid thermal injury by using 420C as a maximum figure. Pressure injuries and mucosal barrier compromise must also be avoided.
With proper vigilance, the use of prophylactic measures and the thermal conservation/replacement technology available, we should be able to minimize the associated morbidity of this common anesthetic problem.