Today's Veterinary Practice

MAY-JUN 2018

Today's Veterinary Practice provides comprehensive information to keep every small animal practitioner up to date on companion animal medicine and surgery as well as practice building and management.

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PEER REVIEWED 72 FOCUS ON METHODS OF REWARMING Patients should be rewarmed quickly but carefully. The recommended rate of rewarming to increase core body temperature is 1.1°C to 2.2°C (about 2°F to 4°F) per hour.3 Rewarming can be by passive surface warming, active surface warming, active core warming, or a combination of techniques. Passive Warming Passive surface warming uses external wraps, blankets, or towels to prevent heat loss while the animal "self- generates" heat. This works best in patients with mild hypothermia and adequate blood volume. Passive techniques reduce heat loss by acting as an insulator and trapping body heat next to the patient, thereby reducing convective and conductive heat loss. 1–3 Because the skin is the major source of heat loss during anesthesia, cutaneous heat loss can be decreased by 30% by simply covering the skin.5 A hypothermic patient should be warmed to 37.2°C (98.5°F), and then passive warming only should continue.3 Active Warming Active warming applies heat to the patient to reduce the gradient between the body and the environmental temperature. This method is recommended for all levels of hypothermia. Active warming with a forced hot air warmer, a water-circulating blanket, towels or blankets around the patient, or socks placed on the paws will help provide external heat to the animal and prevent further heat loss.6 Maintain the body temperature as near to normal as possible. The use of forced warm air blankets seems to be the most efficacious active warming method for thermal management in dogs and cats. These blankets can be placed over or under the patient, with air flow directed toward the patient. Note that infection has been associated with the use of forced warm air blankets placed too near the surgical field and the blowing of particles into the field. This can be prevented with proper blanket placement and isolation of the surgical field.1 The forced air should not be turned on until the surgical drapes have been positioned and clamped into place. Active surface warming can also involve a heating pad, heating lamps, or dryers to offset the animal's inability to generate heat. Heating pads and lamps must be used carefully to avoid patient burns, and lamps should be used from a distance of more than 30 inches.7 A patient should not be left unattended when a heat lamp is used; instead, use of an overhead hospital infant radiant heat warmer is recommended. Be especially careful with active surface warming of cats. Provide IV fluid support and use warm fluids to avoid further heat loss, maintain normotension, and BOX 1 CASE STUDY History and Physical Exam A 1-kg Yorkshire terrier is undergoing general anesthesia for enterotomy to remove an intestinal foreign body. This 2-year-old spayed female ate a hair tie yesterday and has been vomiting since then. On physical exam, she is bright and lively, with a heart rate of 130 beats/min, respiratory rate of 20 breaths/min, normal heart and lung sounds, and body temperature of 38.4˚C (101.2˚F). Diagnostics Abdominal radiographs and ultrasounds show an obstruction due to a foreign body lodged in the duodenum, requiring a surgical exploratory laparotomy. Surgical Procedure The dog is anesthetized with hydromorphone, 0.1 mg/ kg, and midazolam, 0.25 mg/kg IM. Induction is done with propofol, 4 mg/kg IV titrated to effect, and then intubation. The dog is placed on a nonrebreathing circuit with oxygen and isoflurane at a flow rate of 2 L/min. During the procedure, the anesthetist monitoring the dog notes the body temperature, taken via an esophageal probe, is 31.6˚C (89˚F). In our case study, an enterotomy is performed, with successful removal of the hair tie. Before closure, warm saline lavage is instilled into the abdomen, allowed to sit, and then removed via suction; this is done twice. In recovery, the rectal temperature is 32.7˚C (91˚F), a slight improvement over the 31.6˚C (89˚F) body temperature during surgery. This improvement is due to the warm lavage used to rinse the abdomen both before and during the closing of the abdomen. The dog is wrapped in a forced warm air heating blanket with an infant radiant heater positioned over the dog. The dog is kept lightly anesthetized until the body temperature is 35˚C (95˚F) to prevent severe shivering and the associated stress of shivering. When the dog's temperature reaches 35˚C (95˚F), the inhalant is discontinued. Oxygen is used until the temperature reaches 36˚C (97˚F); then the dog is placed on room air. Blood glucose measurement is within normal limits, and the pulse oximeter is monitored until the dog no longer tolerates the probe on the tongue. The dog is extubated when she begins chewing and swallowing. The body temperature is 36.7˚C (98˚F), and the radiant heater is removed. The dog is then placed in a recovery cage with a forced warm air blanket. When the dog's temperature reaches 37.2˚C (99˚F), the forced air blanket is removed and the dog is covered with a towel. The towel is removed when the temperature is 37.8˚C (100˚F). In this case, the dog has an uneventful recovery.

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