Early Heart Rate Response of Normal Human Term Fetuses to Vibroacoustic Stimulation

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Abstract
We examined cardiac reactivity in the first 45 seconds following a single 1 second vibroacoustic stimulus (VAS) to determine if the prestimulus fetal heart rate (FHR) pattern was predictive of the immediate post-VAS heart rate response. Forty-nine normal human fetuses between 37 and 41 weeks of gestation were observed to enter quiet sleep and remain in that state for 3 to 5 minutes before testing. In general, VAS elicited an initial acceleratory response of approximately 20 beats/min above the mean prestimulus heart rate followed by a deceleratory response to an average of approximately 4 beats/min below baseline values. Fetuses with different average prestimulus heart rate patterns generally differed in their early cardiac response to VAS: when compared to a high baseline FHR, a low resting heart rate was associated with a greater increase in heart rate above baseline (r = -0.401; P = 0.03), a greater decrease below prestimulus values (r = -0.312; P = 0.03), and a greater difference between maximum and minimum heart rates (r = -0.465; P = 0.001). Higher baseline heart rates were associated with lower FHR variability in the prestimulus period (r = -0.422; P = 0.009). When we examined the time course of the early response, we found that fetuses that achieved a maximum heart rate quickly exhibited less of an increase above prestimulus values (r = 0.894; P = 0.005), displayed a greater decrease below baseline (r = 0.507; P = 0.0002), and reached the minimum heart rate faster (r = 0.575; P = 0.0001) than fetuses that required longer to achieve a maximum cardiac response. In light of the relationship between heart rate and autonomic nervous system activity, these findings suggest that cardiac reactivity in the human fetus may reflect individual differences in resting sympathetic-parasympathetic balance.

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