Perianesthetic Hypothermia: Why Do We Care?

Hypothermia is the most common perianesthetic complication. Learn from DoveLewis Surgery Technician James Reid, CVT, VTS (Anesthesia & Analgesia) about how to prevent hypothermia in surgical patients.

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Hypothermia occurs when there is a decrease in core temperature, causing the patients temperature to fall below the normal range. The normal range varies, but in general, it should be about 100°F to 102.5°F for dogs and cats, and can be determined by a temperature reading from an oral or rectal thermometer. Hypothermia can also affect specific areas, such as extremities, and be described as local hypothermia.

The most common cause of hypothermia during the perioperative period is the lack of prevention. What else can contribute to hypothermia?

  • The ability to thermoregulate can be depressed as soon as a patient is sedated.
  • Pediatric, neonates, geriatric, and exotic pets generally have a large body surface area to body mass ratio leaving them susceptible to hypothermia.
  • Clipping fur, using copious amounts of scrub and alcohol will lead to an evaporative cooling effect.
  • Prepping patients for surgery on cold tables or floors.
  • Once the patient is anesthetized muscles relax and they also lose the ability to shiver.
  • Vasodilation from pre-operative medications and anesthetic agents is known to cause heat loss.
  • Peripheral vasoconstriction from certain medications can cause a local hypothermia in the extremities.
  • Metabolism during anesthesia slows to about half that of an awake patient.
  • Open body cavities lead to a large surface area where heat loss can occur.
  • Room temperature intravenous fluids.
  • High oxygen flow rates along with non-rebreathing anesthetic circuits.

Patients that are immobile or rehabilitating are good candidates for Bair Hugger warming methods.

 

Most awake and healthy patients can tolerate a drop of a couple degrees. This is not true for the debilitated, sedated, and anesthetized patients.

 

What are some of the effects of hypothermia?

  • Hypothermia slows the metabolism of some preanesthetic drugs and anesthetic agents directly leading to a longer recovery.
  • Hypothermic patients require less gas anesthetic which can easily lead to an overdose.
  • Atrial arrhythmias, coagulopathies, bradycardia, decrease in cardiac output leading to hypotension can be seen as hypothermia progresses.
  • Patients can become unresponsive to anticholinergics.
  • Decreased tissue perfusion leading to delayed wound healing and potential for infection.
  • Shivering post-operatively greatly increases the patient’s oxygen consumption and metabolic demands.
  • Take note that some patients experiencing hypothermia during the recovery period may not have regained the ability to shiver.

 

It is important to be proactive in preventing heat loss. What are some of the treatments and preventative options for hypothermia?

  • Socks and bubble wrap on the feet.
  • Heat loss is generally at its worse during the first 20 minutes of anesthesia
  • Turn down your oxygen flow rates. Use a calculation specific to your anesthetic circuit to determine the ideal oxygen flow rate.
  • Use active heating sources like Bair Huggers, HotDog Blankets, or waterbed heaters.
  • Warmed intravenous fluids.
  • Intravenous fluid line warmers.
  • “Warmies” such as heated fluid bags are not recommended. They have the potential to cause burns and once they cool off, they will pull heat from your patient.
  • Hypothermic patients that are laterally recumbent during recovery, whether they are shivering or not, should have oxygen support.

Hot Dog warmers can warm patients, from above and below, making them an ideal option.

 

Water circulating heat pads are a quiet option to put in kennels or under patient bedding.

 

References

Duke-Novakovski, Tanya, et al. BSAVA Manual of Canine and Feline Anesthesia and Analgesia, 3rd Edition. John Wiley & Sons, 2016.

Grimm, Kurt A., et al. Veterinary Anesthesia and Analgesia: the Fifth Edition of Lumb and Jones. Wiley Blackwell, 2015.

Creedon, Jamie M. Burkitt., and Harold Davis. Advanced Monitoring and Procedures for Small Animal Emergency and Critical Care. Wiley- Blackwell, 2012.

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