Hypothermia is a common complication of general anesthesia in small animal practice. The incidence of postanesthetic hypothermia in dogs1 and cats2 that do not receive heat support during general anesthesia is reportedly 83.6% and 97.5%, respectively. Compared with normothermia, hypothermia has been associated with several complications, including decreased drug metabolism, increased myocardial oxygen consumption from shivering, increased incisional pain from shivering, cardiac arrhythmias, increased bleeding, and increased probability of postoperative wound infections.3–6 Furthermore, hypothermia (vs normothermia) has been associated with an increased interval to regaining consciousness after anesthesia,7 which may be a result of decreased drug metabolism, decreased inhalant anesthetic requirements, and increased solubility of inhalant agents at lower body temperatures.8,9 Additionally, hypothermia may contribute to anesthetic-related death in cats weighing < 2 kg and dogs weighing < 5 kg.10
Redistribution hypothermia is the primary process responsible for heat loss in anesthetized patients.5 Mammalian bodies can be considered as being comprised of 2 thermal compartments: the core and the periphery or shell.5 The core compartment contains organs with high metabolic rates, is warmer than the periphery, and consists of the brain, heart, lung, liver, and kidneys.5 The peripheral compartment acts as a thermoregulatory buffer between the body and the environment and is comprised mainly of blood vessels and arteriovenous shunts.11
Arteriovenous shunts may receive up to 80% of cutaneous blood flow and play an important role in hypothermia during general anesthesia.12,13 The amount of heat lost to the environment can be regulated in conscious animals via constriction of peripheral blood vessels and dilation of peripheral arteriovenous shunts, which are controlled by the hypothalamus, resulting in a core temperature that fluctuates by only 0.5°C.14
During general anesthesia, hypothalamic activity becomes depressed and constriction of arteriovenous shunts does not occur until body temperature has decreased by 1.5°C, resulting in an increase in the amount of heat lost to the environment.5,14 The vasodilatory effects of most anesthetic agents increase blood flow to the extremities, which release heat into the environment from arteriovenous shunts via radiation and conduction. Blood in the extremities cools and mixes with warmer blood in the core compartment, thereby decreasing overall core temperature.5 This process continues, unless the immediate environment surrounding the patient becomes warmer than the patient (eg, through the use of supplemental heat sources or triggering of arteriovenous shunts causing vasoconstriction).14
Hypothermia induced by general anesthesia can be considered to have 3 phases.1,2,5 Phase I or redistribution hypothermia generally occurs during the first hour of anesthesia and is attributed to rapid redistribution of heat from the core to the peripheral compartment. Additionally, radiative and conductive heat loss occurs when the environmental temperature is lower than an animal's body temperature. During phase I, the animal's skin is typically being shaved of hair and prepared for surgery; therefore, evaporative and convective heat losses are likely occurring. Phase II or continual loss of heat occurs as heat is lost to the environment via radiation, convection, evaporation, and conduction. Warming devices can increase the environmental temperature close to the animal and prevent additional heat loss during this phase.5,15 Phase III or the plateau occurs when either hypothalamic-induced vasoconstriction occurs to prevent further heat loss or the patient gains heat from warming devices so that heat loss and gain are equal.
Ketamine hydrochloride, a dissociative anesthetic, is widely used in combination with a benzodiazepine for induction of anesthesia in small animal practice.16 Ketamine administration decreases the extent of redistribution hypothermia in humans, likely because it leads to an increase in the amount of circulating norepinephrine, causing an increase in peripheral arteriolar resistance and muscle activity.17,18 Additionally, propofol (2,6-diisopropylphenol) is routinely used for anesthetic induction and maintenance in dogs and cats.19 Propofol causes a dose-related decrease in arterial blood pressure and vasodilation that has been associated with an increase in heat loss through arteriovenous shunts.18
The purpose of the study reported here was to compare the influence of induction of general anesthesia with a benzodiazepine plus ketamine or propofol on intraoperative rectal and esophageal temperatures and postoperative rectal temperature in dogs undergoing routine ovariohysterectomy. Our hypothesis was that anesthetic induction with the ketamine-benzodiazepine combination would result in higher core and rectal temperatures than would induction with the propofol-benzodiazepine combination.
Supported in part by the Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, Fla.
Presented as a poster at the American College of Veterinary Anesthesia and Analgesia Meeting, Washington, DC, September 2015.
The authors thank Nicole Bengyel and Daniela Isaza for technical assistance.
Body condition score chart for dogs. World Small Animal Veterinary Association Global Nutrition Committee, Dundas, ON, Canada. Available at: www.wsava.org/sites/default/files/Body%20condition%20score%20chart%20dogs.pdf. Accessed Jun 1, 2015.
Adult reusable tubular oral or rectal thermometer (REF403), Measurement Specialties Inc, Dayton, Ohio.
Precision thermometer 4600, Measurement Specialties Inc, Dayton, Ohio.
QUESTemp 32 thermal environment monitor. Quest Technologies, Oconomowoc, Wis.
Random Integer Generator, Randomness and Integrity Services Ltd, Dublin, Ireland.
Previcox, Merial LLC, Duluth, Ga.
Rimadyl, Pfizer Animal Health, Exton, Penn.
Buprenex, Reckett Benckiser Pharmaceuticals, Richmond, Va.
Acepromazine maleate, Boehringer Ingelheim Vetmedica, St Joseph, Mo.
Midazolam injection USP, West-Ward, Eatontown, NJ.
Diazepam injection USP, Hospira Inc, Lake Forest, Ill.
Ketaset, Fort Dodge Animal Health, Fort Dodge, Iowa.
Propoflo, Abbott Animal Health, Chicago, Ill.
Isoflo, Abbott Laboratories, Chicago, Ill.
Pulse oximeter N20-PAV, VetEquip Inc, Pleasanton, Calif.
Gaymar T/Pump, Gaymar Industries Inc, Orchard Park, NY.
Bair Hugger Patient Warmer 500, 3M Health Care, Saint Paul, Minn.
Reusable esophageal/rectal adult temperature probe (REF401AC), Measurement Specialties Inc, Dayton, Ohio.
Durac Plus certified partial immersion thermometer, H-B Instrument, Collegeville, Pa.
SAS, version 9.3, SAS Institute Inc, Cary, NC.
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