MOTOR AND CARDIAC RESPONSE DURING HABITUATION TESTING - DEMONSTRATION OF EXAGGERATED CARDIAC REACTIVITY IN A SUBGROUP OF NORMAL HUMAN FETUSES

In this study we wanted to determine: (1) whether or not a relationship existed between the rate of habituation of the fetal motor response and changes in the fetal heart rate (FHR), and (2) if this relationship could be used to identify a subgroup of fetuses who exhibited exaggerated cardiac reactivity during habituation testing. Thirty-nine normal human fetuses between 34 and 40 weeks of gestation were evaluated. Habituation testing consisted of a 1 second vibroacoustic stimulus with a 10 second interstimulus interval for eight trials. The motor response was observed using real-time sonography, and a score of 0 to 10 was assigned for each trial based on subjective assessment of the fetal response. Motor habituation was evaluated in terms of the rate of response decrement in movement over trials (R(e)). Subjects were initially divided into slow and fast motor habituators based on the median of R(e), and it was found that fetuses whose motor response habituated slowly displayed a significantly greater increase in heart rate compared with fetuses who habituated more rapidly (F1,37 = 8.61; P = 0.0057). Cardiac reactivity was then defined as ''the average increase in FHR above the mean prestimulus value during the last half of the testing period.'' When the data were divided into quadrants based on medians, we identified seven (18%) fetuses who habituated rapidly but who also displayed high cardiac reactivity. We conclude that there is a significant relationship between the magnitude of the increase in FHR above baseline and the rate of motor habituation in normal human fetuses.