The Responses Of Term Fetuses To Different Vibratory Stimulations

BackgroundFreeman and Lee and colleague (1975) introduced the nonstress test to describe fetal heart rate acceleration in response to fetal movement as a sign of fetal health.Currently,nonstress testing is the most widely used primary testing method for assessment of fetal-being.Fetal heart rate is normally increased or decreased on a beat-to beat basis by autonomic influences mediated by sympathetic or parasympathetic impulses from brain-stem centers.Thus,fetal heart rate acceleration is believed to be an indication of fetal autonomic function.Beat-to-beat variability is also under control of the autonomic nervous system.Consequently,pathological loss of acceleration may be seen in conjunction with significantly decreased beat-to-beat variability of the fetal heart rate.Insufficient acceleration does not invariably predict fetal compromise.False-positive nonstress test is most commonly associated with sleep cycles,and may be due to central depression from medications.Because of fetal sleep-wake cycle,false positive nonstress test rate is high and test duration is long.various external stimuli are used for the purpose of waking the fetus.Vibroacoustic stimulation is the most widely used method for waking the fetus in clinic. Human fetal sensing system and central nervous system become more sophisticated and coordinated by the end of pregnancy. Fetal heart rate accelerationthat accompany fetal movements by vibroacoustic stimulation have been considered a sign of fetal well-being, and is intimately close to fetal maturation of the autonomic nervous system and the auditory system. Vibroacoustic stimulation associated with nonstress test offers benefits by increasing the positive-predictiv value and reducing the testing time. However, vibroacoustic stimulation has some potentially adverse effects. Fetal tachycardia, excessively fetal gross body movement and transforming confusion of fetal behavioral state are most common. Obstetricians have been promoted to search a safe and effective method of awaking fetus because of the deficiency in vibroacoustic stimulation.The purpose of the current study was to observe the characteristics of fetal movement and FHR in response to vibratory stimulus and to explore the safe and effectiveness of 3-second external vibratory stimulus on term fetuses.Material and MethodsOne hundred and eighty-three healthy pregnant women at Women's Hospital, School of Medical, Zhejiang University during May 2005 and May 2006 were recruited and randomly divided into two groups: vibratory stimulus I(96) and II(87),and set up the control group and the vibroacoustic stimulation group.Inclusion criteria were(1)Gestational age between 37 and 42 weeks, (2)No maternal medical and obstetric complication, (3)Single birth and cephalic presentation,(4)Nonstress test is positive (5)No uterine contraction. The frequency of vibratory stimulation is 30-80Hz. The vibration strength of vibratory stimulation I is 0.745m/s~2 , and vibratory stimulation II is 1.616m/s~2 ,and vibroacoustic stimulation is 5.56m/s~2 .Stimuli-evoked fetal movements and FHR responses were observed. The fetal heart rate tracing and maternal fetal movement felt was recorded for 10min without any interference, and taken as the control group. The fetus was received a 3s vibratory stimulation in quiet state, and recording the FHR tracing and maternal fetal movement felt.The fetus had not received a 3s vibroacoustic stimulation using artificial electronic larynx until FHR recovering to the baseline level and no maternal fetal movement felt, andobserving the fetal movement and FHR. The criterion of inducing fetal movement was maternal fetal movement felt in 2min after external stimulation. All the data were processed with SPSS 11.5 for windows. The value of P<0.05 was considered significant.Results1. The percentages of fetal movement were found to be 18.0%, 71.9% and 74.4% respectively control 1 group, vibratory stimulus I and vibratory stimulus II. The differences was significant between stimulus groups and control (P<0.001), however, there was no significant difference between vibratory stimulus I and II (P=0.665). The percentages of FHR acceleration in control, vibratory stimulus I, vibratory stimulus II were 22.2%, 84.4% and 97.7%, respectively. The difference was significant between control and vibratory stimulation (P<0.001). The percentages of FHR acceleration, FHR acceleration greater than 15bpm, FHR acceleration duration greater than 15s and the mean peak of FHR acceleration evoked by vibratory stimulus I were significantly different compared with those by vibratory stimulus II.2. The percentage of FHR acceleration evoked by the vibratory stimulus II was not significantly different from that by vibroacoustic stimulation (97.7% vs 100%, P=0.482). The percentages of fetal movement, FHR acceleration above 15bpm, FHR acceleration duration above 15s and the mean peak of FHR acceleration evoked by vibroacoustic stimulation were significantly higher than those by vibratory stimulus (P<0.001).3. The percentages of FHR tachycardia, FHR acceleration above 30bpm evoked by vibroacoustic stimulation and the recovery duration were significantly higher than those evoked by vibratory stimulus II.Conclusion1. It is suggested that the term fetuses respond to vibratory stimulus and vibratorystimulus is a relatively mild stimulation in awaking fetus compared with vibroacoustic stimulus. Vibratory stimulus doesn't conduced to transforming confusion of fetal behavioral state.2. It is also indicated that the fetal responsiveness depends on the intensity of vibrating.3. The effectiveness of the vibratory is lower than that of vibroacoustic stimulation. The continuous study is necessary to explore the proper intensity of vibratory stimulus.