Fetal heart rate response to acoustic stimulation in relation to fetal development and hearing impairment BERTILJOHANSSON', ERIKWEDENBERG'
B J ~ RwESTIN3 N
From the 'Department of Technical Audiology, Karolinska Institutet, 2Department of Oto-Laryngology, Karolinska Hospital and 3Department of Obstetrics and Gynecology, Danderyd Hospital, Karolinska Institutet, Stockholm, Sweden
Actu Obstet Gynecol Scand 1992; 71: 610-615
In twelve healthy pregnant women the fetal heart rate response to sound stimulation was tested every second week between the 22nd and the 34th week of the pregnancy. Habituation to the acoustic stimuli used was also investigated at the last examination. A high risk material for hearing impairment comprised 31 pregnant women. The stimuli were presented by a vibrator placed on the maternal abdomen over the fetal head. As stimuli pure tones were used in the form of pulses of one second at a frequency of 3000 Hz and with an onset and a decay time chosen not to cause audible clicks to a normal ear. The intensity of the tone pulse was set at a level calculated to correspond at the position of the fetal head to 110 dB sound pressure level in free field. Roughly 50% of the fetuses tested started to react with an increased heart rate in the 24th week of the development. Four weeks later all tested subjects responded to the test stimuli. Habituation of the motor 'startle' response was shown but the fetus heart rate change was consistent over 20 consecutive stimulations one min in between each. In 31 high risk subjects the responses from three fetuses must be considered as pathologic. These three children exhibited impaired auditory tests after birth and severe hearing impairment at the age of three years. All the others disclosed normal hearing at birth as well as at three years of age. Repeated significant increases in fetal heart rate following acoustic stimulation may appear as an indication of normal function of the hearing organ. Several consecutive acoustic stimulations without any fetal responses in heartbeat rate is an indication of a possible hearing impairment. Key words: fetal heart rate; acoustic stimulation; fetal development; habituation; hearing
impairment Submitted March 15, 1992 Accepted April 12, 1992
Responses to acoustic stimulations by the human fetus have been reported already during the 19th century. It has earlier been shown that newborn infants
Abbreviations: APR: auro-palpebral startle reflex; SPL: sound pressure level @ A r i a Obstet Cynerol Seand 71 (1992)
react to acoustic stimuli (1.2). At about the 24th week of the intrauterine life the cochlea and the sensory end organ have reached their normal development (3,4). In a previous paper it was shown that human fetuses in late third trimester responded to sound stimuli with a change of the fetal heart rate and with a motor startle response (5). At low frequencies. below 1500 Hz,the tactile stimulation is not negli-
Fetal heart response to acoustic stimulation
every second week during the fetal development in between the 22nd and the 34th week. The problem of possible habituation to the acoustic stimuli was studied at the last examination. The test signal 3000 Hz at a level of 110 dB with 1 sec duration was presented 20 times with an interval of 1 min. The changes of the heart rate and rapid movements were recorded. Pregnant women attending the antenatal care stations in Stockholm were asked about: 1. Maternal hearing impairment or previous births
of infants with hearing impairment. 2. Rubella during the first trimester of pregnancy. In this way 27 cases with maternal hearing impairment or previous births of infants with hearing impairment were collected and in addition 3 cases of maternal rubella. Fig. 1. Location of sound source (V)and microphone (M) The risk material thus comprising 30 women (one of the photocardiograph. of them has given birth twice) i.e. 31 fetuses were gible. In order to exclude response to mechanical examined. The response to acoustic stimulation of vibration (6) a test frequency well above this limit the fetus in this series of the investigation was recorded during the 34th week of the fetal developwas chosen. With the instrumentation available ( 5 ) the test ment. The fetal lie was vertex and all pregnancies were frequency 3000 Hz was used and the signal presented at a level calculated to correspond to a sound pres- singletons. The fetal week of development was calsure level of 110 dB at the position of the fetal head. culated from the estimated time of ovulation. In Fetal responses in heart rate and movements have connection with the delivery the auditory sense of been shown to be related to a subcortical level (7,8). the infants was examined by means of two tests: The last two decades these and similar studies 1. Threshold determination of a startle reflex, the have been overshadowed by other methods of as‘auropalpebral’ reflex (APR). sessing fetal well being such as fetal heart rate mon- 2. Determination of the sound intensity level that is itoring (9), ultra sound studies with biophysical proneeded to awake the child in a specific depth of files (10) and Doppler velocimetry on fetal blood sleep (2). At the age of three years careful tonevessels (1 1). In 1977 (12) the interest in the acoustic audiometric determinations were performed by field was once again revived by a demonstration of play audiometry (14). the fetal heart rate response to acoustic stimuli as being a measure of fetal well being. Recently, fetal The equipment and the procedure for the measuremovements in response to vibro-acoustic stimulation ment of the response by the fetus has been described have been shown to predict fetal well being (13). in detail earlier (5). The mother was placed on a bed The aim of the present investigation was to exam- in a head down position in order to minimize circulaine: tory disturbances due to supine position. The position of the head of the fetus at the start of the 1. At which week of the fetal development does the measurements was marked on the maternal abdohuman fetus start reacting to sound stimuli with a men. The vibrator of the sound source was placed change of the fetal heart rate? with its center over the spot thus indicated and the 2. A clinical material with increased risk of neural contact surface was lubricated with a specific jelly. hearing impairment, in order to see if it was pos- The source impedance of the stimulus generator was sible to diagnose fetal hearing impairment in low compared to the impedance of the abdominal utero. wall in order to establish ‘constant current conditions’. The microphone of a phono-cardiograph was placed in a position where the fetal heart beats were Materials and methods best detected, Fig. 1. In order to avoid any interfeIn 12 apparently healthy women chosen at random rence from the mother, she was supplied with headthe fetal heart rate response to pure tones was tested phones presenting white noise masking sound. 0 Acta Obster Gynecol Scand 71 (1992)
B. Johansson, E. Wedenberg and B. Wesrin
Fig. 2a. Recording of fetal response to a test tone (3OOO Hz,SPL 110 dB, duration 1 sec.). Channel 1. Length of the test tone. Channel 2. Recording of the fetal startle response. Channel 3 and 4. Fetal heart beat recordings.
The subjects were resting 5 to 10 minutes before starting the experiment during which time the heart rate of the fetus and the mother was recorded. The stimuli used were pure tone pulses of the frequency 3000 Hz and at a level of 110 dB as described above. A typical recording is shown in Fig. 2a. The same recording is presented in Fig. 2b as beats per minute determined from the distance between two consecutive beats, and integrated over three and six beats respectively. Apparently the depth of the fetal sleep varied considerably. This resulted in great variations in the response. For the statistical calculations the set of data comprising 60 heart beats immediately before the stimulus onset and 60 beats after the tone pulse have been compared as two independent populations. Using a x2 test the variance quotient has been determined for the judgement of the significant level of the responses.
Furthermore the following criteria for the judgement of positive fetal heart rate changes were decided upon: 1. The significant change of the fetal heart rate must occur within 15 seconds after the stimulus. 2. The initial fetal heart rate just before the test tone pulse must be below 150 beats per minute. 3. At least 50% of the responses should be significant at the 5% level. A series of test tone pulses were presented at each session and at least 3 were recorded. For the risk group test an average of 5 stimulations were used. An example of 4 responses from a healthy test subject is shown in Fig. 3.
equirolent pulse rate 150
xax x y
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0 Arta Obstet Gynecol &and
Fig. 2b. Fetal heart rate change calculated from the recording in Fig. 2a: . - . from the distance between two consecutive heart beats, X - x integrated over three heart beats, 0-0 integrated over six heart beats.
Fetal heart response to acoustic stimulation
p u l r i rote
the recordings, Fig. 2, and was also clearly observed subjectively. The latency time for the first tone pulse was of the order 100 milliseconds. It varied greatly, however, and the latency time increased rapidly and after only a few consecutive stimulations they were no longer readable from the recordings nor could it be detected by subjective observations. The habituation was furthermore studied in a specific series in connection with the development tests comprising 12 subjects. At the last examination of these, 20 consecutive stimuli were given with 1 minute intervals. Generally the motor startle response disappeared after three stimulations. The response in the form of heart rate changes was consistent, however, and occurred throughout the series of the 20 consecutive acoustic stimulations.
. I ~ ~
Fig. 3. Fetal heart rate response in a healthy test subject. Test tone (vertical black line) 3000 Hz, 110 dB SPL for 1 second. Four tests were performed with an interval of about 2 minutes.
Results Fetal development
The fetal heart rate change upon pure tone pulse stimulation as a function of the age of the fetus e.g. the degree of fetal development, is shown in Fig. 4. For the 22nd week of development the response results do not meet the criteria given above but the tests carried out during the 24th week could be interpreted as a threshold, some 50% react positively on the acoustic stimulus. From the 26th week all subjects of this healthy test subject group responded with significant heart rate change. Motor startle response and habituation
In general, in addition to the heart rate change, a distinct motor startle response occurred upon the first acoustic stimulation. It is clearly observed from Positive reactions from3 t o r t s on aoch of 12 rubjacts. ut
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week of davelopmant.
Fig. 4. Fetal heart rate response to the sound stimulation as a function of fetal development. Test tone 3OOO Hz, 110 dB SPL for 1 second. Three tests were given to each of twelve subjects. Level of significance p < 001.
High risk material, hearing impairment
The fetal heart rate response to the acoustic stimuli used, pure tone pulses of the frequency 3000 Hz, duration 1 second, equivalent sound pressure level 110 dB, measured in the 34th week of the pregnancy and with the subjects from the high risk group is reported in Table I. Each of the subjects has been tested with four to nine stimulations with a short rest in between. From the beginning 34 subjects participated but due to different reasons three of them did not fulfil so at the end the series comprised 31 cases. According to the criteria for a positive response described above the fetal response from 21 cases was interpreted as normal. Positive response was shown in 64% of the total number of stimulations at a significance level p < 0.05 to p