Tympanic membrane temperature of term and preterm neonates

Em+ Human Development, Elsevier

241

9 (1984) 241-247

EHD 00553

Tympanic membrane temperature of term and preterm neonates Steven R. Mayfield, Kenneth T. Nakamura, Jatinder Gladys R. Rios and Edward F. Bell Department

Bhatia,

of Pediatrics,University of Iowa, Iowa City, Iowa, U.S.A. Accepted for publication 21 November 1983

Summary Deep body temperatures of 70 term and 24 preterm newborn infants were measured at two sites: deep rectum (5 cm beyond the anus) and tympanic membrane. A significant correlation was found between deep rectal and tympanic membrane temperatures in both term and preterm infants. Mean deep rectal and tympanic membrane temperatures in term infants were 37.Ol”C and 36.83”C, respectively. Mean deep rectal and tympanic membrane temperatures in preterm infants were both 36.69”C. temperature;

tympanic

membrane;

neonate

Rectal temperature at a depth of 5 cm has become the standard for studies of body temperature of infants and children [9]. However, studies in adults [2,3,7,11] have suggested other sites for measurement of deep body temperature. Eichna and co-workers [7] found that deep rectal temperature differed from intracardiac and other intravascular temperatures in some situations; they concluded that rectal temperature may not accurately reflect the average temperature of deep body tissues. Aural (external auditory canal or tympanic membrane) temperature has been offered as another indicator of deep body temperature [2-5,10-121. It changes faster than deep rectal or esophageal temperature in response to changes in environmental temperature [2,3,12]. Moreover, aural temperature may prove useful for investigating body temperature regulation because it is near the hypothalamus, the site of the body’s central temperature receptors [3,8]. Address for correspondence: City, IA 52242, U.S.A. 0378-3782/84/$03.00

Edward F. Bell, M.D., Department of Pediatrics, University of Iowa, Iowa

0 1984 Elsevier Science Publishers B.V

242

The lack of available standards for aural temperature of newborn infants has limited the use of aural temperature measurement in both experimental and clinical practice. This study was designed to determine the relation between tympanic membrane and deep rectal temperatures and to establish standards for normal tympanic membrane temperature of newborn infants.

Methods Seventy term and 24 preterm infants were enrolled in the study. Each infant was studied once. Infants were excluded from the study if they were more than 30 days old; had evidence of necrotizing enterocolitis, blood in the feces, rectal or anal fissures, or major congenital anomalies; or had been placed in strict isolation. Written, informed consent was obtained for all subjects from one or both parents. The study was approved by the Human Subjects Review Committee of the University of Iowa. After admission to the study, demographic information was recorded for each subject. Gestational age was determined according to the method of Ballard and co-workers [l]. Infants with gestational age of 37 weeks or more were considered term. Postnatal age was calculated to the nearest hour. Infants nursed in bassinets were clothed in a cotton T-shirt, disposable diaper, and one to three cotton blankets. Infants in incubators or under radiant warmers were naked or wore only a disposable diaper. Nursing routine for our nurseries dictated that an adjustment be made if axillary temperature was outside the range of 36.5-37.O”C. This range was used for both term and preterm infants. Axillary temperatures outside this range were confirmed by measuring rectal temperature before any adjustment was made. For infants in servocontrolled incubators or radiant warmers, skin temperature was maintained at 36.5”C unless axillary or rectal temperature fell outside the normal range. Prior to each testing period, the room air temperature was recorded. For infants in incubators, the incubator air temperature was measured 10 cm below the center of the top of the incubator. Deep rectal temperature was measured by placing a vinyl-covered, flexible thermistor probe (Yellow Springs Instruments, probe 402, Yellow Springs, OH) into the rectum to a depth of 5 cm [9]. Tympanic membrane temperature was measured with a flexible, thermistor probe (YSI 511). Following inspection of the ear canal with an otoscope to estimate the distance to the tympanic membrane, the probe was slowly inserted into the external auditory canal until resistance was met. Placement of the probe under direct visualization was necessary in a few cases. The probe was then anchored by a cotton wad in the external auditory canal and secured with tape to the external ear. Room and incubator air temperatures were measured with an air temperature probe (YSI 405). All electronic probes were attached to a telethermometer (YSI model 46 TUC). Each probe had been tested against a certified mercury thermometer (U.S. National Bureau of Standards) in a water bath. The rectal and air temperature probes agreed within O.l’C with the certified thermometer throughout

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the range tested (30-40°C for the rectal probe and 20-40°C for the air probe). A linear regression equation was derived to predict true tympanic membrane probe temperature from telethermometer readings between 30°C and 40°C. The tympanic membrane was inspected again with an otoscope after each test. Following simultaneous placement of the thermistor probes, temperatures were recorded at both sites at 1 min intervals until the measurements remained unchanged for two consecutive readings. Infants in incubators had their temperatures measured through the portholes. If infants under radiant warmers had convection barriers pf thin polyethylene film, their temperatures were measured without disturbing the barrier. Data from both temperature sites were analyzed separately for term and preterm infants. The data were analyzed statistically using linear regression analysis and the two-tailed r-test.

Results Demographic data are displayed in Table I. Seventeen preterm infants (71%) and four term infants (6%) were nursed in incubators. The incubators of 12 of these preterm infants but none of the term infants were operated by skin temperature servocontrol. Three preterm infants (13%) and one term infant (1%) were nursed

TABLE I Demographic data Term

Preterm

Total Male Female

70 38 32

24 14 10

Bassinet Incubator Radiant warmer

65 4 1

4 17 3

Birth weight (kg) Mean Median Range

3.46(0/U) = 3.48 2.58-4.56

l.N(O.56) 1.68 0.83-3.19

Gestational age (wk) Mean Median Range

39.5(1.3) 40 37-43

32.9(2.4) 32 27-36

Postnatal age (h) Mean Median Range

52 45 12-240

112 90 15-724

a Mean (S.D.).

244 TABLE

II

Stabilization

temperatures

(“C)

L TV

Pa

Preterm

Term Mean

(SD.)

Range

Mean

(S.D.)

Range

37.01 36.83

(90.33) (0.36)

36.3-38.1 36.0-38.3

36.69 36.69

(0.39) (0.39)

35.4-37.3 35.4-37.4

“Level of statistical significance using two-tailed T,, is deep rectal temperature. TLYis tympanic membrane temperature.

-= 0.001 > 0.100

t-test.

39_ ,” eG 0% $2 25 .o cm zf ge +

_

TERM

PRETERM

38. ,.

*.

. ..&. q.. ..:a. . ,.. .+.,

37.

36-

: * ;: * . . *:.

:

1

35 I

36

*

I

I

37

I

,

J

30

Deep Rectal

. I

t

1

35 Temperature

I

1

36

I

I

37

(“C)

Fig. 1. Tympanic membrane temperature is plotted against deep rectal temperature for TERM and PRETERM infants. Tympanic membrane temperature was measured with an electronic probe placed against the tympanic membrane and insulated from the environment by a cotton wad in the external auditory canal. Deep rectal temperature was measured with an electronic probe 5 cm into the rectum.

35 Temperature

36

37

38

(“C)

Fig. 2. Cumulative number of subjects (W) with stabilized temperature at two measurement sites. T, ) is deep rectal temperature with an electronic probe 5 cm into the rectum. TtY (- - - - - -) is the (tympanic membrane temperature with an electronic probe against the tympanic membrane. The results for TERM and PRETERM infants are displayed separately.

under radiant warmers. Radiant warmers were always operated by skin temperature servocontrol. Room temperature ranged from 20.1”C to 27.2’C for term infants, with a mean of 24.O”C (S.D. 1.5). For preterm infants, room temperature ranged from 21.7 to 27.4’C with a mean of 24.6”C (S.D. 1.4). Mean incubator temperatures for term and preterm infants were 33.5”C (SD. 5.0) and 35.O“C (S.D. 2.6), respectively. Deep rectal and tympanic membrane temperatures are shown in Table II for term and preterm infants. The correlation of deep rectal and tympanic membrane temperatures is displayed in Fig. 1. Correlation coefficients were calculated and regression lines plotted for term and preterm infants. A significant correlation (P < 0.001) between measurement sites existed for both groups (r = 0.84 and 0.87 for term and preterm infants, respectively). This relation is further depicted in Fig. 2 in terms of the temperature measurements at the time of stabilization. No cases of external auditory canal or tympanic membrane trauma were seen in either the term or preterm group.

Discussion The use of aural thermometry was first described in 1948 by Williams and Thompson [13]. Benzinger and Taylor [3] modified the tool and suggested a possible relation between tympanic membrane temperature and the temperature presented to the central temperature receptors in the hypothalamus. Until recently, however, no data were available in newborn infants for either external auditory canal or tympanic membrane temperature measurements. Stratton [12] reported that mean deep rectal temperature of ten term infants was 0.3’C lower than mean aural temperature. In contrast, we found mean deep rectal temperature to be 0.18”C higher than mean tympanic membrane temperature in term infants. The means were equal in preterm infants. Our results are similar to those reported in adults, where aural temperature has generally been found to be lower than rectal temperature [2,3]. Nadel and Horvath [ll] compared the tympanic membrane and rectal temperatures of three young male adults clad only in athletic shorts. When ambient temperature was below 30°C tympanic membrane temperature was consistently lower than rectal temperature. Above 30°C tympanic membrane temperature was higher. This observation is consistent with our findings and may explain why tympanic membrane temperature was lower than rectal temperature in term infants but not in preterm infants. Most term infants (65 of 70) were in bassinets at room temperature below 30°C. Most preterm infants (17 of 24) were in incubators with air temperature above 30°C. Two factors that influence the relation between aural and rectal temperatures can be compared in infants and adults. First, the fraction of the body’s metabolic heat that is produced by the brain is larger in infants [5,12]. This factor should cause the aural temperature to be higher (relative to rectal temperature) in infants than in adults. On the other hand, the infant’s tympanic membrane and external auditory canal are closer to the cool body surface than are the adult’s, which should cause the

246

infant’s aural temperature to be lower (relative to rectal temperature) than the adult’s [6]. With our method both factors were operative, and the tympanic membrane temperature was equal to (preterm infants) or slightly below (term infants) rectal temperature, as in adults [2,3,11]. Stratton’s [5,12] technique of servocontrolled heating of the external ear may have negated the second effect (surface cooling), causing aural temperature to be higher than rectal temperature in his infant subjects. The temperature gradient along the wall of the external auditory canal allows convective heat loss to the environment [4]. Servocontrolled heating of the ear reduces the gradient and reduces or eliminates this convective heat loss [lo]. Thus, servocontrolled heating of the external ear produces higher aural temperatures. This is the most likely explanation for the difference between Stratton’s results [5,12] and ours. The second purpose of this study was to establish standards for normal tympanic membrane temperature in newborn infants. The technique was safe; we saw no evidence of trauma to the external auditory canal or tympanic membrane in 94 subjects. Benzinger and Taylor [3] reported mild discomfort during placement of the tympanic membrane thermistor probe followed by slight loss of hearing, which resolved immediately upon removal of the probe. Placement of our thermistor probe in adult volunteers prior to the onset of this study confirmed this transient discomfort. Our technique, however, was cumbersome and required considerable surveillance during the testing period to assure that the thermistor probe did not become dislodged. While this obviously limits the tool for use in clinical practice, the technique is sufficiently simple and safe when used for experimental purposes. The proximity of the tympanic membrane to the brain suggests that measurement of may ‘be useful in studies of temperature regulation [2,3] and brain aural temperature metabolism [5,12]. We measured the tympanic membrane temperature of 70 term and 24 preterm infants. A significant correlation was found between tympanic membrane and deep rectal temperatures in both term and preterm infants. There were too few patients outside the normothermic range to assess the effects of hypothermia and hyperthermia on this correlation. The potential of aural thermometry to reflect the temperature presented to the hypothalamus and its apparent superiority over deep rectal temperature in reflecting the rate of change in deep body temperature warrant continued investigation.

Acknowledgements We thank Kerry S. Ostlund for her secretarial assistance. This project was supported by Basil O’Connor Starter Research Grant 5-268 and Clinical Research Grant 6-356 awarded to Dr. Bell by the March of Dimes Birth Defects Foundation. Dr. Bell is also the recipient of New Investigator Research Award HD16974 from the U.S. Public Health Service.

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