Mobile Phone Application Versus Artificial Larynx for the Purpose of Vibroacoustic Stimulation in Fetal Non-stress Testing

There are multiple proposed methods of assessing fetal well-being in utero. One such method which has been well accepted for fetal surveillance is the non-stress test (NST), during which the fetal heart rate is monitored with cardiotocography for a period of 20-40 minutes, depending on the time it takes to confirm reactivity. Reactivity is confirmed when there is an increase in fetal heart rate, called an acceleration, the presence of which has been shown to reliably predict the absence of fetal metabolic acidemia. Non-stress testing assumes that with fetal movements, a nonacidotic, neurologically normal fetus will have an acceleration. Fetuses develop reactive tracings between 28 and 32 weeks, with 80% of normally developed fetuses displaying reactive fetal heart rate tracings by 32 weeks.

Two limitations to fetal non-stress testing have been recognized. Due to fetal sleeping periods during which movement is decreased, tests may be falsely interpreted as non-reactive, which can lead to unnecessary intervention such as prolonged monitoring, additional testing such as the biophysical profile, induction of labor, or delivery by cesarean section. In addition, if a tracing is not found to be reactive after 20 minutes, extending the testing time to 40 minutes may eventually produce a reactive tracing. This, however, is taxing on antepartum unit equipment, staff resources, and can create additional patient anxiety.

Vibroacoustic stimulation (VAS) is a method used to arouse the fetus from a state of fetal sleep to fetal activity, reflected as an increased heart rate, called an acceleration, on electronic fetal monitoring. Until now, VAS has been emitted from an artificial larynx called a vibroacoustic stimulator which is held on or above the pregnant woman's abdomen to transmit audible frequency to the fetus. For example, one such device, the Corometrics-146, sends out a vibratory sound stimulus with a frequency of approximately 75 Hz and an intensity of approximately 74db for a duration of 3 seconds. A recent Cochrane Review concluded that the use of VAS in antenatal testing reduced both incidence of non-reactive fetal heart rate tracing and overall testing time. This has implications for clinical use, allowing equipment and staff resources to be available to test more women with less inconvenience for the patient.

Many antenatal testing units only have one VAS device, and these can cost up to $500. In addition, the Cochrane Review highlighted a need for further research into the realm of vibroacoustic stimulation in obstetric care, with particular attention to optimum intensity, frequency, duration, and position of the stimulation. Current devices, however, only come with one set frequency and intensity, making studies outlining optimal response rates with varying acoustic parameters impossible.

The investigators believe that the development of a mobile device (such as a phone) application which produces a vibroacoustic sound wave similar to that produced by the artificial larynx can address the aforementioned issues. This would make vibroacoustic stimulation more available, and allow for further testing with differing parameters such as frequencies, intensities, and duration of stimulation, which are easily adjusted with an application, but not an artificial larynx.