Thermoregulation

Animals frequently become hypothermic during anesthesia because of inhalation of cold gases, exposure of body cavities to the room air, and loss of normal thermoregulatory mechanisms and behaviors. Hypothermia depresses all physiologic functions, including respiration and cardiac function, slows the metabolism of anesthetics and results in prolonged recoveries. All of these can contribute to anesthetic death. Hyperthermia is less common, but may occur because of excessive application of heat, hot surgery lights or malignant hyperthermia in genetically pre-disposed animals. To thermoregulate your patient: 

• Monitor the body temperature frequently using a thermometer during the procedure and during anesthetic recovery. While animal normals vary from species- to-species, in general, when body temperature drops below 99° F, an animal is considered hypothermic. Below 95-96° F an animal cannot regain normal body temperature without supplementation.

• Prevent heat loss by insulating cold surfaces with a blanket

• Prevent heat loss during gas anesthesia by utilizing low flow techniques that conserve heat

• Supplement heat with a thermal blanket (keep blanket temperature below 40 C to prevent burns!) or with pre-warmed fluids

• Treat hyperthermia by administering intravenous fluids or applying water to foot pads or exposed skin. Only use an ice bath as a last resort, as it may cause cardiovascular shock.

 
Water blanket and heater

Monitoring Anesthesia

The depth of anesthesia must be monitored carefully. Animals that are too light will experience pain and may move during the procedure. Animals that are too deep run the risk of experiencing cardiopulmonary arrest. If an animal is too light the anesthesia should be supplemented, if too deep, animals on gas anesthesia can be turned down. Animals given injectable anesthetics can not be lightened directly. Instead respiratory and cardiovascular support must be administered until the anesthetic is metabolized and the animal begins to lighten on its own. 

To monitor the depth of anesthesia, perform the following: 

• Reflexes- these reflexes disappear as the animal becomes deeper in the following order: 

• Palpebral reflex- touching the eyelids causes blinking. The animal is light if it is blinking.

• Toe pinch reflex- pinching the toe or foot web will cause a pain response. If the animal withdraws the toe it is not deep enough. If it doesn't, it is not sensing pain.

• Corneal reflex- touching the cornea of the eye with a tuft of cotton results in a

• blink. Once the animal has lost its corneal reflex, it is too deep.

• Muscle tone increases as the depth of anesthesia decreases, unless the animal is receiving a cataleptic drug like ketamine in the absence of a sedative. Test muscle tone by pulling on the lower jaw or a limb. Rigid tone indicates inadequate depth of anesthesia.

• Monitor cardiopulmonary function and body temperature- As an animal becomes too deeply anesthetized, respiration and cardiac output decrease, resulting in poor blood oxygenation and tissue perfusion and decreased blood pressure and temperature. Likewise, elevations in heart rate and blood pressure may be indications that an animal may be feeling pain and is anesthetized too lightly. Monitor as previously described.

Recovery

Monotoring and support must continue until the animal is completely recovered from anesthesia. Complete recovery means the animal is able to hold itself in a normal upright position, has returned to normal body temperature and all physiological indices are within normal limits. Anesthetic recovery can be rapid for gas agents and short anesthetic episodes. Recovery time can be prolonged if animals were under for a long time or if injectable agents were used.