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TECHNOLOGY, COMPUTING, AND SIMULATION

The Usefulness of an Earphone-Type Infrared Tympanic Thermometer for Intraoperative Core Temperature Monitoring

From the Department of Anesthesiology, Sapporo Medical University, School of Medicine, Sapporo, Hokkaido, Japan.

Address correspondence and reprint requests to Michiaki Yamakage, MD, PhD, Department of Anesthesiology, Sapporo Medical University School of Medicine, South 1, West 16, Chuo-ku, Sapporo, Hokkaido 060-8543, Japan. Address e-mail to yamakage{at}sapmed.ac.jp.

	Abstract

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BACKGROUND: In this study we sought to determine the usefulness of a novel earphone-type infrared tympanic thermometer (IRT) for core temperature monitoring during surgery.

METHODS: Two groups of patients were studied under different surgical conditions. The first group consisted of 18 adult patients (ASA I or II) who had been scheduled for elective surgery under general anesthesia. Before induction of general anesthesia, an earphone-type IRT was inserted into either the left or right ear canal. Tympanic temperature was monitored and recorded along with both rectal and esophageal temperatures during anesthesia. The second group consisted of eight adult patients (ASA II or III) who had been scheduled for cardiac surgery with cardiopulmonary bypass. Similar to the first group, tympanic temperature was measured by the earphone-type IRT and recorded along with the rectal and esophageal temperatures during cooling and rewarming phases of cardiopulmonary bypass.

RESULTS: Study 1—The average temperature (±2 sd) measured with the IRT was +0.08°C (±0.34°C) above the esophageal temperature, and that with the rectal temperature was +0.11°C (±0.55°C) above the esophageal temperature. Study 2—The average temperature (±2 sd) measured with the IRT was +0.72°C (±2.2°C) above the esophageal temperature during cooling and warming phases during cardiac surgery with cardiopulmonary bypass.

CONCLUSIONS: The earphone-type IRT might be used in a clinical setting for reliable and continuous core temperature monitoring during an operation.

	Introduction

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It is important to measure and actively control body temperature during surgery to avoid the complications of perioperative hypothermia (1–4), such as morbid cardiac events (5) and surgical wound infections (6). Core temperature during general anesthesia is usually monitored in the esophagus, rectum, or bladder. However, temperature measurement at these locations is invasive and nonhygienic (7). Furthermore, rectal and bladder temperatures may not reflect the values and changes of central body core temperature (8–10). Particularly during lower abdominal surgery, rectal and bladder temperatures exhibit much lower values than esophageal or tympanic temperatures because they are influenced by operating room temperature and surgical procedures (11).

Because tympanic temperature directly reflects the core temperature of the carotid artery (12), we have developed an earphone-type infrared tympanic thermometer (IRT) that can continuously measure the temperature of the tympanic membrane (Fig. 1). In the current study, we evaluated the usefulness of the IRT for core temperature monitoring during operations with and without cardiopulmonary bypass (CPB).

View larger version (46K): [in this window] [in a new window]   Figure 1. Appearance of the originally developed earphone-type infrared tympanic thermometer. An earphone-type probe of the infrared tympanic thermometer (A) is inserted into either the left or right ear canal (B). The tip of the probe does not come into direct contact with the tympanic membrane.

	METHODS

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This open single-center trial was approved by the IRB of Sapporo Medical University Hospital (Sapporo, Japan), and written informed consent was obtained from each patient. The patients who had esophageal, anal, and/or external auditory canal diseases were excluded from the study.

Study 1: Nonabdominal and Noncardiac Surgery
Eighteen adult patients (ASA I or II, between 18 and 75 yr) who had been scheduled for elective nonabdominal and noncardiac surgery under general anesthesia were enrolled in this study. No premedication was given. Before induction of anesthesia with the patients in the supine position, the probe of the earphone-type IRT was inserted into either the left or right ear canal to measure the temperature of the tympanic membrane (T_Tym). General anesthesia was induced using IV propofol (1.0–1.5 mg/kg) and fentanyl (1.0–2.0 µg/kg), and intubation of the trachea was performed with 0.15 mg/kg vecuronium. Anesthesia was maintained with 1.0%–2.0% sevoflurane in oxygen with intermittent administration of fentanyl (50–100 µg) as an adjuvant analgesic. After induction of general anesthesia, thermistor probes were inserted into the rectum (approximately 8 cm) and esophagus (approximately 30 cm) for measurements of rectal (T_Rec) and esophageal (T_Eso) temperatures, respectively, using a thermometer (CTM-210^TM; Terumo, Tokyo, Japan). These measured temperatures were monitored and recorded automatically to a personal laptop computer at 1-min intervals. The temperatures of operating rooms were kept at 22°C–24°C, and a forced-air warming system (Bair Hugger^TM; Arizant Healthcare, Eden Prairie, MN) was used to warm the patient.

Study 2: Cardiac Surgery with CPB
Eight adult patients (ASA II or III, between 40 and 80 yr) who had been scheduled for elective cardiac surgery with CPB were enrolled in this study. No premedication was given. Similar to Study 1, the probe of an earphone-type IRT was inserted into either the left or right ear canal before induction of anesthesia. General anesthesia was induced by IV midazolam (0.05–0.15 mg/kg) and fentanyl (5–10 µg/kg), vecuronium (0.15 mg/kg) was given and intubation of the trachea was performed. Anesthesia was maintained with 1.0%–2.0% sevoflurane in oxygen with intermittent administration of fentanyl (50–100–200 µg) as an adjuvant analgesic. After induction of general anesthesia, thermistor probes were inserted into the rectum (approximately 8 cm) and esophagus (approximately 30 cm) for measurements of T_Res and T_Eso, respectively. These measured temperatures were monitored and recorded at 1-min intervals. The temperatures of operating rooms were kept at 19°C–21°C, and a conductive warming/ cooling system (Medi-therm III^TM; Orchard Park, NY) was used to control the patient's body temperature.

Statistical Analysis
The T_Tym measured by the earphone-type IRT was evaluated in comparison with T_Eso as a body core temperature (7,13). Bland-Altman plots were used to evaluate the limits of agreement between T_Tym and T_Eso. The mean value of the difference (=bias) >0.4°C, and 2 sd (repeatability or precision) >±1.0°C was considered clinically significant (12).

	RESULTS

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We enrolled 18 patients (aged 18 to 67 yr [mean 46.2 yr]) in Study 1, and eight patients (aged 56 to 78 yr [mean 68.3 yr]) in Study 2. The median durations of the operations were 186 min (range, 50–650 min) in Study 1, and 342 min (range, 250–654 min) in Study 2. The measured ambient temperature ranged from 19.0°C to 24.2°C in both studies. After deletion of obvious artifacts caused by bipolar high frequency coagulation, we obtained 2610 measurements in Study 1 and 521 measurements in Study 2 with each of the three devices. There were no complications related to the site of insertion of the probe in the ear canal.

Study 1: Nonabdominal and Noncardiac Surgery
Figure 2 shows the Bland-Altman plot comparing T_Tym and T_Eso, and T_Rec and T_Eso, in nonabdominal and noncardiac surgery, respectively. The average temperature measured with the IRT was +0.08°C above the T_Eso with ±0.34°C 2 sd, and that with the T_Rec was +0.11°C above the T_Eso with ±0.55°C 2 sd. The bias and repeatability between T_Tym and T_Eso was <0.1°C and <0.5°C, respectively.

View larger version (10K): [in this window] [in a new window]   Figure 2. Bland-Altman analysis between tympanic (TTym) and esophageal temperatures (TEso) (A) and between rectal (TRec) and esophageal temperatures (TEso) (B) in nonabdominal and noncardiac surgery. The average temperature measured with the infrared infrared tympanic thermometer (IRT) was + 0.08°C above the TEso with ± 0.34°C 2 sd, and that with the TRec was + 0.11°C above the TEso with ± 0.55°C 2 sd. The bias and repeatability between TTym and TEso was <0.1°C and <0.5°C, respectively.

Study 2: Cardiac Surgery with CPB
The Bland-Altman plot of the temperature measurements during cooling and rewarming phases with CPB is shown in Figure 3. The average T_Tym was +0.72°C above the T_Eso with ±2.2°C 2 sd, and that with the T_Rec was +0.43°C above the T_Eso with ±3.4°C 2 sd.

View larger version (11K): [in this window] [in a new window]   Figure 3. Bland-Altman analysis between tympanic (TTym) and esophageal temperatures (TEso) (A) and between rectal (TRec) and esophageal temperatures (TEso) (B) in cardiac surgery with cardiopulmonary bypass. The average TTym was + 0.72°C above the TEso with ± 2.2°C 2 sd, and that with the TRec was + 0.43°C above the TEso with ± 3.4°C 2 sd.

	DISCUSSION

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Measurement of T_Rec or bladder temperature is widely used for monitoring body core temperature during surgery. Although measurements of those temperatures have been reported to be reliable for core temperature monitoring (14), it is difficult to follow sudden core temperature changes by such measurements (15) and, therefore, these measurements are not reliable in the case of malignant hyperthermia or sudden decline in body temperature. Rectal or bladder measurements are even less reliable during lower abdominal surgery because they are influenced by operating room temperature (11). Moreover, although the probe for T_Rec monitoring is equipped with a disposable cover, there are problems regarding hygiene and obtaining informed consent for use from the patient. Measurement of bladder temperature is a convenient method for surgery in which a urethral catheter is used, but this method cannot be used for a short operation or during sedation, in which a bladder catheter is not needed.

T_Eso measurement and contact-type T_Tym measurement have high degrees of reliability for monitoring core temperature (7,13). However, because of its invasive nature, T_Eso measurement for core temperature monitoring can only be used for operations under general anesthesia, and the probe is difficult to insert in patients undergoing regional anesthesia (16) and general anesthesia via a laryngeal mask airway. Insertion of the probe may also cause damage to the pharynx or esophagus. Measurement of T_Tym has been established as a useful method for core temperature monitoring because the blood to the tympanic membrane is supplied directly by the carotid artery (7,13). However, a contact-type T_Tym monitoring device can cause pain in the tympanic membrane when inserted in an awake state, and there have been reports of tympanic membrane damage caused by insertion of the device during unconsciousness (17,18). Several infrared noncontact-type tympanic thermometers have been developed (12,19,20). However, these devices cannot be used for continuous temperature monitoring, and there is a problem in discrepancy of measured values (19,20).

To resolve these problems, we have developed an earphone-type iIRT-monitoring device, which is a noncontact-type device that enables continuous temperature monitoring. This newly developed device has two notable features. First, we succeeded in manufacturing a prototype earphone-type temperature sensor with a miniaturized infrared ray measurement component (Fig. 1). Moreover, by using an appropriate algorithm, we succeeded in continuous measurement of infrared rays from the tympanic membrane at 1-s intervals with no drift in measured values (± 0.1°C, 32°C–34°C). The results of Bland-Altman analysis in this study showed very small bias (systemic error) <0.1°C and the repeatability (precision) <0.5°C between T_Tym and T_Eso, meaning that the temperature measurements by using this device are reliable for core temperature monitoring. The average T_Tym was rather high (+0.72°C) above the T_Eso during cardiac surgery with CPB. Furthermore, the bias and repeatability seems to be larger than those reported by Bock et al. (12) This might have been for the following two reasons: 1) number of data from the cooling phase was more than that from the warming phase, resulting in increase of bias, and 2) this device can measure the external ear canal not the direct tympanic membrane in some patients.

Because the device is noninvasive, it can also be used for sedation (21) and regional anesthesia (16), and the probe of the thermometer can be inserted by patients themselves. Another advantage of this thermometer is that it is hygienic because it does not come into contact with the patient's infectious body fluids. A disposable cover is therefore not needed for the device, and the device can be used repeatedly, after only wiping the sensor part with an ethanol cotton swab until sensor membrane damage occurs.

Future issues regarding the newly developed thermometer include generally applicability and problems with measured values just after insertion. First, there is a problem in the material used in the device. Because of the differences in ear canal shape, further consideration must be given to the material used in the device for general applicability. The use of a material with a high degree of plasticity, such as urethane or silicone, or the manufacture of devices with various sizes is expected. There is also a problem in measurement just after insertion of the probe. It took a few minutes (2 to 3 min) after probe insertion to obtain constant values in some of the patients in this study. This may have been because of the influence of the tympanic membrane on the closed external auditory canal, which is where the sensor part of the device is located. However, T_Tym obtained by using the device showed a good correlation with measurements of T_Eso even in these patients after the measured T_Tym had become constant. Consideration will be given to modification of the device to resolve this problem. The device also needs to be evaluated in febrile patients.

In summary, Bland-Altman analysis of temperature data obtained from patients during general anesthesia revealed that temperature measurements obtained by using an earphone-type IRT could be reliable for core temperature monitoring. Tympanic temperature could follow changes in core temperature well, because rather small standard deviation between T_Tym and T_Eso were obtained in the cooling and rewarming phases of cardiac surgery with CPB. Infrared T_Tym membrane measurement is noninvasive and hygienic and, based upon the patients we studied, is suitable for continuous measurement during anesthesia care.

	Footnotes

 
Accepted for publication August 27, 2007.

Supported in part by a grant-in-aid (2005) for clinical research from Sapporo Medical University for the Promotion of Science, Sapporo, Japan.

This paper was presented in part at the annual meeting of the American Society of Anesthesiologists, Chicago, IL, USA, October 14–18, 2006.

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