Hypothermia is the leading prolonged anesthesia side effect in animals. In both dogs and cats, hypothermia, and even hyperthermia, can often happen with any lengthy surgical procedure—whether dental or general—or in a patient who has comorbidity issues.
The use of veterinary-specific multiparameter digital monitors helps to quickly determine when an anesthetized patient is becoming hypothermic during a procedure. Prior to the availability of digital monitoring, patient temperatures could plunge or rise significantly before the animal’s thermal state became visibly noticeable to the veterinary surgeon. This usually occurred through very careful observation of changes to pulse rate and color of mucous membranes, which can be good indicators of a patient’s thermal status. However, the reaction time for intervening in an abnormal loss or gain in temperature was delayed compared to current capabilities afforded by multiparameter monitoring.
The risks associated with thermal deficits or excess are significant, potentially causing adverse effects on the cardiovascular system, inducing respiratory failure or cerebral depression.
Equipped with probes, today’s multiparameter patient monitors can provide constant in-procedure temperature confirmation. Veterinary surgeons are intimately familiar with normal temperature values for both dogs and cats. Monitors that show readings outside those normal values, along with other monitored parameters, allow the veterinarian to quickly determine when a patient is hypothermic or hyperthermic and to begin an immediate intervention strategy.
Despite the accuracy of monitoring temperature digitally, it is important surgical assistants are properly trained and working in concert with both the monitor and visual signs, all of which allow for faster adjustment of anesthesia and other temperature-influencing factors.
As attending surgical assistants or anesthetists maintain close visual monitoring of the patient’s physical condition, they should call out parameters from the monitor that may quickly confirm thermal status to the veterinarian. A decreased heart rate will be seen with hypothermia, while hyperthermia will show the opposite. Likewise, end-tidal carbon dioxide (EtCO₂) will decrease with hypothermia, while hyperthermia influences EtCO₂ upward. As with the other parameters, blood pressure decreases during hypothermia, but in contrast to the increases seen with hyperthermia in the other vitals, blood pressure lowers during significant hyperthermia.
Dental procedure nuances
Hypothermia is most often seen in dental surgeries due to the combination of air-conditioned procedure rooms, the abundant use of water in the patient’s mouth, the long duration of most procedures, and the influence of medications used.
The biggest predisposing factors for hypothermia in cats and dogs during dental procedures are water irrigation during drilling and air-conditioned room temperatures. In dental procedures lasting longer than one hour, it is very common for both dog and cat patients to develop hypothermia if they are not properly monitored.
Clinicians should avidly train for and practice appropriate hypothermia monitoring and mitigation techniques in the dental surgery setting. According to Jan Bellows, DVM, Dipl. AVDC, ABVP, the best approach is for the assisting dental clinician to report and record vital parameters directly from the multiparameter monitor every five minutes during a procedure.
For instance, once a patient’s temperature is prolonged below 93 F (33.9 C) when anesthetized during a dental procedure, he or she will begin to show signs of hypothermic distress in the form of decreased metabolism. These signs include decreased respiratory rate and pulse oximetry (SpO₂) values, decreased heart rate, coagulopathies, and increased arrhythmias.
“Many years ago, a feline patient under anesthesia for full-mouth extractions to care for severe gingivostomatitis gave us a wake-up call,” Dr. Bellows recalls. “About two hours into the surgery, the cat’s heart and respiratory rate were decreasing and her mucous membrane color did not appear as pink as in the beginning of the procedure. At the time, we did not have a digital temperature monitor and by the time we took the cat’s temperature, which was 92 F (33 C), we needed to act. Fortunately, we were able to decrease the anesthesia levels and warm the cat up before completing the procedure. I immediately ordered a temperature probe for our multiparameter monitor.”
General surgery temperature maintenance
For all surgeries, maintaining an ideal temperature is imperative. It is important to note that animals—especially cats—can also experience hyperthermia in the surgical environment when medications such as opioids are involved.
“Keeping the patient’s core body temperature between 97 F and 103 F (36 C to 39 F) is one of the most important parameters to control. I see animals trend toward hypothermia or hyperthermia as a side effect of anesthesia, prolonged procedures, or medications in my practice nearly daily. That is why my mantra is monitor, monitor, monitor,” Bellows explains.
Marco Ruffato, DVM, DACVAA, cVMA agrees. He sees about 20 percent of the surgical cases in his practice migrate outside of normothermia. “It is quite common, especially if certain preventive measures are not adopted perioperatively,” Dr. Ruffato says.
Excessive extremes in temperature during surgery can be avoided. To help deter onset of mild, moderate, and severe hypothermia or hyperthermia, it is important to remember two concepts relating to patient temperature: the size of the animal and understanding the difference between core and peripheral temperatures.
Thin dogs and cats are prone to hypothermia, while their obese counterparts—who are more insulated—are prone to hyperthermia. Due to their body mass and index, cats are more sensitive to temperature changes overall.
“It’s important to be especially careful with small- to medium-sized dogs and cats because most of the hypothermic cases I have encountered were related to these sizes. Small patients tend to drop their temperature quite rapidly and they are usually difficult to bring back to normothermia,” Ruffato explains.
When it comes to patient temperature measurement, there is a difference between the animal’s core temperature and peripheral temperature. Accurate core temperature is obtained with esophageal monitors and deep rectal probes, while peripheral temperature accuracy will be elicited via ear and rectal measurement.
Furthermore, it is critical for veterinarians to watch for overuse of opioids in cats, as hyperthermia can develop in response to these powerful drugs. In this scenario, the heat source should simply be removed, and the cat should return to normal temperature.
Warming up to a solution
If a hypothermic status is identified, specific warming measures should be taken while some strategies should be avoided entirely. It is generally acceptable to use forced warm air systems and radiant energy systems to warm both cats and dogs out of a hypothermic state.
Some approaches that transfer heat through conduction include blankets and electric blankets made of conductive fabrics or those equipped with thermosensors that maintain close monitoring of the surface being warmed. Heated surgical tables work in much the same way, while convective expandable-style blankets into which warm air is forced are also effective. Infrared heating lamps that use an infrared emitting light bulb to warm patients by radiation are also acceptable.
By using warming devices right at the beginning of the procedure, as well as minimizing the exposure of body parts to cold surfaces such as radiology tables, hypothermia incidents can sometimes be avoided.
More importantly, to prevent serious and painful burns to an animal’s skin, clinic staff should avoid using microwaved IV bottles, bags, or defective warm water pads for warming patients. If in doubt, guidelines published online by the American Animal Hospital Association (AAHA) advise against using “supplemental heat sources that are not designed specifically for anesthetized patients, as they can cause severe thermal injury.”
Stay ahead of temperature shifts for better outcomes
According to AAHA’s “2020 Anesthesia and Monitoring Guidelines for Dogs and Cats,” “monitoring decreases the odds of anesthetic death, whereas lack of monitoring increases the odds of anesthetic death.” The association also recommends treatment decisions should be based on information from both electronic multiparameter monitors and hands-on assessment of patients.
Overall, monitoring surgical patients for temperature and corresponding vitals with digital multiparameter monitors during surgical procedures can lead to quicker discovery of hypothermia and hyperthermia, allowing for rapid adjustments in the surgical setting rather than an emergency response.
Eduardo Miranda is president of Digicare Animal Health, a manufacturer of veterinary-specific multiparameter monitoring systems and telemetry systems that help veterinarians achieve better patient outcomes. Based in Boynton Beach, Fla., Digicare works with numerous distributors in the U.S., Canada, and the U.K.