Hypothermia (body temperature below 95ºF) has been shown to occur in up to 80% of anesthetized cats and dogs by the end of anesthesia and surgery. Causes of this problem include small body size relative to body surface area, vasodilation and lack of shivering during anesthesia, breathing cold, dry anesthetic gases from high-flow non-rebreathing systems, and heat loss during surgery from open body cavities.
In their revised anesthesia guidelines (2011), AAHA urges us to provide thermal support and monitor body temperature throughout the procedure, including the peri-anesthetic and recovery periods. Supplemental heat may include warm IV fluids, use of a fluid line warmer, insulation on the patient’s feet (e.g., bubble wrap), circulating warm-water blankets, and/or warm air circulation systems. Do not use supplemental heat sources that are not designed specifically for anesthetized patients as they can cause severe thermal injury. The margin for temperature error that could cause thermal injury is surprisingly narrow. Placing gallon jugs of warm water against a patient, for instance, can court disaster.
Recovery from anesthesia can be prolonged in hypothermic patients, resulting in increased morbidity. So, it’s important to provide adequate thermal support until the patient’s temperature is consistently rising and approaching normal.
There are many ways to minimize patient heat loss during anesthesia. Careful planning to make the anesthetic event as short as possible is one way. For instance, clip the patient for surgery while the patient is sedated but not yet induced. An electric under blanket or circulating warm water blanket can provide support during this time also.
Investigate low-flow rebreathing circuits for your anesthetic gas machine as well. Smaller patients placed on high-flow non-rebreathing circuits waste significant body heat simply by having to warm each inspired breath. Their body temperature and fluids are lost to the scavenge system with every exhale.
Most warming devices used on anesthetized animals are unable to raise the body temperature of hypothermic dogs and cats during surgery. Surface contact alone, such as from electric heat blankets, seems unable to transfer sufficient heat to raise body temperature. And many of those devices have burned animals.
In the early 1990’s forced warm air heating was developed for warming humans, using air-filled blankets with small holes that allow warm air to flow over the patient (Bair Hugger®, Arizant). When placed over patients recovering from anesthesia they were shown to prevent further heat loss and potentially re-warm patients. However, these forced warm air systems are most effective during recovery when patients can shiver, have increased cardiac function and return of their vasomotor function. They have been shown to be only marginally effective in raising the body temperature of hypothermic humans during major abdominal surgery.
Recent innovations in the design of forced warm air heating blankets (Darvall Cocoon®, AAS) have improved their effectiveness before, during, and after anesthesia. These warm air blankets are designed for cats or dogs, to be used during surgery or for caged animals in recovery/ICU. The contact surface is porous, rather than punched with small holes, resulting in low surface air flow. When positioned around or underneath animals, their hair-coat traps the warm air. A recent study in dogs anesthetized for surgery showed that Darvall® blankets consistently increased body temperature to near normal prior to recovery.
Hypothermia during anesthesia continues to be a longstanding challenge for us to manage. But with careful attention paid to keeping each anesthesia as short as possible, and exploring innovations in low-flow breathing circuits and patient warming systems, we can minimize patient heat loss and subsequent morbidity.