Temperature Monitoring - Perioperative Period

An MHAUS Online Brochure
Last Updated: 2/18/2004

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WHY MONITOR TEMPERATURE?

Normal body temperature is near 37°C. Core temperatures just a few tenths of a degree above normal provoke sweating and active vasodilation. Core temperatures just a few tenths of a degree below normal trigger vasoconstriction.

Typical clinical doses of all general anesthetics reduce the threshold for vasoconstriction to 33-35°C. Similarly, general anesthetics increase the thresholds for sweating and active vasodilation by about 1°C.

Anesthesia thus profoundly alters the thermoregulatory system, markedly reducing cold-response thresholds while slightly increasing warm-response thresholds. The result is an approximately 4°C range of core temperatures not triggering thermoregulatory defenses.

DETECTION OF MALIGNANT HYPERTHERMIA

Core temperature monitoring is appropriate during general anesthetics to facilitate detection of malignant hyperthermia (MH) and to quantify hyperthermia and hypothermia. Although MH may be detected by tachycardia and an increase in end-tidal CO2 out of proportion to minute ventilation, core temperature helps confirm the diagnosis of MH.

THERMAL DISTURBANCES

Intraoperative hyperthermia can also be caused by excessive warming, infection, fever, blood in the fourth cerebral ventricle, and mismatched blood transfusions.

Inadvertent hypothermia is the most common perioperative thermal disturbance. Core temperature usually decreases 0.5-1.5°C in the first hour of anesthesia.

Hypothermia results from internal redistribution of heat and a variety of other factors, whose importance is hard to predict in individual patients. Mild hypothermia is associated with adverse outcomes including myocardial ischemia, surgical wound infections, and coagulopathy.

WHEN TO MONITOR TEMPERATURE

Temperature measurements are not usually required during brief general anesthetics because redistribution makes core temperature perturbations during the first 20-30 minutes of anesthesia difficult to interpret. Core temperature should be monitored in patients given general anesthetics exceeding 30 minutes in duration.

MHAUS advocates intraoperative temperature monitoring
during all but the briefest general anesthetics.

REGIONAL ANESTHESIA

Core hypothermia is nearly as common and severe during epidural and spinal anesthesia as during general anesthesia. During regional anesthesia, core temperature should therefore be measured in patients likely to become hypothermic, including those undergoing body-cavity surgery or long, extensive procedures. In contrast, temperature monitoring is not usually required in otherwise healthy patients undergoing minor procedures under regional anesthesia because local anesthetics do not trigger MH.

TEMPERATURE MONITORING SITES

Core

Pulmonary Artery
Distal Esophogus
Tympanic Membrane
Nasopharynx

Intermediate

Mouth
Axilla
Bladder
Rectum
Forehead Skin Surface

OTHER REASONS TO MONITOR TEMPERATURE

Body temperature should be monitored frequently in surgical patients given blood products and in those with overt infections, pre-existing fever, or autonomic instability.

HOW TO MONITOR TEMPERATURE

Most electronic thermometers are thermistors and thermo-couples. Both devices are accurate enough for clinical use and sufficiently inexpensive to be disposable. Most infrared aural canal (so-called "tympanic") thermometers are insufficiently accurate and should not be used clinically.

A thermometer incorporated into an esophageal stethoscope is usually the easiest and most reliable method of measuring core temperature in intubated patients. During regional anesthesia or when patients are mask-ventilated, measurement of axillary, oral, forehead skin surface, or tympanic membrane temperatures at 15-minute intervals can be substituted for esophageal temperature monitoring.

WHERE TO MONITOR TEMPERATURE: FOUR TRUE CORE SITES

The core thermal compartment is composed of highly perfused tissues having a uniform temperature that is high compared with the rest of the body.

Core temperature can be determined by measuring a single temperature adjacent to the tympanic membrane (using a special probe), in the nasopharynx, pulmonary artery, or distal esophagus. These temperature monitoring sites remain reliable, even during rapid thermal perturbations, including cardiopulmonary bypass. However, temperature probes incorporated into esophageal stethoscopes must be positioned at the point of maximal heart sounds, or even more distally, to provide accurate readings.

FIVE INTERMEDIATE SITES

Core temperature can be estimated with reasonable accuracy using oral, axillary, rectal, bladder, or forehead skin surface temperatures, except during extreme thermal perturbations. Rectal temperature is considered an intermediate temperature in deliberately cooled patients and may not prove reliable during MH episodes.

During cardiac surgery, bladder temperature is similar or close to rectal temperature (and therefore intermediate) when urine flow is low, but similar or close to pulmonary artery temperature (and thus core) when flow is high. Since bladder temperature is strongly influenced by urine flow, it may be difficult to interpret in these patients.

SKIN SURFACE TEMPERATURES

Forehead skin temperature tracks core temperature accurately during general anesthesia. Hence forehead skin temperature can be used under clinical conditions to assess core temperature. However, forehead skin temperature is generally 1-2°C less than core temperature.

Therefore, some commercially available skin temperature measurement devices incorpo-rate a 2°C offset in their display. Although forehead skin surface temperature does follow core temperature reliably in such situations as rewarming following cardiopulmonary bypass, data regarding forehead skin temperature trends during human MH episodes is lacking. (In swine, forehead and rectal temperatures fail to track core temperature during MH crises.) Hence, if marked deviations of skin temperature are detected, it is advisable to seek confirmation by measurement of core temperature.

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This brochure was written and produced by the Malignant Hyperthermia Association of the United States (MHAUS). MHAUS, a non-profit organization founded in 1981 by a small group of lay people with a personal interest in MH, is dedicated to reducing the morbidity and mortality of MH. It offers a variety of services and educational materials to medical professionals and to MH-susceptible individuals and their families. For membership, tax-deductible contributions, programs and educational materials, please contact MHAUS at the address provided in this brochure.

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