Keeping any anesthetized animal from hypothermia before, during, and after surgery is a delicate balance of monitoring during treatment and avoiding potential complications when the animal is recovering. This article provides a case study of a Yorkshire terrier under anesthesia during surgery, along with the pathophysiology of hypotension and methods that can be used to manage it during and after surgery.
Hypothermia Under Anesthesia
For any animal under anesthesia for longer than 20 minutes, techniques to minimize heat loss should be applied.1 This is because detrimental effects, such as a decreased ability to recover and an increased incidence of infection, can occur in instances of severe hypothermia, and in the critical patient this could lead to death. Also hypothermia may persist into recovery and affect recovery quality and duration.
Hypothermia can be defined according to the decrease from normal body temperature. Normal body temperature is 37.5˚C to 39.2˚C (99.5˚F to 102.5˚F) for a dog and 37.8˚C to 39.5˚C (100˚F to 103.1˚F) for a cat.2 Mild hypothermia is 37.0˚C to 37.7˚C (98˚F to 99.9˚F); moderate, 35.8˚C to 37.0˚C (96˚F to 98˚F); severe, 33.6˚C to 35.8˚C (92˚F to 96˚F); and critical, less than 33.6˚C (92˚F or less).3,4
Core temperature changes occur in 3 phases during anesthesia.
- In the first hour, an initial rapid decline results from peripheral vasodilation and redistribution of body heat.
- Over the following 2 hours, the temperature declines in a slower linear fashion because of the inhibition of metabolism and heat production by the anesthetic drugs.
- Finally, over the next 3 to 4 hours, core temperature stabilizes and remains relatively unchanged as thermal steady state is achieved.
Body temperature changes in a predictable manner as a result of the core-to-peripheral temperature gradient.4,5 The gradient is maintained mainly via peripheral vasoconstriction, which keeps the body heat in a central pool of blood and minimizes the heat that can be lost through the skin.4 The extremities will be cooler because of shunting of blood to the core.1 The size of the animal plays a role in heat loss. A larger body surface area increases these losses through cutaneous loss. That means that small- and toy-breed dogs have a greater risk for heat loss than large-breed dogs because of their higher surface area.
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.
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 prevent shock. The temperature of warm IV fluids should not be higher than 42.6°C (108°F) to avoid cellular injury.3 Apply heat to the trunk, thorax, and abdomen, but not the extremities (to avoid peripheral vasodilation and persistent hypothermia).
Active Core Warming
Active core warming is the use of heat applied centrally in order to rapidly increase core temperature. This is commonly done before surgical closure of body cavities. Warm saline lavage, 40°C to 45°C (104°F to 113°F), is placed into the peritoneal or pleural cavity and allowed to sit for 2 to 6 minutes. It is removed via suction, and warmer lavage is then added. This process can be done 2 to 4 times before final fluid removal and cavity closure.1–3,6
Complications of Rewarming
Careful warming and close monitoring are essential in managing the hypothermic patient to avoid “afterdrop” or “rewarming shock.” Afterdrop is the continued decrease in the patient’s core body temperature as warming is provided. It is caused by the return of cold peripheral blood to the central circulation. To help prevent this, remember to warm the trunk, chest, and abdomen, but not the extremities.
Rewarming shock develops with extremely rapid warming and is due to the sudden development of systemic vasodilation. This vasodilation leads to hypotension, which may be further aggravated by the increased metabolic demand that develops during rewarming and increases the need for perfusion. The returning colder blood and associated lactic acid carried back to the core can lead to “rewarming acidosis.” To help prevent or reduce rewarming shock, IV fluid therapy should be given. Assess the patient for volume status, systemic blood pressure, tissue perfusion (by evaluating capillary refill time), lactate clearance, mentation, and urine output.2,3 Warm the hypothermic patient to 36.9°C (98.5°F), and then stop use of all warming methods except passive warming.3
Monitoring During Treatment
Monitoring during treatment for hypothermia should consist of blood pressure monitoring and electrocardiography, as well as measurement of blood glucose, electrolytes, and hematocrit. Peripheral vasoconstriction can affect blood pressure and make readings unreliable with use of indirect methods. The hematocrit will increase by 2% for every 1°C (1.8°F) drop in core body temperature. A coagulation panel, lactate levels, and oxygen saturation by pulse oximetry should also be monitored, but vasoconstriction may affect the usefulness of pulse oximetry.2
Do not turn the animal side to side as a form of stimulation because the change in body position results in an acute drop in blood pressure or gastric reflux in some animals.8
Complications From Hypothermia
Because an animal recovering from anesthesia may experience hypothermia, it must be treated appropriately to improve patient recovery and decrease potential morbidity.
Several complications can occur with hypothermia. For example, impaired cardiovascular function may develop, with decreased cardiac output that leads to vasoconstriction. When the animal is rewarmed, vasodilation can enhance hypovolemia and lead to shock. Cardiac arrhythmias can occur at body temperatures of 31°C (87.8°F) or less. Hypothermia can cause hypoventilation, necessitating ventilator support, and liver metabolism and renal excretion of anesthetic drugs are slower because of the decrease in liver and renal blood flow. This may prolong recovery and lead to weakness in recovery. Animals with hypothermia may also have decreased resistance to infection and increased incidence of surgical wound infection.
Other complications include decreased oxidative killing by neutrophils, reduction of phagocytosis, and suppression of leukocyte migration. The vasoconstriction and increased blood viscosity decrease oxygen delivery to tissues, contributing to poor wound healing. The increased blood viscosity and slowing of enzymatic reactions of the coagulation pathways can lead to coagulopathy and increased blood loss. There is also increased postoperative protein catabolism. Shivering increases metabolism and heat production, but also increases myocardial oxygen demands and glucose needs.3,5 These adverse effects can be minimized with appropriate thermal support for the anesthetized patient.
See TABLE 1 for the 4 mechanisms of heat loss. It is also important to remember the following:
- Radiation and convection are the most important causes of heat loss and can account for 80% of the total heat losses in patients.
- Keeping the air temperature around the patient warm will decrease heat loss.
- Most types of heat loss depend on amount of exposed skins; minimizing skin exposure can reduce heat loss.4
References
- Clark-Price S. Inadvertent perianesthetic hypothermia in small animal patients. Vet Clin North Am Small Anim 2015;45(5):983-994.
- Todd JM. Hypothermia. In: Silverstein DC, Hopper K (eds). Small Animal Critical Care Medicine. 2nd ed. St. Louis: Elsevier Saunders; 2015:789-795.
- Lagutchik MS, Ford A. Care of the environmentally injured animal. In: Burkitt-Creedon JM, Davis H (eds). Advanced Monitoring and Procedures for Small Animal Emergency and Critical Care. Ames, IA: Wiley-Blackwell; 2012:799-813.
- Ko JC. Anesthesia monitoring and management. In: Ko JC (ed). Small Animal Anesthesia and Pain Management. London: Manson Publishing; 2013:123-162.
- Posner LP. Troubleshooting hypothermia and hyperthermia. In: Smith LJ (ed). Questions and Answers in Small Animal Anesthesia. Ames, IA: Wiley Blackwell; 2016:147-153.
- Ko JC. Anesthesia monitoring and management. In: Ko JC (ed). Small Animal Anesthesia and Pain Management. London: Manson Publishing; 2013:123-162.
- Hackett TB. Physical examination and daily assessment of the critical ill patient. In: Silverstein DC, Hopper K (eds): Small Animal Critical Care Medicine. 2nd ed. St. Louis, MO: Elsevier Saunders; 2015:6-10.
- Clarke KW, Trim CM, Hall LW. Anesthesia of the dog. In: Clarke KW, Trim CM, Hall LW (eds). Veterinary Anesthesia. 11th ed. St Louis, MO: Elsevier Saunders; 2014:405-498.
- Ko JC. Anesthetic emergencies and cardiopulmonary resuscitation. In: Ko JC (ed). Small Animal Anesthesia and Pain Management. London: Manson Publishing; 2013:173-186.