How Low can You Go? The Effects of Hypothermia on Anesthetized Patients

By:  Tasha McNerney BS,CVT

As technicians monitoring patients under general anesthesia, hypothermia is a common problem encountered in almost every surgical patient. Hypothermia occurs when the temperature drops below 97 degrees F (36 degrees C). So, why does body temperature decrease under anesthesia? There are many reasons including drugs that cause vasodilation (such as acepromazine, propofol, and inhalants) that allow for heat loss at the extremities and limbs, open body cavities during surgical procedures, cold (i.e.: room temperature) intravenous fluids, and cold environment in the operating room. It is also important to remember that small patients lose heat faster because the body surface area is proportionately greater than the surface area of larger patients.[i] Also, patients on non-rebreathing systems or circle systems with high flows of oxygen (which is cold and dry) will expend more energy warming and humidifying this gas.

All of these factors lead to specific mechanisms that lead to hypothermia. There are three phases of heat loss:

1. An initial decrease in temperature due to vasodilation

2. A progressive linear decline in temperature

3. A vasoconstrictive response occurs.

After the initial decline due to vasodilation, temperature continues to fall until vasoconstriction reduces blood flow, and therefore heat distribution, to the periphery.[ii]

The morbidity associated with these stress responses most typically occurs during the postoperative period. Hypothermia impairs platelet function, decreases activity of the coagulation pathways, and increases fibrinolysis. Post-anesthetic shivering results in increased oxygen consumption during recovery (when most animals are no longer on 100% O2). A low temperature must be vigilantly watched in the post-operative period as hypothermia contributes to decreased ability to process anesthetic drugs, and therefore prolonged recovery from anesthesia.

The ACVA makes the following recommendation: “Temperature should be measured periodically during anesthesia and recovery and if possible within a few hours after return to the wards.”[iii] Body temperature should be monitored at least every 15 minutes during anesthesia, using a thermometer. A thermometer probe in the rectum or esophagus allows for constant temperature measurement. Accidental placement in the stomach or mouth can reduce the accuracy of a core body temperature estimate. Rectal thermometers can also be used; however digital thermometers are preferred as mercury thermometers are fragile and if broken cleanup of mercury is hazardous.

 Now that we understand perioperative and postoperative hypothermia is a serious complication, how do we prevent and treat it? There are many resources for preventing and treating hypothermia during the intra operative period. One of the best known products is the Bair Hugger®. The Bair Hugger® warms patients by blowing warm air through porous blankets to surround the patient in warm air. It also has a feature that allows you to use the warm air to actively warm your IV fluids. Blankets for the Bair Hugger® are available in many sizes and can be used under the patient or over top of patient to warm.

The HotDog® warming blanket uses a reusable, easy to clean, heavyweight nylon “blanket” that is flexible and puncture resistant. It can be used over, under, or wrapped around the patient. The flexible blankets feature electrically conductive fabric for even heat distribution.  An advantage of the HotDog® over the Bair Hugger® is that there is no blowing air on the surgical site, and it won’t dry out EKG leads.

Many practices choose to warm their IV fluids. Warmed IV fluids or irrigation fluids must be used with caution. If fluids are warmed in the microwave, temperatures can become damaging to tissues. A device can be purchased to provide a more controlled temperature for IV fluids.  One such device is the I-Warm IV Fluid Warmer using dry heat exchange, the I-Warm quickly brings the temperature level of the fluid up to that of the patient.

Another very cost effective way to keep patients from suffering more heat loss is to use bubble wrap as a blanket over the patient. Heat from the patient’s body is trapped in the “bubbles” of air. Bubble wrap may also be wrapped around the extremities to prevent further heat loss at these sites. (See featured photo of patient recovering with Hot Dog warmer and bubble wrap “booties”)

During recovery patients that suffer from hypothermia will also have a prolonged recovery period because patients will have slower drug clearance. All attempts should be made to return the patient to normal temperature. Take rectal temperature until patient is normothermic. Temperatures can be obtained axially or aurally if peri-rectal surgery had been performed. Normal axial and aural temperature is 100.5 +/- 1°F.

In summary, intra-operative hypothermia must be prevented and aggressively treated when it develops. There are many options on the market available to help maintain a patient’s body temperature while under anesthesia.

[1] Bassert, J.M & McCurnin, D.M. 2010, McCurnin’s Clinical Textbook for Veterinary Technicians (7^th Edition), Saunders. St. Louis, MO

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