Hypothermia and the Veterinary Dental Patient

To highlight the importance of oral health care for pets, the AVMA designated February as National Pet Dental Health Month.  Clients who appreciate the safety and convenience of non-anesthesia dentistry may question the need to have their pet face the risks of anesthesia for a routine dentistry.  In answer, the American Veterinary Dental College (AVDC) states that veterinary dentistry includes scaling the surfaces of teeth both above and below the gum line, followed by dental polishing. Removal of dental tartar only on the visible surfaces of the teeth has little effect on a pet’s health, and provides a false sense of accomplishment. The effect is purely cosmetic.  The 2013 AAHA Dental Care Guidelines for Dogs and Cats says that general anesthesia with intubation is necessary to properly assess and treat the companion animal dental patient. AAHA states, “Cleaning a companion animal’s teeth without general anesthesia is considered unacceptable and below the standard of care.”

Although anesthesia for animals has come a long way and is safer now than it ever has been, we can’t dismiss our clients’ concerns about the risks of anesthesia.  A two-year study of nearly 200,000 pets published in 2006 ranked surgical procedures in the order of those most commonly resulting in anesthetic death.  Dental procedures ranked number three, in the top seven. Age, underlying systemic disease, length of anesthesia, and hypothermia are listed among the probable contributors to the greater anesthetic risk among dental patients.

A hypothermia study conducted by a research team from the Universidad CEU Cardenal Herrera in Spain was published in 2013 in Veterinary Record. The results showed that 83.6 percent of 1,525 dogs developed hypothermia as a result of anesthesia. A previous study performed by the same research team suggests that cats are even more likely than dogs to develop hypothermia while anesthetized. Almost 97 percent of cats develop hypothermia while receiving anesthesia, and kittens are at increased risk. A study published in the Canadian Veterinary Journal evaluating cats undergoing anesthesia for dentistry, supports the Spanish research team’s findings, indicating that unless steps are taken to conserve body temperature, a decrease of nearly 4°F may occur. These studies clearly indicate hypothermia is one of the most predictable complications of anesthesia, and therefore veterinary staff should continuously monitor the body temperatures of anesthetized animals and be proactive in preventing heat loss.

Preventing peri-anesthesia hypothermia has traditionally focused on skin warming and conserving body surface heat, but this often proves to be ineffective and can sometimes burn animals. The margin of safety from causing significant thermal injury is surprisingly narrow in animals. Skin can be burned from devices supplying constant surface heat of as little as 115°F for one hour. Hot tap water can be warmer than that. Sedated and anesthetized animals can’t move away from excessive heat, so containers of warm water, heated wheat bags or ‘on-off’ electric heat pads not specifically designed for anesthetized animals can cause severe burns. Thermostatically controlled constant warming devices with even heat distribution such as warm air blankets are safer.

Premedication drugs can cause mild hypothermia in dogs and cats, typically losing 1 – 2°F over thirty to sixty minutes before anesthesia induction.  This initial drop in core body temperature precedes the precipitous critical heat loss of an additional 2 – 5°F that occurs in the first fifteen to thirty minutes after induction.  If the patient is draped for surgery, then the rate of heat loss slows.  Many of us are surprised to realize that a patient could lose as much as 7°F before it has been anesthetized for very long.

Providing thermal support before anesthesia may sound counter-intuitive, but recent research shows that warming patients effectively from the time of premedication to the time of induction can prevent that initial drop in body temperature prior to induction, and slow the rapid heat loss immediately following induction.

Targeting the rapid heat loss after induction is particularly challenging.  In that early stage, patients are positioned and repositioned which results in poor heat transfer from heating mats beneath them.  Also stimulation during this early stage is often minimal. Combine that with the patient lying on a grate over a water table and having its face and head drenched with cold water, then supporting body temperature seems like a losing battle.  Here are some tips for warming dental patients suggested by Portland’s award winning veterinary hospital and training facility, DoveLewis.

  • Laying a patient on a towel over a water table provides more surface area to lose body heat.  Placing the patient on a solid surface like a mat will help.
  • Placing the patient on any type of approved heating pad will help slow heat loss.
  • Bubble wrap layers over the patient help retain heat
  • Baby socks on their feet retains heat
  • An emergency reflective blanket tented over the patient traps heat
  • Attempt to keep the head as dry as possible and take time to wipe it dry periodically.
  • Administering warm IV fluids can help
  • Warm inhaled gases slow heat loss

Warming inhaled gases can go a long way toward preventing peri-anesthesia hypothermia. Inspiring cold compressed oxygen from an anesthetic gas machine can be a major contributor to cooling anesthetized patients from the inside out, especially right after intubation. Normally the nose and pharyngeal mucosa transfer heat and moisture to inspired air and then recover much of the heat during expiration. Bypassing the nose and pharyngeal mucosa with an endotracheal tube results in the delivery of cold compressed gases directly into the lungs, with no chance of temperature recovery during exhalation. This can cost a 25 pound dog nearly 3000 calories of warming energy in the first hour of anesthesia alone. Warming the inspired gases to near normal body temperature from the moment of intubation is a great way to prevent the loss of core body temperature caused by the body’s attempt to warm cold inspired gases.  It literally warms from within.

The first heated breathing circuit for veterinary use was introduced to the United States in 2013.  Developed by Advanced Anesthesia Specialists of Australia, it is manufactured and distributed under the Darvall brand name.  Darvall’s heated breathing circuits have a heating element imbedded into the tubing of the inspiratory limb of the breathing circuit. A sensor molded in the tubing at the Y piece monitors gas temperature and a microprocessor controls heating. Closed loop feedback is provided by an esophageal temperature probe which enables the microprocessor to monitor the animal’s body temperature, and it turns off the heater if either sensor detects temperatures above the presets.

Anesthesia decreases the metabolic rate and inhibits muscular activity which contributes to hypothermia. Hypothermia may lead to dysrhythmias, hypotension, respiratory depression, bradycardia, coagulopathy, altered blood viscosity, and anesthetic drug overdose. Warming hypothermic animals recovering from anesthesia is a slow and laborious process often taking more than one to two hours. This delay affects the patient, the staff, and the efficiency of your workflow.  Heated breathing circuits offer a new and innovative way to capture control of a dental patient’s body temperature from the moment of intubation, and puts an effective new tool in your hypothermia-management toolbox.

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ken-akedit
Ken Crump AHT, AAS, is a writer and animal anesthetist, and writes Making Anesthesia Easier for DarvallVet, a division of Advanced Anesthesia Specialists.  He makes dozens of Continuing Education presentations on veterinary anesthesia and oncology across the United States and in Canada.  Ken retired from the Veterinary Teaching Hospital at Colorado State University in 2008

 

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