THE RELATIONSHIP BETWEEN AGE-RELATED HEART-RATE CHANGES AND DEVELOPING BRAIN-FUNCTION - A MODEL OF ANENCEPHALIC HUMAN FETUSES IN-UTERO

We attempted to identify the brain segment which controls heart rate c hanges in human fetuses with advancing gestation. Twelve anencephalic and 165 normal fetuses (control-group fetuses) between 25-32 weeks' ge station were studied. The instantaneous fetal heart rate (FHR) data we re obtained from each fetus for a continuous 90-120 min period, using an external cardiotocograph. Calculations included the 'individual pro bability distribution matrices' in which the FHRs at 1 beat/min interv als between 110 and 180 beats/min, the beat-to-beat differences (DFHRs ) between +/- 5 beats/min and the probability values were arranged in rows, columns and the corresponding elements, respectively. Using 2-ge stational-week intervals probability distribution matrices (age-group probability distribution matrices) obtained from 335 normal fetuses in our previous study as a reference, the difference between a given 'in dividual probability distribution matrix' and the corresponding age-gr oup probability distribution matrix' was quantified as the 'difference rate' according to the formula in the text. From 25-26 to 27-28 weeks gestation, the 'difference rates' in four anencephalic fetuses, with only the spinal cord preserved, were significantly higher in value tha n those of control-group fetuses, whereas the rates in four fetuses wi th both the spinal cord and medulla oblongata preserved, indicated no significant differences. From 29-30 to 31-32 weeks' gestation, the rat es of the four fetuses with the spinal cord and medulla oblongata pres erved, showed significant differences from the control-group fetuses. These findings suggest that there is a critical period between 27-28 a nd 29-30 weeks' gestation with regard to the developing brain function pertaining to FHR changes. In the early stage, the medulla oblongata plays a role in FHR changes, whereas, in the latter stage, the brain c ephalad to the medulla also appears to take on the role of FHR regulat or.