Evaluation of a newly developed monitor of deep body temperature

In 1971, Fox and Solman [1] first described the use of an electronic servocontrolled system to achieve almost complete thermal insulation. The temperature measuring probe has two thermistors separated by a thermal insulator, with an electrical heating element mounted at the rear of the probe. The temperatures on the two sides of the insulating layer, as detected by the thermistors, are compared by a differential amplifier. The error signal is used to control the heater current in such a way as to achieve a situation in which no temperature gradient arises across the insulating layer and thus no heat flows out through this layer. This technique is called the “zero-heat-flow” method. As long as a zero-heat-flow condition is maintained, the probe is equivalent to an ideal thermal insulator; i.e., heat loss from the skin surface beneath the probe is prevented and, after a sufficient time, the skin surface temperature will equilibrate with the deep tissue temperature. Monitoring of deep body temperature, especially from the forehead, is now often used in cardiac surgery in Japan. Deep temperature monitoring has also been used in intensive care units [2] and for monitoring of circulatory failure [3]. Although the monitoring of deep body temperature is noninvasive, clinical application of the monitor is limited by the slow initial response time and the slow response time for rapid internal temperature changes. The initial response time of a deep temperature thermometer, extending from the time when the probe is placed on the body surface to the time when the measured temperature become stable, seems to be long. The