Fetal vibroacoustic stimulation (FVS) is a non-invasive device used to evaluate fetal well-being. The aim is to elicit the startle reflex, leading to heart rate acceleration.
Previous studies have demonstrated that fetal VAS is clinically safe, and may improve the efficacy of tests conducted during labor on pregnant woman’s fetuses, such as scalp blood sampling (for pH and lactate measurements) and pulse oximetry.
The device
Fetal vibroacoustic stimulation (FVS) is a simple device consisting of an artificial larynx placed against a mother’s abdomen over the area of her fetus, emitting vibrating sounds for several seconds to trigger startle reflexes in fetuses and result in FHR acceleration or transient tachycardia – changes which provide assurance of well-being while potentially eliminating further tests.
VAS depends on duration, intensity and type of stimulus being administered. Frequency, pitch and duration must be tailored specifically to the fetus and pregnancy; shorter stimuli tend to produce more frequent and intense responses than longer ones. Other factors which might influence response to VAS include prenatal habituation (Edersheim 1987; Smith 1986), gestational age, fetal physiology as well as maternal medication such as steroids which reduce FHR variability and reactivity (Rotmensch 1999).
Women carrying live singleton pregnancies that were cephalic presented and had non-reassuring CTG readings were randomly allocated either VAS activation or mock stimulation – whereby the device would remain placed against their abdomen but would remain silently stimulated – as part of a randomised trial. An investigator blind to each woman’s treatment allocation evaluated her fetal heart rate tracing in the five minutes following stimulus for acceleration, acceleration followed by deceleration or no response at all.
Results demonstrated that fetal vibroacoustic stimulation significantly enhanced the performance of standard tests for assessing fetal wellbeing, yet more research needs to be conducted regarding optimal intensity, duration and location of stimulation, safety issues associated with its use with cardiotocography and other tests of fetal well-being, perinatal outcomes as a result of its combination and definition of optimal metrics for evaluating viability – such as threshold of FHR activity – to help clinicians make decisions about initiating delivery as soon as possible while potentially helping avoid overtreatment of low risk pregnancies altogether.
The sound
Fetal vibroacoustic stimulation uses a handheld electronic device that emits brief sounds into the mother’s abdomen to stimulate her baby and cause it to move vigorously. Although usually only used for brief moments at a time, its impact could make a significant difference for some babies – some researchers even speculate that its response may be more reliable than using heart rate monitors during labor for women who spend long stretches sleeping during labour.
Randomised trials have demonstrated that vibroacoustic stimulation can significantly improve the accuracy of fetal heart rate testing, yet data on its impact on well-being and perinatal outcomes remain scarce. Each study has varied in intensity, frequency, duration and location of stimulation; plus whether or not other tests of wellbeing such as biophysical profiles were also administered at the same time.
Studies comparing low-risk laboring women who had non-reassuring CTG scans with those who did not revealed no differences in time to next reassuring heart-rate pattern or risk of distress during labor (caesarean section for fetal distress or umbilical arterial pH less than 7.20). However, this trial was observational, with little evidence that such testing is useful in identifying which women require immediate delivery; more invasive and complex techniques like fetal scalp blood sampling or pulse oximetry require dilation of the cervix, rupture of membranes, access to presenting part – available only to hospitals equipped with necessary equipment.
The effect on the fetus
Fetal Vibroacoustic Stimulation (VAS) involves placing a handheld electronic device on a mother’s abdomen and emitting brief sounds in order to stimulate her unborn fetus. This technique relies on invoking startle reflex and initiating acceleration or deceleration of heart rate within seconds as evidence that this test has been validly performed, offering peace of mind that there will not be further investigations required in an unfavorable outcome.
However, the effect of fetal VAS tests are still not fully understood; further research must be conducted in order to ascertain if frequency, intensity and duration should be modified to achieve optimal responses from fetuses. Achieved effectiveness may also be impaired by some fetuses not responding at all – this may indicate congenital rubella or hearing impairment in some instances.
Few studies have assessed the effect of fetal VASs on various antepartum outcomes, such as mode of birth (caesarean section for distress), operative delivery and tests of fetal wellbeing like umbilical arterial pH or scalp lactate measurement. These studies found no significant difference in terms of caesarean section or other operative delivery rates or number of fetuses with umbilical arterial pH values below 7.20 or an Apgar score less than seven at five minutes between groups exposed to or not exposed.
Other trials have confirmed the effectiveness of VASs in detecting nonreassuring cardiotocograph (CTG) traces during labor. When an FHR acceleration or transient tachycardia occures after VAS, it could provide assurance of fetal wellbeing thereby eliminating further investigation like taking blood samples or performing fetoscopie (Lin 2001).
Randomised controlled trials have yet to be conducted to assess the clinical effectiveness of using fetal VAS when there is evidence of nonreassuring CTG readings. More trials should be designed in this area as simple yet inexpensive tests for fetal wellbeing may prove useful when more sophisticated devices or procedures cannot be easily available.
The test
Fetal vibroacoustic stimulation uses a portable device to send brief sounds directly into an unborn baby’s abdomen and wake it up so a heart rate trace can be collected and checked for signs of distress. The goal is to develop an easy, affordable test which can be used during labor to identify babies having trouble; while normal heart rates may mask distress in pregnant mothers (Gonzalez-Gonzalez 2009; Leader 1984).
Cardiotocograph (CTG) tracings that do not reassure are found in approximately 18% of labors and can lead to unnecessary operative interventions. Others can help assess fetal well-being and limit unnecessary interventions, including scalp blood sampling for pH with acidosis defined as any value less than 7.20 (Ingemarsson 1989); estimation of antenatal lactic acidosis where intervention should occur when values exceed 4.8 mmol/L (East 2008); oxygen saturation monitoring and electrocardiogram waveform analysis. However, these tests do have limitations. Fetal scalp blood sampling requires dilation of the cervix and access to the presenting part; while fetal oximetry sensors require rupture of membranes with at least 2 cm cervical dilatation (Schenck and Slater 2010).
Numerous studies have explored the efficacy of fetal vibroacoustic stimulation to wake the fetus for heart rate testing during low-risk laboring women who present with nonreassuring CTG tracings. Ratcliffe (2000) conducted an interesting small-scale trial involving 40 low-risk labouring women randomly assigned either five second VAS stimulation or placebo and later assessed by an investigator blind to group allocation; their responses to each stimulus were then assessed later by another blinded investigator, with results showing normal responses being linked with lower risks (caesarean for distress, Apgar scores at 1 and 5 minutes compared with controls).
Another trial conducted in 2012 involved 400 women in the latent phase of labor who were randomly allocated either five second VAS stimulation or no stimulation at all, in random order. Researchers discovered that while VAS reduced non-reactive fetal heart rate tracings, response was not as reliable in high risk labouring women (one trial; response rate (RR) 0.62; 95% confidence interval (0.46 to 0.81); this likely due to differing fetal habituation to vibrationoacoustic stimulation between low risk and high risk pregnancies (RR 0.62; 95%CI 0.46 to 0.81). This difference may explain why high risk pregnancies respond differently; possibly due to differences between high risk pregancies (RR 0.62 95%CI 0.46 to 0.81).
