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Chapter 22: Monitoring Temperature

INTRODUCTION

It is critical to monitor the effects of anesthetic drugs on core and surface temperatures in an attempt to detect and prevent hypothermia, hyperthermia, and malignant hyperthermia.

TYPES OF THERMOMETERS

The most common thermometers used are thermocouples and thermistors. These electrical systems are efficient, accurate, inexpensive, and disposable. The infrared system used to measure temperature from the tympanic membrane or forehead is largely inaccurate and should not be used.

General Anesthesia

Core temperature monitoring is mandatory for patients undergoing more than 30 minutes of general anesthesia. This monitoring is essential in detecting hypothermia, hyperthermia, and, less commonly, malignant hyperthermia. Although increased temperature is usually not the initial diagnostic sign, a rising core temperature may signify malignant hyperthermia. Early signs of malignant hyperthermia are tachycardia and increasing end-tidal carbon dioxide.

General hyperthermia can be caused by fever secondary to infection, inaccurately matching blood products, excessive warming, and presence of blood in the fourth ventricle.

Hypothermia is the most common thermal disturbance and can cause myocardial events like arrhythmias and decreased contractility, wound infections, increased blood loss, and prolonged hospitalization. Intentional hypothermia, however, can be protective against ischemia. It should be noted that 30 minutes after induction, core body temperature decreases by 0.5°C–1.5°C. In most surgical cases, unless hypothermia is indicated, it is important to keep core body temperature greater than 36°C.

Local Anesthesia

Local anesthesia, used for sedation and regional blocks, can frequently cause hypothermia. Local anesthesia does not cause malignant hyperthermia.

TEMPERATURE MONITORING SITES

Measuring the temperature from core thermal sites is necessary, as these regions are well perfused and uniform in temperature. Table 22-1 discusses temperature monitoring sites.

TABLE 22-1Clinical Considerations for Temperature Monitoring Sites
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TABLE 22-1 Clinical Considerations for Temperature Monitoring Sites
Body Site Temperature Accuracy Clinical Correlation
Distal esophagus, tympanic membrane, nasopharynx, and pulmonary artery Highly accurate core temperature site. Good for surgical cases with rapid and frequent temperature changes (ie, cardiopulmonary bypass).
Skin surface Temperature collected is lower than core temperature but can reflect core temperature when adjusted. Fails to confirm malignant hyperthermia.
Oral, axillary Reasonably accurate. Limited when there is extreme thermal disturbance.
Rectal Moderately accurate, “intermediate temperature,” temperature lags behind that measured in core thermal sites. Lags in cooling patients. Fails to rise in malignant hyperthermia.
Bladder Accuracy dependent on urine flow. Low urine output (ie, cardiac surgery) is equal to rectal temperatures. High urine output allows bladder temperatures to equal that of core sites.

SUGGESTED READING

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Insler  SR, Sessler  DI. Perioperative thermoregulation and temperature monitoring. Anesthesiol Clin 2006;24:823–837.  [PubMed: 17342966]

Acknowledgments

Nima Adimi, MD and Christopher ...

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