Vibroacoustic stimulation (VAS) is a noninvasive test used to quickly and cost-effectively assess fetal well-being during nonstress tests or biophysical profile assessments. VAS allows pregnant mothers to quickly waken sleeping or inactive babies during nonstress tests or biophysical profile assessments and is quick and cost-effective way to assess well-being during gestation.
Some studies have reported false negative results, with their sensitivity dependent upon stimulator intensity and duration (Polzin 1988; Ingemarsson 1989). Unfortunately, such outcomes have proven disillusioning.
What is Vibroacoustic Stimulation (VAS)?
Vibroacoustic stimulation (VAS) uses sound and vibration to awaken and elicit responses from fetuses, used as an assessment method when non-stress tests or biophysical profiles don’t provide enough comforting information. The goal is to minimize unnecessary and costly testing by eliminating unnecessary needless tests in babies whose well-being may not require it; unfortunately this test doesn’t always detect distress, especially if an infant is asleep or inactive during testing.
To perform VAS, an artificial larynx device (vibratory stimuli) must be placed over an area of fetal movement on the abdomen, followed by applying vibratory stimulus for several seconds. This should cause movement from both mother and fetus as well as contraction of uterine muscles which will stimulate FHR to increase. A positive reaction indicates that oxygen is reaching all parts of fetus’ development.
Negative fetal stimuli generally indicate that either the fetus is not in an ideal position or that its uterine muscles are contracting incorrectly, leading to complications for both mother and fetus. A negative VAS response has been used in studies as an accurate way of predicting congenital rubella – one study even demonstrated this with three cases predicted accurately using negative VAS responses!
Women in low risk labour with cephalic presentation of singleton fetuses, no heavy meconium, full cervical dilatation and an encouraging CTG were randomly allocated either the fetal vibratory stimulus or sham instrument for labor stimulation. Fetal heart rates were monitored before and after stimulus presentation by an investigator blind to group allocation who assessed its effects; no differences in acceleration/deceleration rates, umbilical arterial pH or Apgar scores at 1 and 5 minutes were detected between groups.
Studies on the diagnostic accuracy of VAS have generally proven accurate (Edersheim 1987; Smith 1986). Most reports observed an acceleration of FHR following stimulus, although some reports described non-reactive responses or FHR not increasing after stimulation (Polzin 1988; Ingemarsson 1989; Lin 2001). To prevent false negatives, authors of these studies recommended a 3-second stimuli and higher intensity sound from artificial larynxes as recommended by these studies.
How is VAS used?
Numerous prenatal medical tests use sound and vibration to wake a sleeping baby or assess its well-being, such as fetal non-stress tests (NST) or biophysical profile assessments using both. A NST or biophysical profile assessment typically employs sound-vibration stimuli to prompt movement during an NST/biophysical profile assessment test; when this happens, heart rates typically increase indicating that all is well. If no movement occurs then vibroacoustic stimulation tests (VASs) have been devised in order to rouse unresponsive babies from restless sleep during testing sessions.
Under this method, a hand-held electronic device sends short sound pulses through a mother’s abdomen to her fetus, in hopes that these short pulses make its eardrum vibrate, stimulating nerve impulses and producing an increase in heart rate that can be recorded by healthcare providers.
This technique has been suggested as an alternative to more invasive approaches such as scalp blood sampling and fetal oximetry, which require dilation of the cervix and access to the fetal presenting part for testing purposes. Unfortunately, test results can often be difficult to interpret when the fetus is asleep or not active enough.
The Cochrane Pregnancy and Childbirth Group conducted a systematic review of randomised trials combining vibroacoustic stimulation (VAS) with tests of fetal wellbeing, with nine trials including 6822 women reporting significant reduction in non-reactive CTG trace in labor (risk ratio 0.62; 95% confidence interval: 0.486 to 0.849).
However, the authors of this review have highlighted that VAS data can be highly questionable due to its ordinal format – not interval – and evidence showing its intervals are not equally spaced. A paper published over two decades ago in British Medical Journal made this point by noting ‘although visual analogue scale has been converted to cm or per cent for convenience purposes, it cannot be considered an interval measure and must therefore remain ordinal only’ (3, 28). Another subsequent study demonstrated this point by showing VAS clustered into seven distinct groups thus invalid interval measure (30), yet its widespread usage persists due to its simplicity and convenience (30). However despite these concerns the visual analogue scale remains popular due to its simplicity and convenience in use across many medical specialties across industries and professions alike (30). Nonetheless, however.
Why is VAS used?
VAS allows physicians to stimulate physical activity in the fetus by applying low-frequency sound and vibration stimulation directly to a mother’s abdomen. The response from this stimulation provides key insight into fetal wellbeing, such as potential distress or health concerns in later gestations stages. VAS is often employed for monitoring high-risk pregnancies to enable early diagnosis.
Studies of fetal responses to VASs have demonstrated its superior sensitivity compared to ordinal scales (e.g.’mild’,’moderate’,’severe’ or ‘agonizing’). It should be remembered however, that VAS does not provide quantitative measurements and must therefore only be used along with objective measures such as contraction tracking (CTG) or biophysical profiles for best results.
Many maternity hospitals employ VAS tests during prenatal assessments of babies’ health. VAS can be helpful when their heart rate patterns do not reassure doctors, raising concern that there may be difficulties for the baby – and in such instances a simple test such as VAS can offer peace of mind about his or her wellbeing, potentially avoiding unnecessary intervention.
Some researchers have raised safety concerns regarding fetal vibroacoustic stimulation, including stress induction or increased intrauterine sound levels that might damage cochlea. However, long-term follow-up of infants exposed to such stimulations have not demonstrated any hearing or neurological impairment by two days old or four years of age.
Concerns surrounding pain VAS include its need for more space than most, and users becoming confused when converting centimeters or percentage. But studies have demonstrated that it’s more sensitive to subtle changes than numeric rating scales (NRSs), easier than standard ordinal scales to understand, with excellent test-retest reliability; also easy for non-literate patients and easy administration and scoring; used extensively in outcomes research as part of medical questionnaires like the WOMAC pain score and Chalder Fatigue Scale among others.
What are the risks of VAS?
Although many tests exist to assess a baby’s health and well-being, their assessment may be complicated by sleep patterns. Furthermore, even “quiet” infants on heart-rate monitors may still be experiencing difficulties; extra tests such as fetal scalp blood sampling or electrocardiograph provide crucial data but can be costly or unavailable when needed – hence there exists a need for noninvasive yet quick evaluation of a child once a nonreassuring cardiotocographic trace (CTG) is recorded during labor.
Vibroacoustic stimulation (VAS) is a noninvasive ultrasound technique. Using brief sounds and vibration to stimulate fetuses, VAS may be conducted as part of nonstress tests or biophysical profile assessments. Different acoustic stimuli have been employed; studies comparing VAS against mock stimuli (placing vibroacoustic device on pregnant woman’s abdomen without activating soundwave) has also been performed.
Animal studies have demonstrated that an acoustic stimulus can induce a startle response and an acceleration in fetal heart rate; however, its exact sensitivity and specificity remain undefined due to various factors that could potentially alter this result – frequency duration intensity stimulus choice etc.
As there is insufficient evidence from randomised controlled trials to support routine use of VAS as a prenatal assessment method, using VAS is currently unadvisable. A Cochrane systematic review (Tan 2001) demonstrated that VAS reduced non-reassuring CTGs significantly and shortened testing time; however only one trial included examination of palpable fetal movement following real or mock stimulation.
Studies assessing the accuracy of VAS to predict fetal scalp pH have yielded mixed results, with some studies reporting high correlations (Edersheim 1987; Irion 1996; Lin 2001). There was also an unexpectedly large proportion of false negatives where stimuli did not prompt an acceleration in FHR but after evaluation fetal scalp pH showed levels higher than 7.20.
