Fetal vibroacoustic stimulation (VAS) has been proposed as an easy and noninvasive test to evaluate fetal well-being during labor when CTG trace is nonreassuring; however, no randomised trial data exist that support its use.
Cochrane systematic reviews involving 12 trials involving 6822 women revealed that VAS significantly reduced the incidence of nonreactive antenatal cardiotocography tests; however, it did not evaluate maternal/fetal outcomes such as mode of birth and perinatal outcome (caesarean for fetal distress, Apgar scores at 1 and 5 minutes) during gestation or the effect of mock stimulation on fetal movements. No trials assessed this aspect.
Background
Fetal Vibroacoustic Stimulation is an efficient, cost-effective method of inducing fetal movement during labor. This involves placing an electronic hand-held device (the Fetal Vibroacoustic Stimulator) against the mother’s abdomen and sending short sounds into her womb that stimulate the baby’s innate recognition of its heart rate patterns and encouraging it to move in response. Nurses may use this tool to identify babies likely experiencing difficulty when an unrereassuring heart rate trace is recorded, and treat these infants appropriately.
Vibroacoustic stimulation of fetal development does not come without risks, with some women reporting severe fetal distress such as tightening of the nuchal cord, bradycardia and even caesarean section following exposure (Sherer 1988 and Sherer 1991). This could be related to stress caused by vibrational stimulation during gestation and vigorous movements stimulated by vibrational stimuli; however, other studies suggest low frequency vibratory stimuli are safe during gestation without having adverse side effects (see also Sherer 1991 and Sherer 1991).
This review of 12 randomised controlled trials with 6822 women showed that using fetal vibroacoustic stimulation reduced non-reactive cardiotocography tests by 9 trials; risk ratio 0.62 (95% confidence interval 0.46 to 0.81). However, caesarean section rates did not decline (two trials; risk ratio 0.80 (95% CI 0.43 to 1.51). There were also no reports of fetal distress or perinatal death in either group of studies utilizing vibroacoustic stimulation or standard non-vibroacoustic stimulation groups of studies.
This device may help enhance antepartum tests of the fetal heart rate, yet more research must be conducted into its intensity, frequency and duration as well as its effects on fetal movement and its safety and perinatal outcome. Ideally it would also be advantageous to test it alongside other tests of fetal wellbeing such as biophysical profiles; and create more reliable techniques for stimulating fetal movements prior to ultrasound examination.
Methods
Fetal vibroacoustic stimulation involves using a hand-held device above a pregnant woman?s abdomen to send brief sounds directly to her baby. If the child responds by altering its heart rate pattern, this indicates their healthiness. Additionally, it’s easy and cost-effective with minimal maintenance or training requirements required to operate the device.
At times of deep sleep, vibration of stimuli may awaken an unresponsive baby from deep slumber and allow additional testing such as ultrasound or counting fetal movements to take place without delay.
Review of 12 randomised trials involving 6822 women demonstrated that, when compared with mock or no stimulation, fetal vibroacoustic stimulation significantly reduced the incidence of non-reassuring cardiotocography tracing and increased the number of accelerations or transient tachycardia recorded in labor (risk ratio 0.62; 95% confidence interval 0.48 to 0.81); it also produced more movement responses in response to non-reassuring heart rate tracings (one trial; risk ratio 1.03, 95% confidence interval 1.05 to 1.12).
Studies comparing real with mock stimulation did not find a difference in terms of acidosis levels in either mother or baby, however they reported less acceleration following real VAS results than following mock stimuli (Skupski 2002a and Skupski 2002b).
Faulty fetal cardiotocography test results may be difficult to interpret due to variations in baby?s heart rate patterns that could arise for various reasons, with non-reassuring test results sometimes signifying genuine worries such as an increase in apical artery pulsation requiring an emergency caesarean section and raising risks such as blood loss, infection or haemorrhage for mother and baby alike. To improve accuracy during labor monitoring more reliably and ensure safe delivery is required. A non-invasive test such as using fetal vibroacoustic stimulator would work wonders!
Results
The Fetal Vibroacoustic Stimulator (VAS) is an electronic device that emits sound at a specific frequency. It is believed that this sound causes acoustic vibrations in the fetus that trigger a startle response with subsequent increases in FHR that provide assurance of fetal well-being and may delay labor onset.
Studies, such as a systematic review of randomised trials of VAS, have reported that VAS can effectively increase FHR acceleration, particularly following nonreassuring CTG tracers. Unfortunately, no data exists to show any correlation between this acceleration and mode of birth or other indicators of fetal wellbeing (such as umbilical arterial pH or Apgar score).
Fetal responses to VAS stimulation vary considerably and have been reported as excessive fetal movements, prolonged tachycardia, disorganization of behavioral states and state transitions not typically seen in healthy fetuses (Anyaegbunam 1994; Marden 1997). A large prospective randomised trial that compared VAS with an artificial larynx activated for five seconds above maternal symphysis to one without larynx revealed no differences in umbilical arterial pH or Apgar score five minutes post stimulation; likely due to an absence of control group and its impact on sensitivity to sound fetuses (Anyaegbunam 1994; Marden 1997).
Another study conducted on term women who presented with single pregnancies that were vertex presented, at early labor at cervical dilatation of less than 4 cm and had intact membranes with reactive fetal heart rate tracings showed no difference in rate of meconium-stained liquor production between groups receiving stimulation or placebo when having their membranes artificially ruptured within one hour (Zimmer 1996).
At present, there have been no randomised controlled trials to provide guidance for use of vibroacoustic stimuli in the evaluation of fetal well-being after nonreassuring CTGs during labor. Additional research should determine optimal intensity, frequency and duration for such stimuli; also testing whether combined stimulation improves cardiotocography in accurately predicting labor progress.
Conclusions
Vibroacoustic stimulation was found to bring biophysical profile scores back into normality for 67% of 81 women with abnormal or uncertain biophysical profile scores, without stillbirths or perinatal deaths, in this group. Fetal vibration also did not increase false-negative rates and may reduce unnecessary obstetric interventions.
Two trials (Marden 1997 and Sood 2007) compared vibroacoustic stimulation with mock stimulation, with mock stimulation consisting of placing an empty stimulator against women’s abdomen in an analogous fashion to real vibroacoustic stimulation but without activating it; due to no fetal movement response resulting from this fake vibrationoacoustic stimulus treatment allocation could not be hidden from women. Saracoglu 1998/Perez Delboy 2002 did not report results fully and neither authors could be reached directly, adding these studies did not alter this review’s overall conclusions in any significant way.
Six trials (Neal-Brown 1989; Newnham 1990; Marden 1997; Saini 2007; Sood 2007; and Saracoglu 1999) evaluated whether reactivation after non-reactive cardiotocography improved results and reduced repeat tests by up to 65% – all using standard neonatal monitoring protocols with second tests after 30 minutes if no qualifying acceleration had occurred and manual fetal reassuring stimulation with both hands simultaneously. Reactivation successfully improved non-reactive cardiotocography results and reduced need for repeat tests by up to 93%.
This review was updated in September 2013 to include two additional trials that did not report any outcomes documented in their methods section. Furthermore, it was observed that the original version included one trial that did not meet criteria for inclusion because it compared a vibroacoustic stimulus with only an FHR monitoring test; future trials evaluating vibroacoustic stimulators should include such comparison in order to increase quality evidence base.
