Dogs and cats commonly lose heat when anesthetized. Maintaining body temperature within a narrow range is important for cardiac function, metabolism, normal enzyme activity, nerve conduction, and hemostasis. Several studies report that small patients, for example cats weighing less than 2 kg, are more likely to die than cats weighing between 2–6 kg and that senior patients and those undergoing long procedures also carry a higher perioperative risk.^1-3 It has been suggested that these increased risks may be related to hypothermia. In cats, Redondo and colleagues correlated intraoperative hypothermia with mortality.⁴ The negative impact of hypothermia is greatly underestimated and its occurrence often goes undetected because intra-operative temperatures are not often monitored.

Thermal Balance

Homeothermy, a balance between heat loss and heat gain involves complex sensing mechanisms that drive the mechanisms controlling heat loss or gain in the correct direction. Heat gains can be obligatory or facultative. Obligatory gains occur independently of thermoregulation and include heat from basal metabolism, eating and exercise. Facultative gains act to restore thermal balance and the most important source is from shivering. Three-quarters of heat loss occurs from the body surface and the remainder is lost from the respiratory tract. Losses occur through convection (transfer of heat to the air), conduction (transfer of heat from the animal to a surface that is cooler), evaporation (heat dissipated by evaporation of moisture from wet skin or the respiratory tract) and radiation (exchange of heat between the body and objects in the environment). Temperature sensors exist centrally (hypothalamus, spinal cord, brain stem, abdominal organs and skeletal muscles) and peripherally (warm and cold receptors in the skin). The hypothalamus acts like a thermostat by integrating thermal input and controlling effector organs.

When an animal is anesthetized many factors interrupt normal thermoregulation. Anesthesia abolishes behavioral responses (e.g., seeking out a warm environment), reduces metabolic rate, alters hypothalamic function, and reduces muscle tone and effector responses (shivering). In addition operating room environments and surgical procedures impose large thermal stresses on patients.

In conscious mammals body temperature rarely fluctuates more than ± 0.2°C. Under anesthesia core temperature varies more widely; often no response is mounted until body temperature has dropped by 2.5°C. Vasoconstriction can occur in anesthetized patients and although it may slow down the rate of heat loss it has a negative effect on tissue perfusion and is usually a late response. This reflex will be also be counteracted if vasodilating agents such as acetylpromazine or isoflurane and sevoflurane are used.

The greatest rate of heat loss is immediately after induction and during the first 20 minutes of anesthesia due to redistribution of heat from the core to periphery. However, it is important to note that heat loss begins immediately after premedication because sedatives and tranquillizers depress the hypothalamus. We can initiate intervention at this time by keeping the animals that are waiting for anesthesia warm. Heat continues to be lost after the initial steep drop but at a lower rate. There is also an increase in the difference between core (esophageal) and peripheral (rectal) temperature over time. The smaller the animal the greater its body surface area:weight ratio and the more prone it is to hypothermia. When no attempt was made to preserve body heat, dogs and cats weighing < 10 kg dropped below their normal temperature by 3.4°C after one hour of anesthesia.⁵ The severity of hypothermia is also influenced by the environmental temperature, duration of anesthesia, and exposure of body cavities.

Ideally core temperature should be monitored, as the core is where vital organs that are negatively affected by hypothermia such as the heart are situated, with a probe placed in the esophagus. Rectal temperature reflects the peripheral temperature and often underestimates core temperature.

Negative Effects of Hypothermia

A drop in core temperature to 34°C is cause for concern. As core temperature falls, the myocardium becomes more irritable and the sino-atrial node beats more slowly. There is a drop in cardiac output and blood pressure and at subnormal temperatures, atropine and glycopyrrolate are unlikely to correct bradycardia. Changes in cardiac rhythm may also be noted and at temperatures approaching 32.2°C asystole or fibrillation may spontaneously occur. Fluid shifts result in hemoconcentration, increased blood viscosity and red blood cell sludging. Increased bleeding occurs secondary to prolonged coagulation times and altered platelet function.

Tissue perfusion is impaired by hypothermia and shifting of the oxyhemoglobin curve to the left decreases oxygen unloading. A metabolic acidosis develops as lactate levels rise secondary to poor perfusion and decreased hepatic metabolism. Blood glucose levels may rise and complicate interpretation of laboratory results.

Metabolism is slowed and liver function is impaired, delaying breakdown of anesthetic drugs which will prolong recovery times. The requirements for inhalant agents drop as temperature decreases and if anesthetic depth is not closely monitored, animals will receive a relative overdose. As a patient cools, the amount of anesthetic required to produce apnea decreases and responses to hypercapnia and hypoxia are blunted.

In human studies, intra-operative hypothermia has been linked to increased post-operative wound infection. This is a result of poor perfusion to the periphery, vasoconstriction and low oxygen tension at the surgical site. Hypothermia also impairs immune function, including the killing ability of neutrophils. One veterinary study linked wound infection to duration of anesthesia⁶ and there is no doubt that maintaining normothermia is in the best interests of the patient.

Cold animals take longer to recover and this is documented in dogs.⁷ During recovery hypothermic animals shiver to generate heat. Shivering increases their metabolic rate and heat production, but also increases oxygen demand. This, combined with a decreased ventilatory drive can lead to hypoxemia. Pain from the surgical incision is likely to be worse when an animal shivers and humans report that waking up cold is extremely unpleasant.

Techniques for Maintaining Body Temperature

Thermal losses should be minimized. Although re-warming is possible in the post-operative period, rapid re-warming can cause vasodilation, which is not well tolerated by some surgical patients.

Suggestions for preventing hypothermia:

 Pre-warm patients by using forced warm air units and blankets after premedication.

 Surgical preparation - avoid cold solutions especially alcohol. Warm sterile saline is a good choice.

 Anesthesia time should be kept to a minimum.

 Ambient temperature - Normal operating room temperatures are often 24–26°C. Warmer temperatures would benefit the patients but may increase discomfort for personnel. The induction and recovery areas should be kept warm.

 Warm inspired gases - this requires specialized anesthetic equipment. If a circle system is used, low flow anesthesia minimizes heat loss from the respiratory tract.

 Circulating warm water blankets are effective in small patients⁸ and are more effective when placed on the limbs than on or under the trunk⁹. Electric blankets must be avoided as severe skin burns can occur in hypothermic animals.

 Forced warm air devices are effective.¹⁰

 Blankets - fleece blankets and thermal insulating blankets can minimize radiation and convective losses. Newer devices that utilize blankets made of thermal conductive materials are durable and effective.

 Infra-red lamps - great care should be used when using these as skin burns can occur.

 Warm intravenous fluids - when large volumes of fluid are given to patients they should be warmed to minimize thermal stress.

References

1.  Brodbelt DC, Blissitt KJ, Hammond RA, Neath PJ, Young LE, Pfeiffer DU, Wood JL. The risk of death: the confidential enquiry into perioperative small animal fatalities. Vet Anaesth Analg. 2008;35(5):365–373.

2.  Brodbelt DC, Pfeiffer DU, Young LE, Wood JL. Risk factors for anaesthetic-related death in cats: results from the confidential enquiry into perioperative small animal fatalities (CEPSAF). Br J Anaesth. 2007;99(5):617–623.

3.  Brodbelt DC, Pfeiffer DU, Young LE, Wood JL. Results of the confidential enquiry into perioperative small animal fatalities regarding risk factors for anesthetic-related death in dogs. J Am Vet Med Assoc. 2008;233(7):1096–1104.

4.  Redondo JI, Suesta P, Gil L, Soler G, Serra I, Soler C. Retrospective study of the prevalence of postanaesthetic hypothermia in cats. Vet Rec. 2012;170(8):206.

5.  Waterman A. Accidental hypothermia during anaesthesia in dogs and cats. Vet Rec. 1975;96(14):308–313.

6.  Beal MW, Brown DC, Shofer FS. The effects of perioperative hypothermia and the duration of anesthesia on postoperative wound infection rate in clean wounds: a retrospective study. Vet Surg. 2000;29(2):123–127.

7.  Pottie RG, Dart CM, Perkins NR, Hodgson DR. Effect of hypothermia on recovery from general anaesthesia in the dog. Aust Vet J. 2007;85(4):158–162.

8.  Evans AT, Sawyer DC, Krahwinkel DJ. Effect of a warm-water blanket on development of hypothermia during small animal surgery. J Am Vet Med Assoc. 1973;163(2):147–148.

9.  Cabell LW, Perkowski SZ, Gregor T, Smith GK. The effects of active peripheral skin warming on perioperative hypothermia in dogs. Vet Surg. 1997;26(2):79–85.

10. Machon RG, Raffe MR, Robinson EP. Warming with a forced air warming blanket minimizes anesthetic-induced hypothermia in cats. Vet Surg. 1999;28(4):301–310.