several studies [ 14 ] showed that fetal sleeping periods can lead to falsely nonreactive tests , therefore , increasing the risk of unnecessary obstetric intervention . attempts to arouse the fetus while is sleeping or in a rest - activity cycle include a change in maternal position , physical activity , maternal glucose ingestion , sound stimulation , light stimulation , and manual fetal manipulation . however , the only stimuli that have consistently evoked responses in normal fetuses are acoustic and vibrotactile ones . fetal vibroacoustic stimulation was first noted in 1947 by bernard and sontag   who observed that the fetal heart rate accelerated after acoustic stimulation   correlated fetal movements with fetal well - being .   in modern obstetrics , fetal vibroacoustic stimulation uses an artificial larynx placed on the mother 's abdomen over the fetal head region . this is expected to induce a startle reflex in the fetus , with subsequent fetal movement and fhr acceleration . many authors have observed an increase in the frequency and size of accelerations after vibroacoustic stimulation during low fhr [ 810 ] . it is hypothesized that fhr acceleration following vas provides reassurance of fetal well - being , obviating the need for further intervention [ 11 , 12 ] . moreover , acoustic stimulation of the fetus has been suggested to improve the efficiency of antepartum fetal heart rate testing [ 13 , 14 ] . some authors in nonrandomized studies [ 1517 ] have reported success using fetal vibroacoustic stimulation to improve the efficiency of antepartum fetal heart rate testing without changing the predictive reliability of the tests . animal studies have revealed that fetal responses to extrauterine sound depend on the peripheral and central components of the fetus 's auditory system . in lambs with bilateral cochlear ablation , no reactions towards vibroacoustic stimulation could be recorded , not even at high intensities , indicating that the auditory apparatus is necessary for fetal heart rate ( fhr ) and fetal movement ( fm ) responses in this animal model [ 18 , 19 ] . some authors have conducted studies to evaluate the existence of possible adverse effects caused by the vas . they concluded that stimulation with the electronic artificial larynx induced excessive fetal movements , a prolonged tachycardia , nonphysiological state changes , and a disorganisation and change in the distribution of fetal behavioural states , therefore , they did not recommend its use routinely . on the other hand , in more recent studies , vibroacoustic stimulation has been demonstrated not to cause any cochlea damage in the fetus , and they have shown that the intensity of sound and vibration are 4000 times less than in the amniotic fluid , compared to that produced in air . moreover , an audiometric and tympanic screening performed on children who had been exposed to vibroacoustic stimulation in utero showed the absence of hearing loss . positive ( reactive ) vas tests appeared comparable to standard reactive nonstress tests ( nsts ) . perinatal outcomes following positive vas appear to be similar to those associated with reactive nsts , while testing time was shorter with vas . since computerized cardiotocography ( cctg ) provides an objective analysis of fhr , our aim of this study was to evaluate fhr before and after vas , in both low- and high - risk pregnancies . moreover , the cctg parameters short - term variability ( stv ) and approximate entropy ( apen ) are considered more closely related to the actual well - being of the fetus , compared to the parameters detectable with the traditional reading . therefore , in the present study , we decided to consider especially them , in an attempt to provide an even more reliable evidence of fetal well - being . a retrospective study was performed in 563 pregnants referring to the  federico ii  university medical school in naples , between november 2007 and october 2010 . the study was approved by ethics committee of the university and all participants gave their written informed consent . patients were selected according to some inclusion criteria : ( a ) availability of data from maternal medical history , ultrasound parameters , amniotic fluid production , doppler velocimetry on umbilical cord ; ( b ) computerized cardiotocographic ( cctg ) analysis from antepartum fhr tracings before and after fetal vibroacoustic stimulation ; ( c ) availability of data about the newborn baby ( kind of delivery , sex , weight , apgar score , malformations at birth , and the possible need of admission in neonatal intensive care ) . moreover , each fhr tracing had to contain ( a ) a period of 10 minutes of continuous quiet before the stimulus was given and ( b ) a period of at least 10 minutes after the stimulus was given , both of acceptable quality , according to the analysis criteria . exclusion criteria for final analysis were ( a ) fhr tracings before the twenty - eighth week of pregnancy , for which the international federation of gynecology and obstetrics does not provide any standard value for the cctg parameters ; ( b ) patient withdrawal from the study and/or unavailability of followup . total population included 216 patients , divided in two groups : low- and high - risk pregnancies . high - risk  pregnancy is generally thought of as the one in which the mother or the developing fetus has a condition that places one or both of them at a higher - than - normal risk for complications , either during the pregnancy ( antepartum ) , during delivery ( intrapartum ) , or following the birth ( postpartum ) . instead , low - risk pregnancy is defined for the absence of significant past medical or obstetric history and of fetal anomalies [ 27 , 28 ] . the characteristics of the population of this study are shown in tables 1 and 2 . all patients underwent the cctg nonstress test in the same conditions , that is , in the same rooms , between 8.30 and 12.00 am and with the women in semirecumbent position . fetal heart rate was recorded using a computer - assisted equipment ( corometrics 170 , general electrics ) . each fhr tracing was analyzed with the sea 2ctg2 system that calculated the short term variability throughout this expression . approximate entropy ( apen ) derives from formulas suggested to estimate the kolmogorov entropy of a process represented by a time series . it is a measurement of the conditional probability that two vectors that are close to each other for m points will remain close at the next point [ 29 , 30 ] . baseline fhr , accelerations , decelerations , long - term irregularity ( lti ) , approximate entropy ( apen ) , and fetal movements ( fms ) were calculated for defined observational periods before and after vas ( figure 1 ) . the cctg parameters short term variability ( stv ) and the approximate entropy ( apen ) were analyzed in reference to the standard values proposed by arduini and associates , for each gestational week [ 31 , 32 ] . the aim of the study was to evaluate the changes in cctg analysis before and after an external vibratory acoustic stimulus over the fetal vertex , by an artificial larynx , was produced . the stimulus consisted of 5 seconds of vibration applied to the maternal abdomen , while the fetus is sleeping , in accordance with the literature and the clinical indications . adverse perinatal outcome included one or more of the following criteria : small for gestational age infant , fetal acidaemia at delivery , 5-minute apgar score < 7 , respiratory distress syndrome , perinatal death , hypoxic ischemic encephalopathy , malformations , or other injuries requiring neonatal intensive care . statistical analysis was performed using chi - square test for independent samples , and the two - tailed student 's t - test . a p value < 0.05 was considered to be statistically significant . fetal vibroacoustic stimulation was performed on 121 high - risk and 95 low - risk pregnancies , while their fhr parameters were monitored and recorded by cctg analysis . significant fetal heart accelerations ( p < 0.05 ) appeared 10 seconds after the stimulation both in low- and high - risk pregnancies . as many authors argued [ 34 , 35 ] , this event can be related to the sudden change of the fetal state from quiet sleep to activity / weakeness . in each group of patients , fhr after vas remained almost unmodified , as differences were not statistically significant ( tables 3 and 4 ) . we considered positive a vas test if the stimulation is follwed by the increase of stv and the decrease of apen . high - risk pregnant women showed an increase of stv in 76 cases ( 62.8% ) , a decrease of apen in 67 cases ( 55.37% ) , and a global positive test in 39 cases ( 32.23% ) , including 31 ( 38.7% ) with a favorable perinatal outcome and 8 ( 20% ) with an unfavorable outcome at birth . in low - risk patients , we found an increase of stv in 72 cases ( 75.79% ) and a decrease of apen in 49 cases ( 51.57% ) , with a positive vas test in 36 cases ( 37.90% ) , including 32 ( 37.20% ) with a favorable perinatal outcome and 4 ( 44.4% ) with an unfavorable outcome . 50 babies ( 23.4% ) showed adverse conditions at birth : 41 ( 33.88% ) in high - risk pregnancies and 9 ( 9.47% ) in low - risk pregnant women . fetal movements significantly increased after vas in both the groups ( p = 0.001 ) . only in the high - risk pregnancies , the increase of stv and the decrease of apen after vas were significantly associated with favorable perinatal outcomes . moreover , chi - square test (  ) showed that only between 28th and 36th week of gestation and only when stv increased more than 3.5  ms and apen decreased more than 0.5 , the relationship with the perinatal outcome was statistically significant ( table 5 ) . the radar graph in figure 2 summarizes the cutoff values found , offering an immediate visual evaluation about the different relationships between stv and apen with the perinatal outcome , respectively . sensitivity was found to be 37% in low - risk pregnancies , and 39% in high - risk pregnancies . instead , specificity was 55% and 80% , respectively , in low- and high - risk pregnancies ( table 6 ) . vibroacoustic stimulation offers a unique opportunity to assess how the fetus responds to the external environment . a metaanalysis performed by skupski et al .   of reports on intrapartum fetal stimulation tests , analyzed different types of fetal stimulation , including scalp puncture for ph testing , the use of an allis clamp to pinch the fetal scalp , digital stroking of the fetal scalp , and vas . moreover , methods like fetal scalp blood sampling , fetal electrocardiography , and fetal pulse oximetry are invasive , their benefit can be uncertain , and are not widely available or used routinely . fetal vas is a noninvasive technique that does not involve puncture of the amniotic membrane , making it particularly useful in antepartum primary care . demonstrated that after vibroacoustic stimulation occurs , a statistically significant increase in basal fhr ( 115  bpm ) during periods of low fhr reactivity and a statistically significant increase in the number of fetal movements happen . bartnicki and dudenhausen   argued that after an acoustic stimulus , fetal movements cause a partial occlusion of the umbilical cord that could be responsible for the reduced fetal pressure and acute hypoxia that would in turn lead to a reflex stimulation of the autonomous nervous system . from the current literature , we know that changes in the beat - to - beat interval are mainly mediated by the regulation mechanisms and by the synergic action elicited by the autonomic nervous system ( ans ) activity , through its sympathetic and parasympathetic branches . as the activities of the ans are closely related with those of the central nervous system , and their contributions are usually quantified through parameters obtained from the spectral analysis of hrv signal [ 39 , 40 ] therefore , the analysis of fhr signals of fetuses submitted to a vibroacoustic stimulation may provide information about the functional aspects of the autonomic nervous system and of the central nervous system , making possible diagnosis , prognosis , and monitoring of major pathologies of the central or peripheral nervous system . cardiotocography ( ctg ) provides a noninvasive monitoring of fetal heart rate ( fhr ) . it is the most popular analysis method in clinical practice allowing the recording and the analysis of fhr , via ultrasonographic techniques . indices obtained from the cardiotocographic fhr signals have been adopted to study the autonomic nervous system activity of the fetus during the intrauterine life both in physiologic and pathologic conditions . .   showed a significant relationship between fhr linear and nonlinear parameters and fetal state , as linear indices increased significantly with rising fetal activity whereas the opposite occurred with nonlinear indices . used the computerized cardiotocography to assess the fetal heart rate in healthy term fetuses subjected to vas . he found a statistically significant increase not only for fm and number of fhr accelerations , but also in stv . moreover , they showed that the response was influenced by fetus ' behavioural state at the time of the stimulus . interestingly , a study   performing a linear and nonlinear fetal heart rate analysis in normal and acidemic fetuses before delivery showed that , with the progression of the labor , a significant increase in linear frequency domain indices and a significant decrease in nonlinear indices became evident . bernardes et al .   hypothesized that fetal sex differences could affect the fhr response to vas . from our results , we can state that fetuses respond to vibroacoustic stimulation accelerating their fhr . however , the analysis of stv and apen after vas shows that the difference is significant only in high - risk pregnancies . this result make these cctg parameters a useful tool in the evaluation of fetal neuronal response . especially noteworthy is the significant association of a positive vas test with perinatal conditions in high - risk pregnancy , between the 28th and the 36th week of gestation . this result could be explained with the development of the auditory apparatus , which occurs in that period . indeed , vibroacoustic stimulation elicits a neural response in the fetus , from the auditory sensory system to the central level , whose response is able to trigger the autonomic mechanisms responsible for the regulation of the fhr . the identification of cutoff values of stv and apen for the association with the perinatal outcome in high - risk pregnancies could provide useful tools for better understanding and management of pathophysiological neural development of the fetus . in addition , the sensitivity and the specificity of the results of the present study were analyzed . the low sensitivity values found , both in low- and high - risk pregnancies , show that vas test was positive only in a small number of patients delivering healthy babies . on the other hand , the difference between the two groups , found in the specificity analysis , is very meaningful . the high percentage of specificity found for high - risk pregnancies ( 80% ) suggested that a negative vas test was observed in a large percentage of patients whose babies show negative outcome at birth , in contrast with the lower specificity ( 55% ) found in low - risk pregnancies . this is to confirm the importance of the use of vas test inside the cctg analysis as a predictive tool of conditions at birth . moreover , our findings can be seen as a confirmation of what gonalves et al .   hypothesized , that is , an increase in the autonomic nervous system activity and a decrease in the central nervous system activity , when a stress condition occurs ( the final minutes of labor in gonalves ' study and the vibroacoustic stimulation in the present study ) , even if we found differences between low- and high - risk pregnancies . differently from previous studies [ 4 , 5 , 810 ] , our aim was not to study only the role of vas in relationship with the modifications of fetal biophysical profile , but we wanted to analyze how its cardiotocographic effect relates with the perinatal outcome , in order to verify its predictive value . moreover , the present study tried to find , for the first time , values of stv and apen from which it is possible to define a statistical significance of the relationship found . further randomized trials are surely needed to determine reliable quantitative indices that could help a nearly recognition of central and peripheral pathologies . for this purpose
the aim of this study was to evaluate the impact of vibroacoustic stimulation ( vas ) on computerized cardiotocography short - term variability ( stv ) and approximate entropy ( apen ) in both low- and high - risk pregnancies . 
 vas was performed on 121 high- and 95 low - risk pregnancies after 10 minutes of continuous quiet , while their fhr parameters were monitored and recorded by cctg analysis . 
 fetal heart rate was recorded using a computer - assisted equipment . 
 baseline fhr , accelerations , decelerations , stv , long - term irregularity ( lti ) , apen , and fetal movements ( fms ) were calculated for defined observational periods before vas and after 10 minutes . 
 data were also investigated in relationship with the perinatal outcome . in each group of patients , 
 fhr after vas remained almost unmodified . 
 fetal movements significantly increased after vas in both groups . 
 results show that only in the high - risk pregnancies , the increase of stv and the decrease of apen after vas were significantly associated with favorable perinatal outcomes .