Effect of vibroacoustic stimulation on fetal heart rate during nonstress tests

Vibroacoustic stimulation of the fetus with an artificial larynx has been shown to cause an immediate fetal startle response, and increase in fetal activity and an increase in fetal heart rate (FHR) accelerations that persist for 10 to 60 minutes or longer. This randomized prospective study compared the two most common techniques of vibroacoustic stimulation tests (VST): one 5-second stimulus and four intermittent 3-second stimuli, with a control group of traditional nonstress tests (NST). The FHR tracings in the two experimental groups had more accelerations, and were of greater amplitude and duration, than in the control group. The length of tine required to obtain a reactive NST was almost 15 minutes shorter for VSTs than for NSTs. VSTs elicited a heightened reactivity that persisted beyond the observation period in 66% of the experimental group subjects. Of this 66%, another two-thirds of the subjects displayed prolonged, marked accelerations. If the baseline FHR is not determined prior to stimulus, these periods of prolonged accelerations may necessitate additional observation time.