Vibroacoustic stimulation (VAS) is an easy test used during labor if a cardiotocograph trace (CTG) becomes nonreassuring; its primary aim is reducing unnecessary cesarean deliveries while preventing acidaemia in newborns.
VAS involves applying low-frequency sounds and vibrations directly to the uterus, prompting physiological responses such as increased heart rate acceleration and transient tachycardia in fetuses.
Background
Fetal vibroacoustic stimulation (VAS) is an easy, noninvasive procedure that utilizes vibration and sound waves from a device attached to a mother’s abdomen to emit vibration and sound, which activate resonant frequencies in cells and cause them to vibrate at an increased frequency, possibly startling the fetus into initiating FHR acceleration or transient tachycardia responses that provide assurances that all is well with her baby.
Vibration is essential to human functioning, from heartbeats and nerve impulses to recoil impulses. Resonant frequencies can be detected through tactile stimuli on the skin and activated within cells of the body by sonic vibrations. Not only can biosignals elicited by such vibration be measured objectively; subjective stress measures like Perceived Stress Scale and Electroencephalographic (EEG)/Heart Rate Variability (HRV) measures can also be measured concurrently – providing an invaluable insight into its effects on physiological and psychological stress effects of vibration on physiological and psychological stress effects of these waves of vibration.
An effective test to quickly and reliably identify non-reassuring cardiotocographic traces (CTGs) during labor is well recognized. Unfortunately, current methods for determining fetal distress like scalp blood sampling or electrocardiograph are invasive, cumbersome and sometimes uncertain of benefit; not to mention not always available or easily utilized.
Multiple studies have explored the use of sonic vibration as an assessment tool for non-reassuring CTG patterns during labor. One such randomized controlled trial design involved only women eligible based on an encouraging pattern on their CTG; Zimmer 1996 randomly assigned women in early labour at cervical dilatation less than 4 cm with intact membranes to receive either real or mock stimulus testing whether sonic stimulation would accelerate rupture of meconium-stained liquor (MSL), however no significant difference was discovered in MSL rupture rates between those receiving and those not receiving stimulation versus those not receiving stimulus; no significant differences were discovered between women receiving stimuli and those not receiving stimulation; no statistical difference could be identified in terms of rate of MSL rupture between those receiving and those not receiving stimulus in terms of rate MSL rupture between those receiving stimulus and those not receiving one;
Other trials have demonstrated that sonic vibration reduces the frequency of operative deliveries performed in response to non-reassuring CTG results without impacting rates of fetal acidaemia or neonatal morbidity. While these results aren’t statistically significant, they should still be taken with care in interpretation.
Methods
Fetal vibroacoustic stimulation (VAS) is a noninvasive technique whereby an artificial larynx or commercial acoustic stimulator is placed over the area of fetal head on mother’s abdomen and sound is emitted at a predetermined level for several seconds to induce startle reflex in fetus that leads to FHR acceleration or transient tachycardia, providing assurance of well-being of fetus while potentially avoiding unnecessary interventions such as cesarean section or intervention in labour.
A Cochrane systematic review of nine trials concluded that Fetal VAS significantly decreased the frequency of nonreactive CTG tracings during labor (risk ratio 0.62; 95% confidence interval 0.46-0.87) while simultaneously increasing palpable fetal movement in comparison with mock tests. Only one trial directly compared vaginal rupture following VAS with ruptured uterus in control group and reported no difference (Tan 2001).
Zimmer 1996 conducted a small prospective randomized study where term singleton fetuses in established labor were randomly assigned either an artificial larynx activated for five seconds above maternal symphysis or a sham stimulus; no significant differences were noted in heart rate tracings over one hour of follow up; umbilical arterial pH levels and Apgar scores also did not vary between groups.
Murphy 1993 conducted another randomised controlled trial involving singleton pregnancies with cervical dilatation of less than 4 cm and intact membranes admitted to a labour ward, receiving either an artificial larynx or sham stimulus in order to measure meconium-stained liquor at delivery time. No significant differences were noted in meconium-stained liquor at delivery; thus leading them to conclude that positive responses to artificial larynx stimulation did not predict successful outcomes.
Skupski 2002a and 2002b reviews covered two studies which estimated fetal scalp pH after exposure to VAS stimuli. Both reported an average scalp pH of 7.20 after experiencing reassurance responses to VAS stimuli; when false negative responses occurred the mean pH dropped below this value.
Results
Fetal vibroacoustic stimulation (VAS) is a noninvasive procedure in which vibrating sound waves from an electrolarynx are directed over an unborn baby’s abdomen. When exposed to this sound vibration, it causes startle reflex reactions in fetuses which result in transient acceleration or transient tachycardia on fetal heart rate monitors; these responses serve as assurance of their wellbeing without further interventions (Perez-Delboy 2002).
VAS has been studied through various trials with different design characteristics, with results that do not support its use to predict fetal compromise in labor. One such trial involved women at term with single pregnancies presenting in vertex position who had reassuring CTG tracings and intact membranes being randomly assigned either a three second electrical stimulation from an electrolarynx or no stimulation at all; if neither did spontaneous rupture within an hour artificial rupture was attempted and blood samples tested for meconium presence – 24 of 101 stimulated fetuses contained meconium which then had to be transferred directly into labour ward care whereas only three out of 101 sham stimulations did have meconium present within their amniotic fluid and were transferred whereas only three out of 101 stimulations had meconium present within their amniotic fluid while only three from 101 stimulation groups did!
In other trials, VAS results were defined as any acceleration or transient tachycardia of fetal heart rate recorded after application of stimulus. One of these studies defined a satisfactory fetal heart rate result as an increase in fetal heart rate from baseline of 15 beats per minute, lasting 60 seconds after stimulation. Conflicting studies employed different durations and intensities of stimuli stimuli; studies that produced no false positives used a three-second stimulus; those which showed negative reactions employed a longer stimulus (Edersheim 1987; Smith 1986). There also varied intensity of vibrational stimulation devices employed during trials with some studies employing higher stimulation than others (Edersheim 1987; Smith 1986).
One trial compared reactivity of fetal heart rate response to stimulus with that to mock test and found significant difference (nine trials; risk ratio 0.62; 95% confidence interval 0.42-0.84). Unfortunately, no trials assessing incidence of nonreactive CTG test were included and attempts at reaching authors proved futile; hence further randomised controlled trials are necessary to confirm these findings.
Conclusions
Vibroacoustic stimulation to induce fetal accelerations as a test of fetal well-being has become more and more widespread, though research on this technique remains limited. Studies using vibroacoustic stimulation demonstrate its efficacy at turning nonreactive heart rate tracings into reactive ones, but more evaluation is necessary regarding its effectiveness, safety and predictive reliability. It appears that using this test may help decrease unnecessary interventions during labor thus saving patients both time and expense.
Randomised controlled trials (RCTs) have examined the performance of fetal vibroacoustic stimulation against traditional nonstress tests for pregnant women admitted to labour wards. Nine of 10 trials revealed that using vibroacoustic stimulus reduced incidences of non-reactive CTG tests as well as shortening testing times before reaching an acceptable CTG result; its sensitivity is however lower than with movement monitors; CTG may still be necessary before an acceptable nonstress result can be reached.
Studies have reported that the type of acoustic stimulus used may influence results: trials that did not report false negatives used a three-second stimulator, while those reporting them utilized five second stimulators (Edersheim 1987; Smith 1986). Both intensity and duration likely played a part in shaping results, therefore further investigation should focus on finding optimal frequencies, strengths, positions and durations for vibroacoustic signals to ensure reliability and sensitivity during this test.
The Cochrane Pregnancy and Childbirth Group conducted a systematic review of published and unpublished randomised controlled trials comparing vibroacoustic stimulation with mock or no stimulation, or halogen light stimulation; this review was updated in May 1994.
