HEART-RATE CHANGES FOLLOWING HABITUATION TESTING OF THE MOTOR RESPONSE IN NORMAL HUMAN FETUSES

We examined the heart rate (HR) response immediately following habitua tion testing in 54 normal human fetuses between 34 and 40 weeks of ges tation to determine if cardiac self-regulation was a function of the r ate of motor habituation. All fetuses received eight trials of a 1-s v ibroacoustic stimulus (VAS) with a 10-s interstimulus interval. A scor e of 0-10 was assigned for each trial based on subjective assessment o f intensity and duration of the fetal motor response. Motor habituatio n was evaluated in terms of the rate of response decrement over trials . For each fetus the average post-VAS HR was calculated in 1-min block s for the first 7 min following habituation testing. When subjects wer e divided into slow and fast motor habituators, we found that fetuses who were slow habituators displayed a significantly greater increase i n HR above baseline at each of the first 7 min following habituation t esting as compared to fetuses whose motor response habituated more rap idly, F(1,52) = 4.88, P = 0.03. However, there was no difference betwe en slow and fast motor habituators in the rate at which the HR returne d toward prestimulus levels. To further examine these results, we divi ded the 54 fetuses into four groups based on the medians of the slope of the post-VAS HR regression line and the variability in HR about thi s line. We found that fetuses whose HR decreased slowly after habituat ion testing displayed significantly greater variability in their cardi ac response as compared to fetuses whose HR decreased more rapidly (X( 2) = 4.74, P = 0.03). These findings could not be explained in terms o f either gestational age, features of the prestimulus HR pattern, or t he rate of motor habituation. We conclude that, although there were si gnificant differences between slow and fast motor habituators in the m agnitude of the HR response following testing, the rate and variabilit y in return of HR toward prestimulus values provided a marker for indi vidual differences in cardiac self-regulation which was independent of the rate of motor habituation.