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Intrauterine noise: A component of the fetal environment
Intrauterine noise: A component of the fetal environment DAVID
WALKER
JAMES
GRIMWADE
CARL Melbourne,
WOOD Victoria,
Australia
Low-frequency random noise measured inside the pregnant human uterus is shown to be related to maternal cardiovascular dynamics. The attenuation of sound by maternal t&es is considerable, and environmental noise does not contribute greatly to the intrauterine sound energy. The fetus is subjected to a unique sound field of internal origin which may have important implications for sensory development.
IT HAS BEEN ASSERTED thatvarious factors such as sound, movement, and light may influence the development of basic reflex systems in the fetal brain.l While there is limited information available concerning the functional development of sensory receptors in fetal life, there is no description of the nature of the stimuli which may act at the level of the fetus.l The following is a report of measurements made of the sound field in the uterus adjacent to the fetus and of the extent to which external sound and vibration contribute to this sound energy. Measurements were taken in 16 pregnant women at term but not in labor and 7 nonpregnant women undergoing uterine curettage. A yd inch condenser microphone (Bruel and Kjaer, Type 4136) and a frequency analyzer/sound level meter (Bruel and Kjaer, Type 2107) permitted measurement of sound above 55 db sound pressure level (SPL) in narrow bandwidths from 20 to 40,000 Hz. The microphone was covered by a rubber sleeve and sterilized in alcoholic hexachlorophene solution. In pregnant pa-
tients measurements were taken before rupture of the fetal membranes with the microphone in the uterus adjacent to the fetal head; the membranes were then ruptured, and the microphone was passed to a position lateral to the fetal head and near the ear. In nonpregnant patients the microphone was passed into the uterine cavity after dilatation of the cervix. In all cases the vagina was packed with sterile cotton packing to exclude room noises. In these experiments the condenser microphone operates under conditions less than ideal. Error must be introduced for two reasons: (1) There is an impedance mismatch across each of the fluid/rubber/air interfaces between the amniotic fluid and the diaphragm of the microphone; (2) the microphone is designed to operate with normal barometric pressures behind the condenser diaphragm, which in this case is sealed by a rubber membrane. An estimate of the magnitude of the error has been attempted by calibrating the microphone covered by its rubber sleeve against a pressure-sensitive transistor (Pitran, Stow Laboratories) in a saline-filled tube maintained at 37’ C. The Pitran is sensitive to
From the Department of Obstetrics and Gynaecology, Monash University and Queen Victoria Hospital.
91
92
Walker,
Grimwade,
and Wood Amer.
----i
I mECG
J
J-
I
January J. Obstet.
1, 1971 Gynec.
r---r
1 ret
trace: output from sound level meter; biphasic wave corresponds to the 95 db pulse in the background noise level. Lower trace: maternal ECG, reversed polarity. Fig.
1. Upper
pressure down to 2 dynes cm? and has a frequency response from zero to more than 100 kHz. Sound pressure levels from 80 to 110 db SPL were passed through the fluid by vibrating a rubber diaphragm at one end of the tube. Readings from both the microphone and Pitran transistor were taken simultaneously, and it is thus possible to plot the response of the microphone against that of the transistor. The Pitran itself was first calibrated in air where acoustic pressures were measured to within + 1 db SPL by a calibrated condenser microphone. The correction provided is an approximate one in that only pure tone frequencies between 100 and 1,000 Hz were used. It is more desirable to use random noise since this is similar to that found in the fluid of the uterus. The corrected data are offered as being nearer to the measurements that would be achieved if a fluid-measuring microphone could be inserted into the intrauterine space. In the 16 pregnant patients the mean noise level was 85 db SPL (+ 2.5 S.D.). A peak of 95 db (+ 3.5 S.D.) occurred 300 msec. after the “R” wave of the maternal electrocardiogram (ECG) (Fig. 1) . There was no significant difference between measurements taken before rupture of the membranes with the microphone in the cervical canal or after rupture of the membranes with the microphone lateral to the fetal head. In the 7 nonpregnant patients the mean
noise level was 84 db SPL (+ 5.5 S.D.) with the maternal pulse being associated with peak readings of 94 db (+ 5.0 S.D.). The difference between the pregnant and nonpregnant situation is not significant, though the range encountered in the latter was greater (82 to 94 db SPL) . The readings are estimated to be in error (as previously discussed) by 10 db SPL, and the mean noise level would then be in the region of 95 db SPL. The frequency analysis of the noise is presented in Fig. 2. The instrument used is of the. constant percentage bandwidth type, the actual bandwidth increasing as the center frequency of analysis is increased. The filter characteristic selected gave a bandwidth 18 per cent of the center frequency with a 30 db/octave cutoff, so that at 20 Hz the bandwidth is 3.6 Hz, whereas at 200 Hz it is 36 Hz. In Fig. 2 the data are expressed as the SPL for a bandwidth of one Hertz; this is termed the “sound pressure spectrum leve1.“2 The dashed lines are for the data corrected for microphone impedance mismatch. The error is estimated at * 16 db at 20 Hz and -t 12 db at 200 Hz. Frequencies above those plotted could not be measured since the threshold of the instrumentation was 55 db SPL. The mean value of 85 db SPL (range 82 to 90 db, uncorrected values) is higher than the previously published figure of 72 db obtained from one patient in Iabor.3 The discrepancy may be because of the positioning
Volume Number
Intrauterine
109 1
-m--
.
.
.l.l.,l..
..L..^.-..+.
noise
93
94
Walker,
Grimwade,
and Wood Amer.
of the microphone which, in the earlier study, was situated in the vagina. All measurements in this study were taken from the uterine cavity. From studies made in other tissues,4 it is likely that the background noise is generated by turbulent blood flow and muscle movement. That blood flow is a determinant of the noise in the uterus would seem to be confirmed by the proper association of the sound pressure peaks with the maternal ECG. It is possible in an ideal situation to relate the mean velocity of turbulent flow to the vibrations caused by the fluid in the walls of a pipe.5 There is a known increase in the blood flow to the pelvic organs during pregnancy, which would seem to be such that turbulence is increased, leading to the divergence of the pregnant and nonpregnant spectra at higher frequencies as seen in Fig. 2, although the total noise level does not increase significantly. The contribution of environmental sound
Fig. 3. Attenuation microphone.
Solid
of line
external sound =
uncorrected
through values;
January J. Obstet.
1, 1971 Gym.
to the uterine noise was assessed by measuring the attenuation of external sound and vibration through the maternal tissues. The following procedure was employed for 14 pregnant patients. External sounds from 100 to 3,000 Hz were generated by a 6 inch loudspeaker driven by a sine wave function generator. The loudspeaker was placed over the maternal umbilicus. The external sound pressure level was measured by placing the microphone between the loudspeaker and abdomen, a procedure which takes into account the reflection of sound at the air/tissue interface. Frequencies below 100 Hz were generated by an electromechanical vibrator (Pye-Ling, Type V47), having a displacement up to 0.1 inch and driving a rubber pad 2!/4 square inches in area. Measurements were made after rupture of the membranes. The frequency analyzer was tuned to the frequency of the external source, and the following frequencies were used: 50, 100, 500, 1,000, 2,000, and 3,000 Hz. The
maternal dashed
tissues as measured by an intrauterine line = corrected values as in Fig.
2.
Intrauterine
Volume 109 Number 1
attenuation of these frequencies is shown in Fig. 3. The dashed line is the data corrected for microphone error; since the microphone underestimates the true intrauterine sound pressure, the actual loss must be somewhat less than is shown. Estimation of the error was not done at frequencies above 1,000 Hz. We have used the curve of Fig. 3 in conjunction with published data for environmental noise spectra6 and conclude that the attenuation is such that there will rarely be an internal value for an environmentally generated noise that will exceed the intrauterine noise level at any particular frequency, and the external sound can then be said to be masked. It is possible that in certain situations (e.g., within 20 feet of a subway train or close to aircraft) the fetus will be exposed to additional noise, mainly of low frequency. The great attenuation suffered by frequencies above 1,000 Hz and the low level of these in most environmental situations make it unlikely that the fetus normally receives external sound of this nature. This does not exclude the possibility that external sounds have an indirect effect on the fetus by causing maternal cardiovascular changes. The nature of the attenuation curve
REFERENCES
1. Klosovskii, B. N.: The Development of the Brain, Oxford, 1963, Pergamon Press, Inc., pp. 117-118. 2. Young, R. W.: In Harris, C. M., editor: Handbook %f Noise Control,. chap. -2, New York, 1957. McGraw-Hill Book Comuanv. Inc. 3. Bench, J.: J. Genet. Psychol. i13:‘83, 1968. 4. Faber, J. J., and Purvis, J. H.: Circulation Res. 12: 308, 1963. 5. Bull, M. K.: J. Fluid Mechanics 28: 719, 1967. 6. Stevens, K. N., and Baruch, f. J.: In Harris, C. M., editor: Handbook of Noise Control,
noise
95
below 50 Hz is uncertain. However, von Gierke and associates7 have shown that the behavior of tissues to vibration does not change markedly for frequencies up to 100 Hz, and one may thus still expect a loss of some 20 db, i.e., only a hundredth of the external sound pressure reaches the fetus. There remains to be established whether the internal noise incident upon the fetus has any implications for the development of sensory function. Does the intrauterine noise provide a background of afferent neural “noise”? It is necessary to determine whether the fetus has sensory apparatus of sufhcient sensitivity to detect these pressure fluctuations. The possibility that the maternal heartbeat is an imprinting stimulus to which the fetus has a defined response has been described,s and in this there is the clear implication that the fetus can sense the pressure and pulsatile changes that surround it. Grimwade and colleagues9 have shown that the term fetus is sensitive to abnormal vibratory and acoustic changes produced in a number of ways. The fact that fetal responses can be elicited by frequencies below the audible range would implicate sensory pathways other than the cochlea, such as the cutaneous and vestibular organs.
chap. 35, New York. 1957, McGraw-Hill Book Co&pa&, Inc. . . 7. Von Gierke. H. E.. Oestreicher. H. L.. Francke, E. K., Parrack, H. D., and’Von Wit: tern, W. W.: J. Appl. Physiol. 41: 886, 1952. 8. Salk, L.: Trans. N. Y. Acad. Sci. 24: 753, 1962. 9. Grimwade, J. C., Walker, D. W., Bartlett, M., Gordon, 8, and Wood, C.: AMER. J. OBSTET. GYNEC. 109: 86, 1970. 172 Lonsdale St. Melbourne, Victoria
3000,
Australia
WALKER
JAMES
GRIMWADE
CARL Melbourne,
WOOD Victoria,
Australia
Low-frequency random noise measured inside the pregnant human uterus is shown to be related to maternal cardiovascular dynamics. The attenuation of sound by maternal t&es is considerable, and environmental noise does not contribute greatly to the intrauterine sound energy. The fetus is subjected to a unique sound field of internal origin which may have important implications for sensory development.
IT HAS BEEN ASSERTED thatvarious factors such as sound, movement, and light may influence the development of basic reflex systems in the fetal brain.l While there is limited information available concerning the functional development of sensory receptors in fetal life, there is no description of the nature of the stimuli which may act at the level of the fetus.l The following is a report of measurements made of the sound field in the uterus adjacent to the fetus and of the extent to which external sound and vibration contribute to this sound energy. Measurements were taken in 16 pregnant women at term but not in labor and 7 nonpregnant women undergoing uterine curettage. A yd inch condenser microphone (Bruel and Kjaer, Type 4136) and a frequency analyzer/sound level meter (Bruel and Kjaer, Type 2107) permitted measurement of sound above 55 db sound pressure level (SPL) in narrow bandwidths from 20 to 40,000 Hz. The microphone was covered by a rubber sleeve and sterilized in alcoholic hexachlorophene solution. In pregnant pa-
tients measurements were taken before rupture of the fetal membranes with the microphone in the uterus adjacent to the fetal head; the membranes were then ruptured, and the microphone was passed to a position lateral to the fetal head and near the ear. In nonpregnant patients the microphone was passed into the uterine cavity after dilatation of the cervix. In all cases the vagina was packed with sterile cotton packing to exclude room noises. In these experiments the condenser microphone operates under conditions less than ideal. Error must be introduced for two reasons: (1) There is an impedance mismatch across each of the fluid/rubber/air interfaces between the amniotic fluid and the diaphragm of the microphone; (2) the microphone is designed to operate with normal barometric pressures behind the condenser diaphragm, which in this case is sealed by a rubber membrane. An estimate of the magnitude of the error has been attempted by calibrating the microphone covered by its rubber sleeve against a pressure-sensitive transistor (Pitran, Stow Laboratories) in a saline-filled tube maintained at 37’ C. The Pitran is sensitive to
From the Department of Obstetrics and Gynaecology, Monash University and Queen Victoria Hospital.
91
92
Walker,
Grimwade,
and Wood Amer.
----i
I mECG
J
J-
I
January J. Obstet.
1, 1971 Gynec.
r---r
1 ret
trace: output from sound level meter; biphasic wave corresponds to the 95 db pulse in the background noise level. Lower trace: maternal ECG, reversed polarity. Fig.
1. Upper
pressure down to 2 dynes cm? and has a frequency response from zero to more than 100 kHz. Sound pressure levels from 80 to 110 db SPL were passed through the fluid by vibrating a rubber diaphragm at one end of the tube. Readings from both the microphone and Pitran transistor were taken simultaneously, and it is thus possible to plot the response of the microphone against that of the transistor. The Pitran itself was first calibrated in air where acoustic pressures were measured to within + 1 db SPL by a calibrated condenser microphone. The correction provided is an approximate one in that only pure tone frequencies between 100 and 1,000 Hz were used. It is more desirable to use random noise since this is similar to that found in the fluid of the uterus. The corrected data are offered as being nearer to the measurements that would be achieved if a fluid-measuring microphone could be inserted into the intrauterine space. In the 16 pregnant patients the mean noise level was 85 db SPL (+ 2.5 S.D.). A peak of 95 db (+ 3.5 S.D.) occurred 300 msec. after the “R” wave of the maternal electrocardiogram (ECG) (Fig. 1) . There was no significant difference between measurements taken before rupture of the membranes with the microphone in the cervical canal or after rupture of the membranes with the microphone lateral to the fetal head. In the 7 nonpregnant patients the mean
noise level was 84 db SPL (+ 5.5 S.D.) with the maternal pulse being associated with peak readings of 94 db (+ 5.0 S.D.). The difference between the pregnant and nonpregnant situation is not significant, though the range encountered in the latter was greater (82 to 94 db SPL) . The readings are estimated to be in error (as previously discussed) by 10 db SPL, and the mean noise level would then be in the region of 95 db SPL. The frequency analysis of the noise is presented in Fig. 2. The instrument used is of the. constant percentage bandwidth type, the actual bandwidth increasing as the center frequency of analysis is increased. The filter characteristic selected gave a bandwidth 18 per cent of the center frequency with a 30 db/octave cutoff, so that at 20 Hz the bandwidth is 3.6 Hz, whereas at 200 Hz it is 36 Hz. In Fig. 2 the data are expressed as the SPL for a bandwidth of one Hertz; this is termed the “sound pressure spectrum leve1.“2 The dashed lines are for the data corrected for microphone impedance mismatch. The error is estimated at * 16 db at 20 Hz and -t 12 db at 200 Hz. Frequencies above those plotted could not be measured since the threshold of the instrumentation was 55 db SPL. The mean value of 85 db SPL (range 82 to 90 db, uncorrected values) is higher than the previously published figure of 72 db obtained from one patient in Iabor.3 The discrepancy may be because of the positioning
Volume Number
Intrauterine
109 1
-m--
.
.
.l.l.,l..
..L..^.-..+.
noise
93
94
Walker,
Grimwade,
and Wood Amer.
of the microphone which, in the earlier study, was situated in the vagina. All measurements in this study were taken from the uterine cavity. From studies made in other tissues,4 it is likely that the background noise is generated by turbulent blood flow and muscle movement. That blood flow is a determinant of the noise in the uterus would seem to be confirmed by the proper association of the sound pressure peaks with the maternal ECG. It is possible in an ideal situation to relate the mean velocity of turbulent flow to the vibrations caused by the fluid in the walls of a pipe.5 There is a known increase in the blood flow to the pelvic organs during pregnancy, which would seem to be such that turbulence is increased, leading to the divergence of the pregnant and nonpregnant spectra at higher frequencies as seen in Fig. 2, although the total noise level does not increase significantly. The contribution of environmental sound
Fig. 3. Attenuation microphone.
Solid
of line
external sound =
uncorrected
through values;
January J. Obstet.
1, 1971 Gym.
to the uterine noise was assessed by measuring the attenuation of external sound and vibration through the maternal tissues. The following procedure was employed for 14 pregnant patients. External sounds from 100 to 3,000 Hz were generated by a 6 inch loudspeaker driven by a sine wave function generator. The loudspeaker was placed over the maternal umbilicus. The external sound pressure level was measured by placing the microphone between the loudspeaker and abdomen, a procedure which takes into account the reflection of sound at the air/tissue interface. Frequencies below 100 Hz were generated by an electromechanical vibrator (Pye-Ling, Type V47), having a displacement up to 0.1 inch and driving a rubber pad 2!/4 square inches in area. Measurements were made after rupture of the membranes. The frequency analyzer was tuned to the frequency of the external source, and the following frequencies were used: 50, 100, 500, 1,000, 2,000, and 3,000 Hz. The
maternal dashed
tissues as measured by an intrauterine line = corrected values as in Fig.
2.
Intrauterine
Volume 109 Number 1
attenuation of these frequencies is shown in Fig. 3. The dashed line is the data corrected for microphone error; since the microphone underestimates the true intrauterine sound pressure, the actual loss must be somewhat less than is shown. Estimation of the error was not done at frequencies above 1,000 Hz. We have used the curve of Fig. 3 in conjunction with published data for environmental noise spectra6 and conclude that the attenuation is such that there will rarely be an internal value for an environmentally generated noise that will exceed the intrauterine noise level at any particular frequency, and the external sound can then be said to be masked. It is possible that in certain situations (e.g., within 20 feet of a subway train or close to aircraft) the fetus will be exposed to additional noise, mainly of low frequency. The great attenuation suffered by frequencies above 1,000 Hz and the low level of these in most environmental situations make it unlikely that the fetus normally receives external sound of this nature. This does not exclude the possibility that external sounds have an indirect effect on the fetus by causing maternal cardiovascular changes. The nature of the attenuation curve
REFERENCES
1. Klosovskii, B. N.: The Development of the Brain, Oxford, 1963, Pergamon Press, Inc., pp. 117-118. 2. Young, R. W.: In Harris, C. M., editor: Handbook %f Noise Control,. chap. -2, New York, 1957. McGraw-Hill Book Comuanv. Inc. 3. Bench, J.: J. Genet. Psychol. i13:‘83, 1968. 4. Faber, J. J., and Purvis, J. H.: Circulation Res. 12: 308, 1963. 5. Bull, M. K.: J. Fluid Mechanics 28: 719, 1967. 6. Stevens, K. N., and Baruch, f. J.: In Harris, C. M., editor: Handbook of Noise Control,
noise
95
below 50 Hz is uncertain. However, von Gierke and associates7 have shown that the behavior of tissues to vibration does not change markedly for frequencies up to 100 Hz, and one may thus still expect a loss of some 20 db, i.e., only a hundredth of the external sound pressure reaches the fetus. There remains to be established whether the internal noise incident upon the fetus has any implications for the development of sensory function. Does the intrauterine noise provide a background of afferent neural “noise”? It is necessary to determine whether the fetus has sensory apparatus of sufhcient sensitivity to detect these pressure fluctuations. The possibility that the maternal heartbeat is an imprinting stimulus to which the fetus has a defined response has been described,s and in this there is the clear implication that the fetus can sense the pressure and pulsatile changes that surround it. Grimwade and colleagues9 have shown that the term fetus is sensitive to abnormal vibratory and acoustic changes produced in a number of ways. The fact that fetal responses can be elicited by frequencies below the audible range would implicate sensory pathways other than the cochlea, such as the cutaneous and vestibular organs.
chap. 35, New York. 1957, McGraw-Hill Book Co&pa&, Inc. . . 7. Von Gierke. H. E.. Oestreicher. H. L.. Francke, E. K., Parrack, H. D., and’Von Wit: tern, W. W.: J. Appl. Physiol. 41: 886, 1952. 8. Salk, L.: Trans. N. Y. Acad. Sci. 24: 753, 1962. 9. Grimwade, J. C., Walker, D. W., Bartlett, M., Gordon, 8, and Wood, C.: AMER. J. OBSTET. GYNEC. 109: 86, 1970. 172 Lonsdale St. Melbourne, Victoria
3000,
Australia