Nine pregnant women near term, with medical indications for internal monitoring during labor, were studied to determine the transmission of sound across maternal soft tissues during vibroacoustic stimulation with an electronic artificial larynx. A miniaturized hydrophone was placed transcervically at the level of the fetal neck under ultrasound guidance. Intrauterine sound pressure levels were measured before and during stimulation applied at 1 and 0 cm from the surface of the maternal abdomen. When the electronic artificial larynx was placed at a distance of 1 cm from the maternal abdomen, the intrauterine sound pressure level averaged 75 dB at frequencies between 0-5000 Hz. However, when the electronic artificial larynx was firmly applied on the maternal abdomen, intrauterine sound pressure levels were enhanced by more than 20 dB (P < .001) and averaged 95 dB at all frequencies between 87-20,000 Hz. Baseline intrauterine sound pressure levels consisted of low-frequency sound with intensity of 85 dB at 12.5 Hz, decreasing to 60 dB at 100 Hz and less than 40 dB above 200 Hz. There was a significant correlation (r = 0.78, P < .02) between the duration of the first fetal heart rate (FHR) acceleration following stimulus and the overall intrauterine sound pressure level at the time of the stimulus. Our data suggest that a threshold of 94 dB intrauterine sound pressure level is necessary to produce a consistent FHR response during active labor.
(C) 1992 The American College of Obstetricians and Gynecologists