Continuous measurement of tympanic temperature
with a new infrared method using an optical fiber
MANABU SHIBASAKI,1 NARIHIKO KONDO,1 HIROTAKA TOMINAGA,1 KEN AOKI,1
EIICHI HASEGAWA,2 YOSHIYUKI IDOTA,3 AND TOSHIMICHI MORIWAKI3
1Laboratory for Applied Human Physiology, Faculty of Human Development, Kobe University,
Kobe 657-8501; 2Production Engineering Laboratory, Shimadzu Company, Atsugi 243-0213;
and 3Faculty of Engineering, Kobe University, Kobe 657-8501, Japan
infrared sensor; device; core temperature; thermoregulation;
humans
CORE TEMPERATURE is one of the most important indexes
for clinical practice and thermoregulatory research in
humans. It is necessary that the gold standard for
measuring core temperature responds to rapid changes
in the core temperature. Esophageal (Tes ) and tympanic
(Tty ) temperatures have been used to estimate core
temperature in clinical practice and thermoregulatory
research. Benzinger (1, 2) proposed that Tty reflected
the core temperature because of the proximity of the
tympanic membrane to the carotid artery, where it
flows into the hypothalamus. Although some researchers agree with his opinion (6, 7, 18, 24), others do not
accept that Tty is a true measurement of core temperature because Tty is affected by a decrease in skin
temperature resulting from facial cooling or fanning (5,
14, 15, 20). In a study directly measuring brain temperature, Shiraki et al. (21) indicated that Tty did not reflect
the brain temperature during hyperthermia. On the
other hand, Mariak et al. (12) suggested that the
changes in brain temperature were better approximated by Tty than by Tes. This discrepancy may result
from the imperfections in the method used for measuring Tty. Brinnel and Cabanac (6) and Sato et al. (18)
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reported that when a probe was successfully placed on
the tympanic membrane, and the external canal was
covered with cotton or a thermal insulating material,
then Tty was not affected by ambient temperature or by
a decrease in skin temperature on the face or head.
Generally, the traditional methods for measuring Tes
and Tty require that the thermometry keeps in contact
with the esophagus or tympanic membrane, respectively. However, these methods have some risks (spread
infection, injure the esophagus or tympanic membrane,
and cause subjects discomfort or pain). Infrared tympanic thermometry has been developed to solve these
problems, as a method of measuring Tty more safely and
conveniently (19). Terndrup noted that this method has
been mentioned in many publications since the late
1980s (see Ref. 23 for review). However, the accuracy
and reliability of existing devices are not high enough
to allow their use in clinical medicine and thermoregulatory research (3, 10, 13, 17, 18, 22, 23). The reasons
for such inadequate performance in measuring Tty are
attributed to the characteristics of the infrared sensor
and its size. An infrared sensor measures the average
temperature in its ‘‘field of vision.’’ Existing devices
may measure the infrared radiation from the auditory
canal as well as the tympanic membrane. Furthermore,
existing devices are not suitable for continuous monitoring, although continuous information on the core temperature during experiments is valuable. We have
developed an infrared tympanic thermometry with an
optical fiber (optical fiber thermometry) to detect the
limited amount of infrared radiation from the tympanic
membrane. A chalcogenide glass fiber was used for the
optical fiber, and this made it possible to detect the
temperature of the tympanic membrane selectively.
In this study, we investigated whether the optical
fiber thermometry accurately registered the temperature of the tympanic membrane during different experimental maneuvers. Head cooling and facial fanning
tests were used to determine whether the temperature
registered by the optical fiber thermometry was affected by a decrease in skin temperature. The passiveheat-stress and progressive-exercise tests were used to
compare Tty measured by the optical fiber thermometry
(infrared-Tty ), Tty measured with a thermistor (contactTty ), and Tes measured by a thermocouple. We also used
the relationship between the rate of sweating and the
two core temperatures in these experiments as an
additional test of the utility of the optical fiber thermometry.
8750-7587/98 $5.00 Copyright r 1998 the American Physiological Society
921
Downloaded from http://jap.physiology.org/ by 10.220.33.4 on June 18, 2017
Shibasaki, Manabu, Narihiko Kondo, Hirotaka Tominaga, Ken Aoki, Eiichi Hasegawa, Yoshiyuki Idota, and
Toshimichi Moriwaki. Continuous measurement of tympanic temperature with a new infrared method using an
optical fiber. J. Appl. Physiol. 85(3): 921–926, 1998.—The
purpose of this study was to investigate the utility of an
infrared tympanic thermometry by using an optical fiber for
measuring tympanic temperature (Tty ). In the head cooling
and facial fanning tests during normothermia, right Tty
measured by this method (infrared-Tty ) and esophageal temperature (Tes ) were not affected by decreased temple and
forehead skin temperatures, suggesting that the infrared
sensor in this system measured the infrared radiation from
the tympanic membrane selectively. Eight male subjects took
part in passive-heat-stress and progressive-exercise tests. No
significant differences among infrared-Tty, the left Tty measured by thermistor (contact-Tty ), and Tes were observed at
rest or at the end of each experiment, and there was no
significant difference in the increase in these core temperatures from rest to the end. Furthermore, there were no
significant differences in the core temperature threshold at
the onset of sweating and slope (the relationship of sweating
rate vs. infrared-Tty and vs. contact-Tty ). These results suggest that this method makes it possible to measure Tty
accurately, continuously, and more safely.
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TYMPANIC TEMPERATURE MEASUREMENT
MATERIALS AND METHODS
Fig. 1. Schematic illustration of left external auditory canal and
infrared tympanic thermometry and optical fiber. Infrared rays from
tympanic membrane were transmitted to a pyroelectric infrared
sensor along a chalcogenide glass fiber. DC, direct current; A/D,
analog to digital.
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Subjects. Three healthy untrained male subjects participated in the head cooling and facial fanning tests, and these
three and five other male subjects took part in the passiveheat-stress and progressive-exercise tests. Their mean age,
height, weight, and body surface area were 22.8 6 0.9 (SD) yr,
1.70 6 0.05 m, 64.5 6 9.8 kg, and 1.74 6 0.14 m2, respectively.
Each subject was informed of the nature of the procedures
and the risks associated with the experiments, and each
signed an informed consent agreement when he first visited
our laboratory.
Procedures. For the first group of tests, each subject sat on
a chair in an environmental chamber (SR-3000, Nagano
Science, Osaka, Japan) maintained at an ambient temperature of 25°C and 50% relative humidity. After a 5-min
baseline period, the subject’s temple was cooled by application of an ice pack or his forehead was cooled by air blown
from an electrical fan. Cooling lasted for 15-min followed by a
10-min recovery period. The two tests were performed on
different days.
For the second group of tests, the subjects wore only shorts
and entered an environmental chamber maintained under
the same conditions. They remained seated for at least 1 h to
reach equilibrium, during which time the measuring devices
were attached. After baseline values were measured for 3
min, they then performed either the passive-heat-stress test
or the progressive-exercise test. The passive-heat-stress test
involved immersion of the subjects’ lower legs in a hot water
bath at 42°C for 60 min. The progressive-exercise test involved pedaling on a cycle ergometer (model M50, Combi) at a
frequency set to 60 rpm. The subjects cycled at 50 W for 1 min,
and then the exercise intensity was increased by 1 W every 15
s for 30 min, for a total of 31 min. One subject was exhausted
27 min after the start of exercise, so only the first 27 min of
data are used for all of the subjects. These two tests were
performed in a random order, with at least 2 days between
tests.
Measurement of Tty and Tes. In this study, two types of
tympanic thermometry were used to measure Tty. The first
was an optical fiber thermometry, consisting of an infrared
sensor and an optical fiber. The infrared sensor used was a
pyroelectric infrared detector (DLATGS, Shimadzu). This
infrared sensor can detect infrared energy independent of the
wavelength of the radiation emitted by an object (11), but the
incident infrared radiation must be interrupted at regular
intervals. Therefore, this system is equipped with a mechanical chopper, inserted between the infrared sensor and the
optical fiber (Fig. 1), which has two blades that rotate at 120
Hz.
The optical fiber used was the most suitable for detecting
human body temperature. The core temperature in humans
is normally between 36 and 40°C, and the wavelength of
infrared radiation corresponding to these temperatures is
,9.38–9.26 µm. A chalcogenide glass fiber (NTEG, NOG) is
the most suitable optical fiber for transmitting infrared
radiation at these wavelengths. This fiber has been used in
some machines: CO laser, thermal imaging, and so on. This
optical fiber transmits the infrared radiation from the tympanic membrane better than do other optical fibers (16). The
transmission loss below 1 dB/m for the infrared wave with a
wavelength of 9.35 µm is matched to ,37°C. The fiber we
used is 0.63 mm in diameter and 500 mm in length. NTEG
makes it possible to detect the infrared radiation from the
tympanic membrane without the measurement being influenced by the temperature of the surrounding ear canal wall
and without contacting the tympanic membrane.
The basic characteristics of the optical fiber thermometry,
such as stability (sensor and system drift) and linearity, were
investigated by using a blackbody temperature that was
controlled by a Peltier element. The optical fiber thermometry
was calibrated with the blackbody before each experiment.
To position the optical fiber in the extra-auditory canal, a
polyethylene sponge was glued to the fiber. This also insulated the auditory canal from the ambient temperature
(Fig. 1). Each subject compressed the polyethylene sponge
and carefully inserted the optical fiber into his right ear canal
while he listened to the spinning noise of the mechanical
chopper that traveled through the fiber and we monitored the
temperature. The noise seems louder as the fiber approaches
the tympanic membrane, and the temperature of tympanic
membrane is maximal in the auditory canal. We used these
parameters to determine whether the optical fiber thermometry was properly positioned to accurately measure Tty. The
temperature from the infrared sensor was sampled every 5 s
and sent to a personal computer (9801T, NEC).
The other method of thermometry was a thermistor sensor
(Sensor Technica), which measured the temperature of the
contralateral tympanic membrane (contact-Tty ). This information was sent to the personal computer (9801NST, NEC) with
a data logger (K730, Techno Seven) every 4 s. The thermistor
probe was gently inserted into the left ear canal by the test
subject while we monitored the temperature. We used the
sharp pain felt by the subject and the scratching noise heard
before and after each experiment to determine that the sensor
was in contact with the tympanic membrane. The auditory
canal was then filled with cotton wool and taped to prevent
movement of air inside the auditory canal.
Tes was measured by a copper-constantan thermocouple
with silicon coating at the tip. The thermocouple was inserted
through the nose to a distance equal to one-quarter of each
subject’s height. The temperature was recorded every second
and was stored in the personal computer (9801RA, NEC) with
a data logger (HR2300, Yokogawa). Average values for infrared-Tty, contact-Tty, and Tes were calculated for each minute.
The control and final temperatures were the average values
for the 3-min before the start and at the end of each
experiment, respectively. The change in the core temperatures (DTcore ) was the difference between the control and final
values.
Measurement of skin temperatures, sweating rate, and
heart rate. During the tests of head cooling and facial fanning,
the skin temperature of the forehead and right temple was
measured with the copper-constantan thermocouple. These
temperatures were sampled every second. During the passiveheat-stress and progressive-exercise tests, the local sweating
rate and heart rate were measured. Sweating rate was
TYMPANIC TEMPERATURE MEASUREMENT
RESULTS
The results of the stability and linearity tests of the
optical fiber thermometry, carried out with the blackbody, are shown in Fig. 2. The stability tests that used
the blackbody calibrated at 37°C were carried out at an
ambient temperature of 25°C for 60 min. There is no
long-term drift in this system, and the maximum
difference between the temperature the optical fiber
thermometry measured and the temperature of the
blackbody was 60.2°C (Fig. 2A). The temperature
measured by the optical fiber thermometry shows a
good linear relationship with that of the blackbody (Fig.
2B).
The head cooling and facial fanning tests were used
to investigate whether the optical fiber thermometry
accurately measured only the limited amount of infrared radiation from the tympanic membrane. The skin
temperatures on the right temple and forehead decreased remarkably when the ice pack was pressed
against the right temporal region (Fig. 3A) or the
electrical fan blew air in the subject’s face (Fig. 3B).
Infrared-Tty and Tes, however, are not influenced by the
decrease in these temperatures. The differences in
infrared-Tty between the control and final values in the
cooling and fanning tests are 0.0026 6 0.0480 (SD) and
0.0411 6 0.0731°C, respectively.
No significant differences between infrared-Tty, contact-Tty, and Tes are observed for the control and final
values and for DTcore in the passive-heat-stress and
progressive-exercise experiments (Table 1). During both
experiments, there are no significant differences in the
changes in infrared-Tty, contact-Tty, and Tes over time
Fig. 2. Results of blackbody stability test at constant temperature
(A) and linear response between infrared tympanic thermometry
using an optical fiber (optical fiber thermometry) and blackbody
temperature (B).
(Fig. 4). In the progressive-exercise experiment, there
was a significant delay in the rise in infrared-Tty and
contact-Tty compared with Tes. There was no significant
difference in the onset of the elevation in infrared-Tty,
contact-Tty, and Tes in the passive-heat-stress experiment (Table 1). The correlation between infrared-Tty
and contact-Tty in both experiments is statistically
significant [correlation coefficient: 0.82 6 0.10 (passiveheat-stress test) and 0.87 6 0.07 (progressive-exercise
test)].
There are highly positive correlations, which range
from 0.864 to 0.950, between sweating rate on the chest
and forearm and the core temperatures. There are no
significant differences in the slopes of sweating rate vs.
infrared-Tty and sweating rate vs. contact-Tty at each
site in both experiments. No significant differences in
the threshold values of infrared-Tty, contact-Tty, and Tes
for sweating were observed for either site.
DISCUSSION
Although many thermometries have been developed
and used to measure core temperature, use of these
devices involves great risks, technical difficulties, and
controversies (4). Recently, a method of infrared tympanic thermometry that did not involve contact with
the tympanic membrane was developed to measure Tty
safely and easily. The accuracy of predecessors has
been questioned (3, 10, 13, 17, 22). The insufficient
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recorded continuously at two sites, the left side of the chest
and the forearm, by the ventilated capsule method. Dry
nitrogen gas was pumped through the capsules (chest: 8.54
cm2, forearm: 5.31 cm2 ) at a rate of 1.5 l/min. The humidity of
the nitrogen gas flowing out of the capsules was measured
with a capacitance hygrometer (HMP 133Y, Vaisala, Finland). The skin temperatures and hygrometer output signals
were stored in the same system used for measuring Tes and
were calculated every minute. Heart rate was measured by
using lead V5 of an electrocardiogram.
Regression equations between sweating rate and the core
temperature were calculated for each subject during each test
from the data sampled every minute. Whenever the relationship showed a sigmoidal pattern, the points representing the
steepest portion of the curve were fitted by regression analysis to obtain the slope. Infrared-Tty, contact-Tty, and Tes
thresholds for sweating were the values for each core temperature after which sweating rate increased above its equilibrium value.
Statistical analysis. A one-way analysis of variance was
performed to assess the difference between calculated and
measured values for the thermal parameters, e.g., the control
values, the final values, DTcore, and the slope of sweating rate
vs. core temperature. Repeated two-way analysis of variance
with repeated measures was used to assess the change in
thermal parameters with time. Values of infrared-Tty, Tes, and
contact-Tty sampled every 5 min were used for the statistical
analysis. In addition, Sheffé’s test was used to test treatment
mean values among the parameters when there were significant differences. Statistical significance was set at P , 0.05
for all statistical tests.
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TYMPANIC TEMPERATURE MEASUREMENT
Fig. 3. A: time course of tympanic temperature measured by an
infrared tympanic thermometry with an optical fiber (infrared-Tty ),
esophageal temperature (Tes ), and right-side temporal skin temperature (Ttemple ) during head cooling test. Ice pack was positioned
against right temporal area for 15-min after a 5-min baseline period.
Values of infrared-Tty and Tes were 36.88 6 0.03 (SD) and 36.72 6
0.06°C for 3–5 min before cooling and were 36.93 6 0.05 and 36.71 6
0.07°C for 18–20 min during cooling. B: time course of infrared-Tty,
Tes, and forehead skin temperature (Tforehead ) during facial fanning
test. Electrical fan with a 300-mm-diameter blade was placed 150
mm in front of the face. Facial skin was cooled for 15 min after a
5-min baseline period. Values of infrared-Tty and Tes were 36.75 6
0.03 (SD) and 36.67 6 0.05°C for 3–5 min before cooling and 36.75 6
0.05 and 36.66 6 0.06°C for 18–20 min during fanning.
accuracy is caused by the fact that these devices may
measure the temperature of the auditory canal. To
prevent this, we used a chalcogenide glass fiber to
detect infrared radiation solely from the tympanic
membrane (Fig. 1). Because the optical fiber thermometry accurately measured the regulated temperature of
a blackbody (Fig. 2), it is believed that the optical fiber
thermometry measurements are stable and have a
linear response, making the optical fiber suitable for
thermometry.
Because the tympanic membrane is near the point
where the carotid artery enters the hypothalamus, Tty
is believed to reflect the brain and core temperatures
Table 1. Thermal parameters at rest and end periods of each test, increase in core temperature from the rest
period to the end period, onset time of rising core temperature from the control value, slopes
between sweating rate and core temperature, and threshold for onset of sweating during
a passive-heat-stress and a progressive-exercise test
Control,
°C
End,
°C
DTcore ,
°C
Onset of
Rising
Tcore min
Slope
Chest
Forearm
Threshold,
°C
1.00 6 0.44
0.71 6 0.24*
1.31 6 0.50
0.79 6 0.23*
0.57 6 0.10*
1.04 6 0.26
36.74 6 0.35
36.58 6 0.24
36.84 6 0.20
1.17 6 0.60
0.90 6 0.33
1.54 6 0.78
0.69 6 0.22
0.55 6 0.15
1.08 6 0.56
36.83 6 0.35
36.73 6 0.26
37.04 6 0.28
Passive heat stress
Infrared-Tty
Contact-Tty
Tes
36.69 6 0.21
36.49 6 0.34
36.62 6 0.14
37.51 6 0.32
37.39 6 0.39
37.40 6 0.25
0.81 6 0.31
0.91 6 0.26
0.78 6 0.19
Infrared-Tty
Contact-Tty
Tes
36.91 6 0.24
36.80 6 0.25
36.86 6 0.19
37.75 6 0.54
37.76 6 0.46
37.86 6 0.49
0.84 6 0.41
0.96 6 0.29
1.00 6 0.36
5.4 6 2.5
5.1 6 2.0
3.0 6 0.9
Progressive exercise
6.9 6 1.9*
7.0 6 1.4*
3.5 6 1.2
Values are means 6 SD for 8 men. Control and end values represent average values for 3 min before start and end of each thermal stress,
respectively. Tty , tympanic temperature; Tes , esophageal temperature measured by thermocouple; Tcore , core temperature; infrared-Tty, Tty
measured by infrared tympanic thermometry with optical fiber; contact-Tty, Tty measured by thermistor; Control, rest; End, end period of each
test; Slope, relationship between sweating rate and Tcore . * Significant difference from Tes , P , 0.05.
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Fig. 4. Comparison of core temperatures during passive-heat-stress
(A) and progressive-exercise (B) tests. Time course of infrared-Tty,
tympanic temperature measured with a thermistor (contact-Tty ), and
Tes during both experiments. One subject became exhausted 27 min
after the start of exercise, so data plotted for all the subjects’ data are
for 27 min.
TYMPANIC TEMPERATURE MEASUREMENT
Thermoregulatory sweating responses are generally
assessed by two parameters, the core temperature
threshold for the onset of sweating and the slope of the
relationship between core temperature and the rate of
sweating (i.e., sensitivity). We used these two parameters to investigate whether infrared-Tty could be used
to evaluate the thermoregulatory sweating responses
as well as contact-Tty and Tes. In this study, there was a
remarkable positive relationship between the core temperatures measured by the three methods and sweating rate on the chest and forearm. In both experiments,
there were no significant differences in the sensitivity
of measurement determined by sweating rate vs. infrared-Tty and sweating rate vs. contact-Tty at each site.
Furthermore, there were no significant differences in
infrared-Tty, contact-Tty, and Tes thresholds for sweating in both experiments (Table 1), although the increase in infrared-Tty and contact-Tty lagged behind the
rise in Tes during exercise (Fig. 4). Consequently, these
results also suggest that infrared-Tty accurately reflects
the temperature that determines the thermoregulatory
sweating response and can be used in thermoregulatory research and clinical medicine.
In conclusion, the optical fiber thermometry performs extremely well, and it accurately measures the
Tty. Head cooling and facial fanning tests proved that
the optical fiber thermometry accurately measured
only the limited amount of infrared radiation from the
tympanic membrane. The changes in infrared-Tty during the passive-heat-stress and progressive-exercise
experiments were the same as those in contact-Tty.
Furthermore, the changes in infrared-Tty and contactTty were similar to those in Tes during both experiments. There were also no significant differences between infrared-Tty and contact-Tty and the changes in
the two parameters for monitoring sweating responses
(threshold temperature and sensitivity). Therefore, we
can recommend the optical fiber thermometry as a
device for continuously measuring Tty.
The authors thank the test subjects for their wonderful cooperation in our research.
Address for reprint requests: M. Shibasaki, c/o N. Kondo, Laboratory for Applied Human Physiology, Faculty of Human Development,
Kobe University, 3-11 Tsurukabuto, Nada-Ku, Kobe 657-8501, Japan
(E-mail: [email protected]).
Received 1 December 1997; accepted in final form 28 April 1998.
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tympanic membrane throughout exercise. Two subjects
had a low correlation between infrared-Tty and contactTty during exercise. These subjects reported that they
felt that the fiber was slipping out of their auditory
canals. It is necessary to improve the method of attaching the fiber, perhaps by fashioning an attachment
similar to a hearing aid. On the other hand, this system
accurately measured Tty, even in three subjects whose
heart rate reached 200 beats/min at the end of the
exercise, suggesting that the optical fiber thermometry
could accurately measure the Tty during intense exercise when the fiber was positioned in external canal.
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