Position of the vibroacoustic stimulator does not affect fetal response Daniel P. Eller, MD, Lucinda J. Robinson, MD, and Roger B. Newman, MD Charleston, South Carolina OBJECTIVE: This study was designed to compare the fetal response to the vibroacoustic stimulation test when applied alternately over the fetal vertex or breech. STUDY DESIGN: Two hundred five patients with a nonreactive nonstress test after 10 minutes were prospectively randomzied to receive the vibroacoustic stimulation test over the fetal vertex (n = 115) or the fetal breech (n = 90). Fetal heart rate reactivity was evaluated. Two patient subsets were evaluated for fetal movement (n = 37) and the fetal startle response (n = 20) after the vibroacoustic stimulation test. RESULTS: Virtually identical fetal heart rate responses and increases in tetal movement were observed after vibroacoustic stimulation over the fetal vertex or breech. The fetal startle response was uniformly observed in both groups. CONCLUSION: Vibroacoustic stimulation over the fetal breech elicits an identical short-term fetal response compared with stimulation over the fetal vertex, potentially reducing the intensity of sound exposure at the fetal ear. (AM J OSSTET GVNECOL 1992;167:1137-9.)
Key words: Location of vibroacoustic stimulation test, fetal hearing, fetal heart rate reactivity, fetal movement, fetal startle response
The vibroacoustic stimulation test has become an established adjunct to the nonstress test (NST) for assessment of fetal well-being.l~3 Positioning of the stimulator over the fetal vertex was empirically selected as a standard. The effect on fetal response of placement has not been studied. Gerhardt et al. and other investigators4~6 have cautioned that, given the responsiveness of the fetus to low frequencies and the possibility of increased potential for damage caused by overstimulation of an immature ear in utero, care should be taken to verify safety and determine the best method of implementing the test. The purpose of this study was to determine if fetal acoustic stimulation over the fetal breech would produce the identical fetal response as stimulation over the fetal vertex, providing a basis to investigate theoretic differences in sound exposure to the fetus.
Material and methods Between December 1990 and May 1991, 205 volunteers with a nonreactive NST after 10 minutes in the antepartum fetal testing unit at the Medical University of South Carolina were prospectively randomized. The vibroacoustic stimulation test was performed over the
From the Medical University of South Carolina, Division of Maternal! Fetal Medicine, Department of Obstetrics and Gynecology. Presented at the Twelfth Annual Meeting of the Society of Perinatal Obstetricians, Orlando, Florida, February 3-8, 1992. Reprint requests: Daniel P. Eller, MD, Depa1~tment afObstetrics and Gynecology, Medical Univresily of South Carolina, Charleston, SC
29425. 6/6/39675
fetal vertex (n = 115) or the fetal breech (n = 90). Fetal acoustic stimulation was performed by placing the Corometrics (model 145) fetal acoustic stimulator on the maternal abdomen over the fetal vertex in patients with an odd Medical University Hospital number and over the fetal breech for patients with an even number. The force of application was sufficient to indent the skin approximately 1 cm when the fetal acoustic stimulator was applied. Acoustic stimulation was applied for 3 seconds. Fetal presentation was determined by Leopold maneuvers with ultrasonographic backup if Leopold maneuvers were indeterminate. Fetal heart rate responses (FHR) were considered reactive if an FHR acceleration of 15 beats over the baseline for 3 minutes was present or if two accelerations of 15 beats I min over baseline of at least 15 seconds occurred within 10 minutes after the vibroacoustic stimulation test. I A subset of 33 patients (17 breech and 16 vertex) was evaluated for changes in fetal movement after the vibroacoustic stimulation test, as recorded by a Doppler activity monitor (MT-320 Actograph, Toitu Corp.). Fetal activity was calculated as described by Reddy et aJ.7 beginning 5 minutes before vibroacoustic stimulation and for 10 minutes after stimulation. The fetal startle response (sudden gross body movements and flexion-extension of all extremities lasting 1 to 2 seconds after the vibroacoustic stimulation test) was observed ultrasonographically in 20 additional patients (10 breech and 10 vertex) as described by de Vries et al. B Differences among proportions were evaluated with
1138
Eller, Robinson, and Newman
October 1992 Am J Obstet Gynecol
Table I. Demographic characterization of study and control group Location of stimulator*
Maternal age (yr, mean ± SD) Gestational age at stimulation (wk, mean ± SD) Gravidity (mean ± SD) Parity (mean ± SD)
Head
Breech
24.6 ± 5.7
26.1 ± 6.3
36.0 ± 3.1
35.8 ± 2.9
2.8 ± 1.6
2.5 ± 1.9
1.4 ± 1.3
1.2 ± 1.7
Observed Breech
Nonreactive Rective with two accelerations Reactive with tachycardia
1
Expected*
1
Head
Head
Breech
1 25
7 28
7.8 23.0
10.2 30.0
30
51
35.2
45.8
*p = Not significant.
*p = Not significant.
Table II. Indications for procedure Intrauterine growth retardation (%) Postdates (%) Hypertension (%) Previous intrauterine fetal death (%) Decreased fetal movement (%) Class A diabetes (%) Other diabetes (%) Other (%)
Table III. Contingency tables comparing observed with expected values for FHR response to vibroacoustic stimulation test
Breech
Vertex*
11.3
1l.l
12.2 23.2 5.6
12.2 17.3 10.4
6.7
8.7
14.4 ILl 15.5
8.7 3.5 28.1
*p = Not significant.
the X2 analysis. Continuous variables were evaluated with the paired Student t test. Results are expressed as mean ± SD and significance was established at p < 0.05. On the basis of 205 volunteers enrolled for evaluation of FHR response, a power analysis was performed. The power of the test was 0.91, indicating a 9% risk of a type II or ~-error. Results
The demographics of each group were similar with respect to gestational age, maternal age, gravidity, parity, and indication for NST (Tables I and II). The observed and expected distribution of NST results for each group are presented in Table III; they demonstrated no significant difference between the two groups (p = 0.26). The change in percent of fetal movement before and after the vibroacoustic stimulation test was not significantly different between those stimulated at the fetal breech and those stimulated at the fetal head (Table IV). The fetal startle response was virtually identical in both groups and was identified in every fetus observed. Comment
A reactive fetal acoustic stimulation test was defined according to the guidelines set forth by Serafini et al. I
Table IV. Average percent fetal movement (± SD) before and after vibroacoustic stimulation test Prestimulation Poststimulation
Head
Breech*
17.4 ± 3.6 30.3 ± 6.4
17.1 ± 3.3 28.9 ± 3.7
*p = Not significant.
There is some controversy regarding whether fetal tachycardia represents a reactive test. Thomas et al. 9 caution that it is unknown whether the development of fetal tachycardia represents a reactive NST. However, the purpose of this study was to compare the FHR reaction in each group to determine if a difference existed in the response. No difference was found when evaluating fetal tachycardia or fetal accelerations. At this time there are no data specifically supporting any risk to the fetus from the vibroacoustic stimulation test. There is, however, controversy over theoretic risks. Lenoir et al. lO notes that the cochlea acquires adult functional properties between 28 weeks' gestation and I year of life. Fetal acoustic stimulation is also usually used between 28 weeks and delivery, which is theoretically the point of maximal fetal susceptibility to nonphysiologic sound pressures. Ohel et al. " tested 40 neonates for auditory acuity at I to 2 days of age after exposure to the vibroacoustic stimulation test in utero. These investigators found no evidence of acoustic trauma; however, only a small number of patients were involved and there was no description of the fetal acoustic stimulation that the patients received. Arulkumaran et al. 12 evaluated 465 children exposed to vibroacoustic stimulation in utero. They were screened at 4 years of age for high-frequency hearing loss and none was found. The investigators conceded that there could be an effect on hearing acuity that is mild enough not to cause an abnormal hearing test at 4 years of age. Most of the fetuses in their study were exposed between 38 and 43 weeks' gestation and received only a single
Volume 167 Number 4, Part 1
test, The effect of vibroacoustic stimulation on preterm infants receiving multiple stimulations has not been evaluated, The intrauterine sound environment after the vibroacoustic stimulation test has been studied in humans and in sheep with differing results. Smith et al. also examined intrauterine sound pressures in nine patients after amniorrhexis in labor. 13 They found sound pressures ranging from 91 to III dB after the vibroacoustic stimulation test with an electronic artificial larynx. They concluded that sound pressures generated from the electronic artificial larynx pose no major risks to the fetus. Nyman et al. 14 examined intrauterine sound pressures in 16 humans receiving vibroacoustic stimulation. They found the highest recorded sound pressure levels to be 129 dB. These investigators found no obvious relationship between the distance from the stimulator to the hydrophone and the intrauterine sound pressure level. In pregnant ewes, Gerhardt et aU was able to record sound pressures as high as 135 dB. Sound pressures fell as a function of distance away from the hydrophone (114 dB at 3 inches, 101 dB at 6 inches). Nyman et al. 14 have suggested that these elevated sound pressure levels are reduced before reaching the cochlea of the fetus because of the dampening effect of the surrounding amniotic fluid and the fluid in the middle ear. However, Gerhardt6 states that with frequencies
Key words: Location of vibroacoustic stimulation test, fetal hearing, fetal heart rate reactivity, fetal movement, fetal startle response
The vibroacoustic stimulation test has become an established adjunct to the nonstress test (NST) for assessment of fetal well-being.l~3 Positioning of the stimulator over the fetal vertex was empirically selected as a standard. The effect on fetal response of placement has not been studied. Gerhardt et al. and other investigators4~6 have cautioned that, given the responsiveness of the fetus to low frequencies and the possibility of increased potential for damage caused by overstimulation of an immature ear in utero, care should be taken to verify safety and determine the best method of implementing the test. The purpose of this study was to determine if fetal acoustic stimulation over the fetal breech would produce the identical fetal response as stimulation over the fetal vertex, providing a basis to investigate theoretic differences in sound exposure to the fetus.
Material and methods Between December 1990 and May 1991, 205 volunteers with a nonreactive NST after 10 minutes in the antepartum fetal testing unit at the Medical University of South Carolina were prospectively randomized. The vibroacoustic stimulation test was performed over the
From the Medical University of South Carolina, Division of Maternal! Fetal Medicine, Department of Obstetrics and Gynecology. Presented at the Twelfth Annual Meeting of the Society of Perinatal Obstetricians, Orlando, Florida, February 3-8, 1992. Reprint requests: Daniel P. Eller, MD, Depa1~tment afObstetrics and Gynecology, Medical Univresily of South Carolina, Charleston, SC
29425. 6/6/39675
fetal vertex (n = 115) or the fetal breech (n = 90). Fetal acoustic stimulation was performed by placing the Corometrics (model 145) fetal acoustic stimulator on the maternal abdomen over the fetal vertex in patients with an odd Medical University Hospital number and over the fetal breech for patients with an even number. The force of application was sufficient to indent the skin approximately 1 cm when the fetal acoustic stimulator was applied. Acoustic stimulation was applied for 3 seconds. Fetal presentation was determined by Leopold maneuvers with ultrasonographic backup if Leopold maneuvers were indeterminate. Fetal heart rate responses (FHR) were considered reactive if an FHR acceleration of 15 beats over the baseline for 3 minutes was present or if two accelerations of 15 beats I min over baseline of at least 15 seconds occurred within 10 minutes after the vibroacoustic stimulation test. I A subset of 33 patients (17 breech and 16 vertex) was evaluated for changes in fetal movement after the vibroacoustic stimulation test, as recorded by a Doppler activity monitor (MT-320 Actograph, Toitu Corp.). Fetal activity was calculated as described by Reddy et aJ.7 beginning 5 minutes before vibroacoustic stimulation and for 10 minutes after stimulation. The fetal startle response (sudden gross body movements and flexion-extension of all extremities lasting 1 to 2 seconds after the vibroacoustic stimulation test) was observed ultrasonographically in 20 additional patients (10 breech and 10 vertex) as described by de Vries et al. B Differences among proportions were evaluated with
1138
Eller, Robinson, and Newman
October 1992 Am J Obstet Gynecol
Table I. Demographic characterization of study and control group Location of stimulator*
Maternal age (yr, mean ± SD) Gestational age at stimulation (wk, mean ± SD) Gravidity (mean ± SD) Parity (mean ± SD)
Head
Breech
24.6 ± 5.7
26.1 ± 6.3
36.0 ± 3.1
35.8 ± 2.9
2.8 ± 1.6
2.5 ± 1.9
1.4 ± 1.3
1.2 ± 1.7
Observed Breech
Nonreactive Rective with two accelerations Reactive with tachycardia
1
Expected*
1
Head
Head
Breech
1 25
7 28
7.8 23.0
10.2 30.0
30
51
35.2
45.8
*p = Not significant.
*p = Not significant.
Table II. Indications for procedure Intrauterine growth retardation (%) Postdates (%) Hypertension (%) Previous intrauterine fetal death (%) Decreased fetal movement (%) Class A diabetes (%) Other diabetes (%) Other (%)
Table III. Contingency tables comparing observed with expected values for FHR response to vibroacoustic stimulation test
Breech
Vertex*
11.3
1l.l
12.2 23.2 5.6
12.2 17.3 10.4
6.7
8.7
14.4 ILl 15.5
8.7 3.5 28.1
*p = Not significant.
the X2 analysis. Continuous variables were evaluated with the paired Student t test. Results are expressed as mean ± SD and significance was established at p < 0.05. On the basis of 205 volunteers enrolled for evaluation of FHR response, a power analysis was performed. The power of the test was 0.91, indicating a 9% risk of a type II or ~-error. Results
The demographics of each group were similar with respect to gestational age, maternal age, gravidity, parity, and indication for NST (Tables I and II). The observed and expected distribution of NST results for each group are presented in Table III; they demonstrated no significant difference between the two groups (p = 0.26). The change in percent of fetal movement before and after the vibroacoustic stimulation test was not significantly different between those stimulated at the fetal breech and those stimulated at the fetal head (Table IV). The fetal startle response was virtually identical in both groups and was identified in every fetus observed. Comment
A reactive fetal acoustic stimulation test was defined according to the guidelines set forth by Serafini et al. I
Table IV. Average percent fetal movement (± SD) before and after vibroacoustic stimulation test Prestimulation Poststimulation
Head
Breech*
17.4 ± 3.6 30.3 ± 6.4
17.1 ± 3.3 28.9 ± 3.7
*p = Not significant.
There is some controversy regarding whether fetal tachycardia represents a reactive test. Thomas et al. 9 caution that it is unknown whether the development of fetal tachycardia represents a reactive NST. However, the purpose of this study was to compare the FHR reaction in each group to determine if a difference existed in the response. No difference was found when evaluating fetal tachycardia or fetal accelerations. At this time there are no data specifically supporting any risk to the fetus from the vibroacoustic stimulation test. There is, however, controversy over theoretic risks. Lenoir et al. lO notes that the cochlea acquires adult functional properties between 28 weeks' gestation and I year of life. Fetal acoustic stimulation is also usually used between 28 weeks and delivery, which is theoretically the point of maximal fetal susceptibility to nonphysiologic sound pressures. Ohel et al. " tested 40 neonates for auditory acuity at I to 2 days of age after exposure to the vibroacoustic stimulation test in utero. These investigators found no evidence of acoustic trauma; however, only a small number of patients were involved and there was no description of the fetal acoustic stimulation that the patients received. Arulkumaran et al. 12 evaluated 465 children exposed to vibroacoustic stimulation in utero. They were screened at 4 years of age for high-frequency hearing loss and none was found. The investigators conceded that there could be an effect on hearing acuity that is mild enough not to cause an abnormal hearing test at 4 years of age. Most of the fetuses in their study were exposed between 38 and 43 weeks' gestation and received only a single
Volume 167 Number 4, Part 1
test, The effect of vibroacoustic stimulation on preterm infants receiving multiple stimulations has not been evaluated, The intrauterine sound environment after the vibroacoustic stimulation test has been studied in humans and in sheep with differing results. Smith et al. also examined intrauterine sound pressures in nine patients after amniorrhexis in labor. 13 They found sound pressures ranging from 91 to III dB after the vibroacoustic stimulation test with an electronic artificial larynx. They concluded that sound pressures generated from the electronic artificial larynx pose no major risks to the fetus. Nyman et al. 14 examined intrauterine sound pressures in 16 humans receiving vibroacoustic stimulation. They found the highest recorded sound pressure levels to be 129 dB. These investigators found no obvious relationship between the distance from the stimulator to the hydrophone and the intrauterine sound pressure level. In pregnant ewes, Gerhardt et aU was able to record sound pressures as high as 135 dB. Sound pressures fell as a function of distance away from the hydrophone (114 dB at 3 inches, 101 dB at 6 inches). Nyman et al. 14 have suggested that these elevated sound pressure levels are reduced before reaching the cochlea of the fetus because of the dampening effect of the surrounding amniotic fluid and the fluid in the middle ear. However, Gerhardt6 states that with frequencies
