Perioperative temperature monitoring is an essential factor as with other vital signs—blood pressure, heart rate, and percutaneous oxygen saturation. For accurate temperature measurements, we must select the kind of thermometers, insertion, or attachment regions [18, 19]. Perioperative hypothermia causes a higher risk of SSI , massive hemorrhage , and postoperative shivering [8, 9]. For prevention of evitable hypothermia during surgery, we must accurately monitor core body temperature and effectively perform body warming.
Usefulness of zero-heat-flux thermometer has been already reported in perioperative temperature monitoring. Fox and Solman first reported a principle of an electronic servo-controlled system to achieve almost complete thermal insulation . Zero-heat-flux thermometer sensor contains two thermistors, separated by a thermal insulator and covered by an electric heater. BHTMS, one of the zero-heat-flux thermometers, also has some advantages to use in its accuracy, ease of use, and disposable sensor compared with former types. Eshraghi showed that the overall difference between the temperature obtained from BHTMS and that of the pulmonary artery was − 0.23 °C (95% LOA of ± 0.82 °C) . However, this thermometer is unsuitable for use in certain clinical situations. In patients undergoing craniotomy, it is impossible to attach the BHTMS sensor to the patients’ forehead. During surgery of the neck and face, as well, surgeons do not permit the BHTMS sensor to be attached due to the proximity of the forehead to the surgical site. Use of a BIS monitor sensor and/or INVOS™ sensor on the patients’ forehead is common during cardiovascular surgery, which interferes with the attachment of the BHTMS sensor on the forehead. Since temperature monitoring is more significant in the surgeries described above, an alternative method to use the BHTMS for the measurement of core body temperature is required. Judging from the principle of the zero-heat-flux thermometer, we hypothesized that core body temperature would be measurable by attaching the BHTMS sensor to other areas with good vascularity. In this pilot study, we could prove a high correlation and accurate performance in the neck group. Eshraghi also presented that bias and precision values for neck site were similar to the forehead values . It was almost comparable performance between standard forehead attachment and neck attachment. Furthermore, the systematic error was absent in the neck group. There are several reasons why core temperature monitoring in the neck is appropriate. The neck is anatomically close to the heart and is a route to the cerebral blood flow; therefore, the blood temperature hardly decreases. Besides, blood vessels in the neck run 1–2 cm below the skin, which has little influence by fatty tissues, making it easier to measure core temperature accurately.
Meanwhile, the mean bias was − 0.55 °C in the chest group, which had a systematic error. LOA in this systematic error ranged from − 1.04 °C (lower coefficient limit) to − 0.06 °C (upper coefficient limit), it is necessary to consider how to interpret numerical values under real clinical use. One reason for systematic error with anterior chest application might be influenced by the effect of the rib bones and pericardial fatty tissue, and movements of the thoracic cage and aerated lung during breathing.
Our study has certain limitations. First, we monitored and analyzed core body temperature within a limited range, almost between 35.5 and 38.0 °C, in laparoscopic surgery cases. Thus, we cannot judge whether our monitoring method is proper in hyperthermia cases over 38.0 °C and hypothermia cases below 35.5 °C. Second, we collected 10 cases and data from each group, but the sample size was too small. Third, we did not measure peripheral temperature in this study, to serve as a comparison. We need further verifications to resolve these limitations; however, the knowledge obtained from this study would be one option in monitoring core body temperature.