Fetal Reactivity to Acoustic Stimulation has been observed since 1925 and used in multiple clinical settings. Fetoacoustic stimulation offers several distinct advantages over other tests: It is simple, inexpensive and non-invasive.
This randomised trial seeks to compare the effects of vibroacoustic stimulation and halogen light stimulation on an unsatisfying nonstress test result (NST).
Background
Fetal vibroacoustic stimulation (VVS) is a noninvasive procedure in which a device is placed on the maternal abdomen over the region of fetal head and sound is released at a predetermined level for several seconds, in an attempt to trigger startle reflex in fetuses, resulting in heart rate acceleration or transient tachycardia and providing assurance of well-being during labor when used alongside nonstress testing (NST).
There are various methods used to assess fetal well-being during labor. This may include using methods such as scalp blood sampling, an electrocardiograph and pulse oximeter sensor to monitor fetal wellbeing. While these tests provide additional information, they also have their own specific limitations: for instance fetal scalp blood sampling requires dilation of the cervix and rupture of membranes in order to obtain an adequate sample (East 2008); an electrocardiograph has an extremely high false positive rate; while pulse oximeter sensors must only be applied after rupture of amniotic membranes when at least 2 cm has dilated (Westgren 1998).
To minimise time spent and biophysical profiles (BPP) required due to non-reactive tests, an NST that is capable of inducing responses in all pregnant women would be useful. Previous studies have demonstrated the fetal vibroacoustic stimulation test can generate significant responses in most pregnancies.
Fetuses may be able to detect acoustic stimuli through vibrations that resonate within their mother’s uterus, which then reach through to them via membranous larynx and into their bodies, where they cause an increase in heart rate acceleration.
However, the accuracy of fetal vibroacoustic stimulating test has come under question due to an increase in false negatives and differences in sound pressure levels used by different researchers. Studies that observed false negatives may have utilized stimulators with higher sound pressure levels compared with others or used stimulators that produced different sounds with equal vibration (Edersheim 1987; Smith 1986), leading them to conclude there is no difference between acoustic stimulation and mock stimulation despite prior research efforts (Smith 1986). According to Erdersheim 1987 and Smith 1986), no such evidence of difference was observed between real acoustic stimulation and mock stimulation.
Methods
Fetal Acoustic Stimulation Test involves attaching a small electronic device to a pregnant mother’s abdomen which emits brief sounds that reach the baby through her abdomen. Fetal heart rates are then monitored in response to this stimulus and compared against those recorded from pregnant women not exposed to vibrations; when reactive patterns change from non-reactive, that indicates healthy babies. This method provides a simple and safe means of testing foetuses, providing guidance as to when it is safe to continue pregnancy.
Oxytocin challenge tests used to be the standard for prenatal fetal surveillance; however, their labor intensive requirements and contraindications caused many women to spend hours sitting on a monitor waiting for reactive results of nonstress tests (NST). Since then, several less invasive techniques have been created by obstetricians that enable them to assess women’s uteri more efficiently; such as biophysical profiles/NST, kick counts or heart rate tracings.
Previous studies have demonstrated that fetal acoustic stimulation stimulates the fetus and can alter its nonstress test results, however methods of stimulation differ according to sound frequency, amplitude and duration; furthermore studies found both significant effects and non-significant ones.
To test whether vibroacoustic stimulation could reduce time spent on NST monitor and increase patient satisfaction with this assessment tool, 64 primigravida patients were randomly allocated into groups that received either vibration or music during their 20-minute NSTs. The primary outcome was defined as the first adequate FHR acceleration during an NST, defined as two accelerations greater than 15bpm within three seconds period – this determined if vibroacoustic stimulation made any difference at all in patient satisfaction with this assessment tool.
Both halogen light stimulation and vibroacoustic stimulation groups were successful at changing 68% of non-reactive patterns into reactive patterns following stimulation; NST times were significantly shorter in vibroacoustic stimulation groups than control groups.
Results
Fetal heart rate monitoring is a critical diagnostic tool to assess fetal wellbeing. Unfortunately, however, results of such tests can often be affected by sleeping patterns of unborn infants, making interpretation more challenging. Vibroacoustic stimulation has been used as one method to waken unborn children and make them more responsive during NIPT testing sessions.
Vibro-acoustic stimulation uses a hand-held device which transmits brief sounds directly into the fetus’s environment. A study demonstrated that vibrations stimulated an early and more frequent response than in the control group, reaching maturity at around 32 weeks gestation.
Acoustic stimulation was found to increase positive predictability of NST results while simultaneously decreasing false nonreactive results, according to its authors. Unfortunately, however, acoustic stimulation could not serve as a standalone test for fetal well-being as it was not reliable enough to replace traditional NST or oxytocin challenge tests.
Further research should be undertaken to ascertain the optimal intensity, frequency, duration, and location of acoustic stimulation during NST. Further, the authors recommend conducting further randomized trials to explore the effectiveness, predictive accuracy, safety and perinatal outcomes associated with acoustic stimulation with cardiotocography or other tests of fetal wellbeing. They suggest that halogen light stimulation might be an acceptable alternative to acoustic stimulation for inducing an earlier and faster fetal heart rate response and hastening the timeframe required to achieve a nonstress test result. Halogen light stimulation differs from acoustic stimulation in that it does not involve dilation of the cervix or rupture of membranes, making it a safe procedure for pregnant women. Therefore, this should be considered an alternative to ultrasound or oxytocin challenge tests when in labor. These invasive techniques not only cause maternal discomfort but are also associated with high error rates and the potential risk of complications for mother and fetus. Furthermore, they cannot be performed on pregnancies of all gravidities; therefore a simpler, user-friendly test such as NST is essential to reduce labor induction times more quickly and easily integrated with other antenatal tests. Regardless of its limitations however, it offers great potential as part of an overall pregnancy testing regimen.
Conclusions
The vibroacoustic stimulation test may provide a simple and noninvasive means of assessing fetal wellbeing during labor when combined with reassuring cardiotocographic traces (CTG). It seems to increase frequency and duration of FHR accelerations while decreasing non-reactive tests; however its predictive value remains uncertain; further randomised trials should be conducted to establish its ideal intensity, frequency, duration vibration parameters as well as evaluate efficacy, reliability, safety in relation to cardiotocography or other measures of fetal wellbeing tests (CTG or otherwise).
Different tests exist to help detect babies experiencing difficulty during labor, but many require invasive procedures like taking a scalp blood sample or accessing the fetus via the cervix (East 2010). As these methods have their limitations, an easier test with less restrictions would be ideal.
Fetal vibroacoustic stimulation is one such test, consisting of a device that produces vibrations within the uterus that convert to electronic signals that can be measured by a computer and recorded as changes to beat-to-beat patterns of fetal heart rate (FHR) measurements, then analysed for any indications that they indicate problems for your unborn baby.
Recent clinical research compared the acoustic stimulation test with standard non-stress testing (NST). The trial involved 121 high risk and 95 low-risk women admitted to labour ward, randomly assigned either acoustic stimulation or sham stimulation before receiving CTG scanning and CTG recordings of the CTG results. Following these, women were asked whether their results indicated distress – defined as 2 or more FHR accelerations of 15bpm over 20 minutes, or 1 prolonged acceleration over 2 minutes as determined by FHR accelerations being 15bpm over 20 minutes or 1 prolonged acceleration over 2 minutes from baseline – during CTG monitoring of CTG recordings taken after testing completed CTG monitoring was completed and CTG recorded results were collected for analysis.
Acoustic stimulation was associated with significantly fewer non-reactive NSTs; however, reactive NSTs did not differ between groups. Mean baseline FHR was similar for both groups prior to stimulation; however, first FHR acceleration after acoustic stimulation was lower in the acoustic group than in the sham group and this difference wasn’t statistically significant.
