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.
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.
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 0.5°C to 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, 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 23-1 discusses temperature monitoring sites.
TABLE 23-1Clinical Considerations for Temperature Monitoring Sites ||Download (.pdf) TABLE 23-1 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. |
DI Perioperative thermoregulation and temperature monitoring. Anesthesiol Clin 2006;24:823–837.