The nonstress test (NST) has achieved widespread popularity as a safe and simple method of evaluating fetal health.1-2 Considerable health care resources are being allocated to the use of this technology. The development of a method to shorten the duration of this test is highly desirable and would make an immediate and worthwhile contribution to perinatal health care. Vibratory acoustic stimulation when applied to the maternal abdominal wall has been shown to result in fetal heart rate (FHR) accelerations.3-8 This intervention is often referred to as fetal acoustic stimulation (FAS). It has been shown in several studies that clinical application of FAS can reduce the proportion of antepartum FHR tests that fail to achieve reactivity and may thus improve the efficiency of antepartum testing.5-6 The purpose of this study was to investigate, by means of a randomized controlled trial, the hypothesis that FAS can reduce the overall time for antepartum FHR testing. METHODS

One hundred and seventy-two pregnant women of at least 34 weeks' gestation and in whom there were no contraindications to contraction stress tests were recruited into this study. The protocol had previously been approved by the institutional ethics

committee. After informed written consent had been obtained, women were randomized by draw of sealed envelopes either to NST (n = 82:150 tests) or to FAS (n = 90:150 tests). This sample size had been calculated to have a 90% power of increasing by 15% the proportion of tests that achieve reactivity within 20 minutes (alpha = 0.05). Women randomized to NST were tested at the first and subsequent visits as follows: The FHR and uterine activity were monitored for an initial 20 minutes. If the trace was nonreactive, the fetus was stimulated manually and the test was continued for a further 20 minutes. If satisfactory accelerations were not found, the woman was then sent for a meal. On her return to the department, a nipple stimulation contraction stress test was performed if the subsequent NST remained nonreactive after a further 20 minutes.9 Women randomized to FAS were monitored for an initial 5 minutes. If the pattern was nonreactive, a 3-second vibratory acoustic stimulus was applied to the maternal abdomen in the region of the fetal head. The stimulus was repeated a second and a third time, also at 1-minute intervals, if satisfactory FHR accelerations had not occurred. Reactivity was defined as the presence of two accelerations of greater than 15 beats/minute and of 15 seconds' duration or more within a 20-minute period. In tests in which accelerations had been provoked by FAS, the definition of reactivity required one of the two accel-

The Departments of Obstetrics and Nursing, King Edward Memorial Hospital for Women, Perth, Western Australia Reprint requests: Dr. Newnham, King Edward Memorial Hospital, Bagot Road, Subiaco, Perth, Western Australia 232

Copyright © 1990 by Thieme Medical Publishers, Inc., 381 Park Avenue South, New York, NY 10016. All rights reserved.

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The ability of vibratory acoustic stimulation to shorten the duration of antepartum fetal heart rate monitoring was investigated by a randomized controlled trial. Vibratory acoustic stimulation did not shorten the overall duration of testing. This failure to improve the performance of antepartum monitoring appeared to result from prolonged accelerations, which complicated one third of the tests in which vibratory acoustic stimulation was employed. Further investigation is warranted using less profound methods of fetal stimulation.




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I I.I 1-10

One hundred and fifty tests were performed in 82 women who had been randomized to the NST protocol and an equal number of tests were performed in 90 women who had been randomized to the FAS protocol. The primary indications for the initial test in the two study groups are shown in Table 1. These two groups were comparable for maternal ages, parities, gestational ages, modes of delivery, and Apgar scores. There were no perinatal deaths. The times taken to complete the tests for the two groups are shown in Figure 1. There were no significant differences between the two groups in the overall times taken to complete these tests. Tests performed with the NST protocol were completed within a median (range) time of 20.6 (5 to 117) minutes. Of these NSTs, 25 (16.7%) were interpreted as nonreactive. These cases were tested again later in the day, at which time eight required further investigation by contraction stress test. Six of these contraction stress tests were negative, one was positive, and one was equivocal. Tests performed with the FAS protocol were completed within a median (range) time of 22.5 (7 to 115) minutes. Of these tests, 15 demonstrated reactivity before FAS was employed. Seventeen (11.3%) tests were interpreted as nonreactive, of which three cases required a contraction stress test later in the day. Of these contraction stress tests, two were interpreted as negative and one as positive. The proportion of tests classified as nonreactive (NST 25 of 150 and FAS 17 of 150) and the proportion of tests requir-

Table 1. Comparison of the Primary Indications for the Initial Test in the Two Study Groups

Small for dates Hypertension Reduced fetal movement Post-term gestation Glucose intolerance Other*


NST (n = 82)

FAS (n = 90)

14 23 13 8 9 15

19 20 17 9 13 12

*Other includes premature rupture of membranes, antepartum hemorrhage, medical disorders, poor obstetric history, and maternal weight loss.






D >60

Time to complete test (minutes) Figure 1 . The overall time to complete the antepartum FHR tests in minutes for the two study groups. • : NST; D : FAS.

ing a contraction stress test (NST 8 of 150 and FAS 3 of 150) were less in the FAS group than in the NST group, although the differences were not statistically significant. Of the tests in which FAS was employed, 45 (33.3%) cases involved prolonged accelerations exceeding 2 minutes' duration and 32 (23.7%) cases involved accelerations exceeding 5 minutes' duration. In five cases these FHR accelerations persisted for as long as 30 minutes. The median (range) testing time for FAS cases in which the provoked accelerations were less than 2 minutes' duration was 17.0 (11 to 48) minutes; this time was significantly less than that observed with the NST protocol (p < 0.05). In order to determine the effect of our criteria for reactivity, a retrospective review was made of the FHR traces in which FAS had been employed. Relaxation of our criteria, so that an unprovoked acceleration was no longer required, did not significantly reduce the overall testing times (median, 22.0; range, 5 to 115 minutes). DISCUSSION

There is no doubt that vibratory acoustic stimulation induces accelerations of the fetal heart rate. A variety of studies have demonstrated reproduceable effects on basal heart rate, the duration of accelerations, and the total time during which accelerations occur.1011 When introduced into clinical practice, vibratory acoustic stimulation has been shown to reduce the proportion of antepartum FHR tests that remain nonreactive after 40 minutes of monitoring.5'6 In the present study, FAS also was associated with a reduction in the proportion of tests classified as nonreactive, and a reduction in the proportion requiring contraction stress tests, although these differences did not achieve statistical significance. However, the results of the present study indicate that despite profound effects on FHR accelerations, use 233

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erations to have been unprovoked. All tests were performed with Corometrics 115 monitors using Doppler FHR transducers. The tests were performed by specially trained fetal intensive care midwives with a nurse:patient ratio of 1:1. The vibratory acoustic stimulus had an audio frequency of 75 Hz, a sound intensity of 74 db at 1 m in air and a stimulation duration of 3 seconds. The effectiveness of randomization was assessed by comparison of several parameters within the two groups. Statistical tests were by chi-square for proportions and by Wilcoxon's rank sum test.



tory acoustic stimulus, but also to evaluate fully the efficacy and safety of these stimuli within the context of controlled trials of rigorous design. It is vital that research in this area should continue. Antepartum FHR testing is used throughout the developed world and the introduction of an inexpensive and effective method of shortening the duration of the NST would make an important contribution to the cost of providing perinatal health care. REFERENCES

1. Keegan KA, Paul RH: Antepartum fetal heart rate testing IV: The nonstress test as a primary approach. Am J Obstet Gynaecol 136:75-80, 1980 2. Thacker SB, Berkelman RL: Assessing the diagnostic accuracy and efficacy of selected antepartum fetal surveillance techniques. Obstet Gynecol Surv 41:121-141, 1986 3. Birnholz JC, Benaceraff BR: The development of human fetal hearing. Science 222:516-518, 1983 4. Read JA, Miller FC: Fetal heart rate acceleration in response to acoustic stimulation as a measure of fetal well-being. Am J Obstet Gynaecol 129:512-517, 1977 5. Smith CV, Phelan JP, Paul RH, Broussard P: Fetal acoustic stimulation testing: A retrospective experience with the fetal acoustic stimulation test. Am J Obstet Gynecol 153:567-568, 1985 6. Smith CV, Phelan JP, Platt LD, Broussard P, Paul RH: Fetal acoustic stimulation testing II. A randomized clinical comparison with the nonstress test. Am J Obstet Gynecol 155:131-134, 1986 7. Phelan JP: Antepartum fetal assessment—newer techniques. Semin Perinatol 12:57-65, 1988 8. Gagnon R, Morrow R, Ritchie K, Hunse C, Patrick J: Umbilical and uterine artery blood flow velocities after vibratory acoustic stimulation. Am J Obstet Gynecol 159:574578, 1988 9. Newnham JP, Burns SE, Szczygielski C, Roberman BD: Nonstress and contraction stress fetal heart rate monitoring. A randomized trial to determine which is the faster primary test. J Reprod Med 33:356-360, 1988 10. Gagnon R, Hunse C, Carmichael L, Fellows F, Patrick J: External vibratory acoustic stimulation near term: Fetal heart rate and heart rate variability responses. Am J Obstet Gynecol 156:323-327, 1987 11. Gagnon R, Hunse C, Carmichael L, Fellows F, Patrick J: Fetal heart rate and fetal activity patterns after vibratory acoustic stimulation at thirty to thirty-two weeks' gestational age. Am J Obstet Gynecol 158:75-79, 1988 12. Romero R, Mazor M, Hobbins JC: A critical appraisal of fetal acoustic stimulation as an antenatal test for fetal well-being. Obstet Gynecol 71:781-786, 1988 13. Ohel G, Horowitz E, Linder N, Sohmer H: Neonatal auditory acuity following in utero vibratory acoustic stimulation. Am J Obstet Gynecol 157:440-441, 1987 14. Goodlin RC: Possible deleterious effects of sound or vibratory fetal stimuli. (Letter.) Am J Obstet Gynecol 159: 1016, 1988

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of this stimulus does not shorten the overall duration of testing. This failure to improve the performance of FHR testing appeared to result from the prolonged accelerations that complicated one third of tests in which FAS was employed. In those tests in which the FAS-induced accelerations were of less than 2 minutes duration, the overall duration of testing was less than that observed for regular NSTs, suggesting a possible role for a less profound vibratory acoustic stimulus. The failure of the present study to support a role for FAS in antepartum testing is in contrast to the studies of Smith and coworkers.5-7 In their institution FAS was shown to reduce both the rate of nonreactive tests and to produce a reduction in overall testing time. These studies defined reactivity as the presence of two accelerations of at least 15 beats/ minute, lasting 15 seconds in a 10-minute period and allowed both accelerations to be induced by the vibratory acoustic stimuli. In view of the paucity of information on the relative predictive powers of provoked and unprovoked FHR accelerations,12 the protocol employed in the present study demanded one of the two accelerations to be unprovoked. In an attempt to offset this more rigid criterion for reactivity, the period during which accelerations were expected to occur was increased from 10 to 20 minutes. We have attempted to assess the impact of our criteria for reactivity on testing times by retrospective review of the FHR traces in which FAS had been employed and by removing our requirement for an unprovoked acceleration. This review suggested that overall testing times were not significantly altered by our more rigid criteria. However, it is not possible to assess completely the testing times using criteria that allow both accelerations to be provoked, since our study design precluded further use of FAS once a provoked acceleration had been observed. A role for vibratory acoustic stimulation in antepartum testing remains controversial. Much of the conflict in the literature results from differing forms of stimulation, different criteria for completion of the test, and differing end points with which the pregnancy outcome is judged. 12 Although external vibratory acoustic stimulation has been shown to have no apparent effect on neonatal auditory nerve and brainstem-evoked responses,13 concern for the safety of such stimuli has been expressed for many years and continues today.14 Clearly, further investigation is required to determine not only the optimum intensity, frequency, and duration of the vibra-

July 1990

Effect of vibratory acoustic stimulation on the duration of fetal heart rate monitoring tests.

The ability of vibratory acoustic stimulation to shorten the duration of antepartum fetal heart rate monitoring was investigated by a randomized contr...
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