Fetal responses to vibratory acoustic stimulation: influence of basal heart rate.

Forty-three healthy pregnant women between 26 and 40 weeks' gestation were studied to determine the influence of prestimulation basal heart rate on the maximum amplitude of the first fetal heart rate acceleration after external vibratory acoustic stimulation. A significant negative correlation was found between the maximum amplitude of the first fetal heart rate acceleration and the prestimulation basal fetal heart rate from 30 weeks to term. In the presence of fetal tachycardia (basal fetal heart rate greater than 160 beats/min), more than 50% of fetal heart rate accelerations after stimulus were less than 15 beats/min amplitude. There was also a significant maturational process in the immediate fetal heart rate response to vibratory acoustic stimulation that occurred between 26 to 28 and 30 to 32 weeks characterized by a prolonged increase in basal fetal heart rate and an increase in the maximum amplitude of the first acceleration after stimulation.