EARLY HEART-RATE RESPONSE OF NORMAL HUMAN TERM FETUSES TO VIBROACOUSTIC STIMULATION

We examined cardiac reactivity in the first 45 seconds following a sin gle 1 second vibroacoustic stimulus (VAS) to determine if the prestimu lus fetal heart rate (FH R) pattern was predictive of the immediate po st-VAS heart rate response. Forty-nine normal human fetuses between 37 and 41 weeks of gestation were observed to enter quiet sleep and rema in in that state for 3 to 5 minutes before testing. In general, VAS el icited an initial acceleratory response of approximately 20 beats/min above the mean prestimulus heart rate followed by a deceleratory respo nse to an average of approximately 4 beats/min below baseline values. Fetuses with different average prestimulus heart rate patterns general ly differed in their early cardiac response to VAS: when compared to a high baseline FHR, a low resting heart rate was associated with a gre ater increase in heart rate above baseline (r = -0.401; P = 0.03), a g reater decrease below prestimulus values (r = -0.312; P = 0.03) and a greater difference between maximum and minimum heart rates (r = -0.465 ; P = 0.007). Higher baseline heart rates were associated with lower F HR variability in the prestimulus period (r = -0.422; P = 0.009). When we examined the time course of the early response, we found that fetu ses that achieved a maximum heart rate quickly exhibited less of an in crease above prestimulus values (r = 0.894; P = 0.005), displayed a gr eater 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 r equired 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.