THE HEART-RATE DECELERATORY RESPONSE IN LOW-RISK HUMAN FETUSES - EFFECT OF STIMULUS-INTENSITY ON RESPONSE TOPOGRAPHY

The purpose of this study was to determine the effect of stimulus inte nsity on heart rate response in 18 low-risk human fetuses between 37 a nd 40 weeks gestation. Each fetus was stimulated in quiet sleep with a 30-s voice sound at intensities of 80 dB and 90 dB. The fetal cardiac electrical signal was captured transabdominally at a rate of 1024 Hz and fetal R-waves were extracted using adaptive signal-processing tech niques. We found that fetuses generally exhibited a 5- to 10-s decreas e in heart rate following stimulus onset at an intensity of 80 dB. The response pattern changed from deceleratory to acceleratory when stimu lus intensity was increased to 90 dB. Our findings suggest that a hear t rate deceleration at low-stimulus intensity may be a component of th e orienting reflex in the human fetus. (C) 1997 John Wiley & Sons, Inc .