Anesthesia is a very fragile time for animals because they easily can become hypothermic, putting them at risk of infection and delaying their recovery.
“When you anesthetize a patient, one of the functions it loses is its ability to affect thermoregulatory control,” explained Kelson Danielson, DVM, Dipl. ACVS, a surgeon at Blue Pearl Veterinary Partners in New York. “For instance, as you get cold, you shiver to help keep warm, but under anesthesia, they are unable to do that.
“In addition, their brain loses its ability to keep that set point that keeps the temperature normal.
They are also less metabolically active, so the body is burning less energy during anesthesia and that will decrease the amount of heat that is produced.”
Many of the drugs used during anesthesia compound the problem because they cause vasodilation, which the body normally uses to stay cool when it is overheated. So, if it is already cool and drugs cause vasodilation, the body will become hypothermic.
“Human research shows that once surface cooling occurs, blood moves from the extremities to the core blood compartment, which can make it very difficult to rewarm the patient,” explained Sandra Perkowski, VMD, PhD, Dipl. ACVA, head of the anesthesia service at the University of Pennsylvania, College of Veterinary Medicine.
In both human and animal patients, heat loss occurs through radiation, conduction, convection and evaporation, but radiation loss—loss to the environment—is the major source of heat lost during anesthesia. When an animal is anesthetized, vasodilation occurs almost immediately, which increases heat loss to the environment.
Research shows that hypothermia is a frequent complication of anesthesia in dogs and cats. A recent retrospective study analyzed more than 1,500 cases of anesthetized dogs in the University Clinical Hospitals of the CEU Cardenal Herrera and Cordoba, Spain. They found that 83.6 percent of 1,525 dogs presented this complication, whereas in humans, the percentage is between 30 and 60 percent.
In this study, published in the Veterinary Record in 2012, the variables directly related to hypothermia in dogs included the duration of the pre-anesthesia and anesthesia, the physical condition of the animal and, also, their posture during surgery. Sternal and dorsal recumbencies showed lower temperatures than lateral recumbency, according Jose Ignacio Redondo Garcia, DVM, Ph.D., senior lecturer in the department of animal medicine and surgery at the University CEU Cardenal Herrera in Spain.
“In our clinical experience, hypothermia is a very frequent complication during the postoperative period, but there are only a few studies in veterinary medicine about postoperative hypothermia, and to our knowledge, there are no published epidemiological studies of this complication and its clinical predictors,” said Redondo Garcia.
In a previous study, the same research team determined that this prevalence rate is even higher in cats: 96.7 percent of cats suffer from hypothermia in procedures requiring anesthetic. In this case, the study showed that abdominal and orthopedic interventions generate a greater decrease in cat’s body temperature.
For most animals, the loss isn’t necessarily significant, but the body temperature can become too low for the very old, young and small, and those with concurrent conditions. The type of surgery is also a risk factor because surgeries like thoracic or abdominal procedures open a much larger portion of the animal’s body for a longer period, Danielson said.
Therefore, the researchers believe that temperature should be continuously monitored, and veterinarians should take preventive measures to avoid heat loss during procedures.
Remember how heat is lost when deciding what to use to keep the patient warm, suggested Perkowski.
“The way you lose heat is primarily through radiation into the air. Many people will try to protect the animal by placing it on a blanket instead of covering it. But having a heating pad under them, while it will keep them from contact with the cold metal table, will not be enough to keep them warm because you are not giving them heat where they need it,” she said.
Because they lose radiant heat, the best way to keep patients warm would be to keep the room warm, but that is not practical for many reasons, including concern over bacterial growth and infection.
But there are still steps one can take to keep the patient warm. Providing heat with an air-circulating blanket (such as the Bair Hugger by Arizant) and draping the patient will help keep it warm during anesthesia. It is also a good idea to cover the animals’ paws because heat is lost through the extremities.
“Forced-air blankets are the most commonly used intraoperative warming technique and the efficacy of the method is well established,” Redondo Garcia said. “Forced air heating is both inexpensive and remarkably safe.”
Some surgeons use electric blankets, hot water bottles or IV saline solution that has been heated in the microwave, but Danielson recommends against using these devices because there is no way to regulate their temperature and they can burn the patient, even if the warming devices don’t feel too hot to the touch.
“When a patient is anesthetized, it often does not have good perfusion to the skin,” Danielson said.
“Placing a warm water bottle next to the skin, even it if does not feel too hot to you, can cause thermal burns because it will sit there for a long time and the blood is not circulating very well through the skin and cannot dissipate the heat.
“I would recommend being very careful with something that does not have a built-in automatic temperature regulation system and an automatic shut-off if it gets over temperature.”
Danielson also warned against overcorrecting, because an animal can become too warm too quickly, which is also dangerous. This is really important with small animals, whose temperature can become too cold or too hot very quickly.
“Any time a patient is anesthetized, they are at risk for hypothermia,” added Danielson. “We should be monitoring for that, so we know what is going on, and we should try to prevent it. Monitoring is very important. Although it is dangerous to become hypothermic, treating blindly without monitoring the effects of the treatment and then having patients become hyperthermic would also be dangerous.”
Therefore, patients should be monitored from the time they are given the anesthetics until they are fully recovered from the anesthesia, said Perkowski, who recommended using either an esophageal temperature probe or rectal thermometer and checking it every 5 to 15 minutes.
“It is really important to keep them warm right away,” Perkowski said. “Many times, people will not be actively warming the patients while they do their preparations to get them into surgery.”
Even putting them under a heat lamp during the preparation can help, she added, especially in very small patients.
“We see problems with hypothermia when patients have been placed under general anesthesia for an imaging procedure, and they’ve had an MRI or CT scan and then gone into the operating room. They are under general anesthesia. They are in a room that is typically kept cold to keep the equipment functioning properly, and they can lose heat rapidly. We should do everything we can to provide heat support immediately,” said Danielson.
Animals can lose a lot of heat during abdominal or thoracic procedures, so many surgeons will lavage with warm—not hot—saline during and after the procedure to minimize the degree of hypothermia and to start correcting it before closing.
Keep patients warm in recovery, too, Danielson recommended, and continue to monitor the patient until it is fully awake and able to control its body temperature.
“In human medicine, patients often indicate that feeling cold in the immediate postoperative period is the worst part of their hospitalization, sometimes rating it worse than surgical pain. Given the appropriate efforts to treat surgical pain, it would seem similarly appropriate to prevent and treat thermal discomfort,” said Redondo Garcia.