Body Temperature Monitoring

The term body temperature usually means the body core tem­perature, which is the temperature of the central part of the body. Many different techniques have been used for monitor­ing body temperature (22). Although different body parts have different temperatures, such differences are small when the temperature is stable, so that body temperature can be monitored fairly well at many measurement sites. Body tem­perature is usually measured by a clinical thermometer at the oral cavity. Recently, the tympanic thermometer is also used. For continuous monitoring, it is measured at the rectum, esophagus, bladder, auditory canal, tympanum, nasal cavity, or digestive tract. However, when body temperature varies, significant differences in observed temperatures may occur between sites. Thus when rapid changes of body temperature have to be monitored the measurement site used is important.

Rectal temperature has been used widely in patient moni­toring because the rectum is a convenient site into which a thermometer probe can be inserted far enough to protect it from heat loss. Rectal temperature is always higher than oral temperature as well as temperatures of other sites, and has been considered to be a reliable indicator of body core temper­ature. However, when body temperature varies, changes in rectal temperature are delayed comparable to those of other, more central parts of the body, and thus rectal temperature cannot be accurate enough for monitoring in such conditions.

The esophagus has been used most frequently as a site for body temperature monitoring during anesthesia. Esophageal temperature is measured by inserting a probe through the mouth or nose so that the sensor tip is positioned at near­heart level. Under stable conditions, esophageal temperature is intermediate between oral and rectal temperature, and fol­lows internal temperature changes rapidly.

Bladder temperature can be monitored using a thermistor – tipped bladder catheter as shown in Fig. 7(a). Although blad­der temperature is close to rectal temperature in stable condi­tions, it follows internal temperature changes rapidly. Blad­der temperature is recommended as a measurement site for

Body temperature can also be monitored across the skin using the zero-heat-flow method as shown in Fig. 7(b) (23). The probe that is used in this method has two thermistors to detect heat flow across the probe. It also has a heater, and the heater current is controlled so that the temperatures of two thermistors are equal, which means that we can compen­sate for the heat flow from the skin to the outer air. Under such conditions, the probe can be regarded as an ideal ther­mal insulator. When the skin surface is insulated, the tem­perature gradient in the tissue near the surface will vanish, and finally the temperature of the surface of the skin will reach that of the deep tissue. A commercial model (Coretemp, Terumo Co., Tokyo) has now been developed for which disc­shaped probes of different sizes, from 15 mm to 80 mm in diameter, are available. By applying a probe to the forehead, chest, or abdomen, body temperature can be monitored con­tinuously for several days in intensive care units (24). Simul­taneous monitoring of body core and peripheral temperatures by applying probes to the forehead and to the sole of a pa­tient’s foot, temperature differences between the body core and the limbs can be observed which can be a useful index of peripheral circulation (25).

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