Gestational age related changes in cardiac dynamics of the fetal baboon

To provide insight into the maturation of neural mechanisms governing fetal heart rate and rate variability, seven chronically instrumented fetal baboons were monitored under steady state conditions between 120 and 165 days gestation (term 175 d). Forty records of 24 h duration (5-7 records/fetus) were evaluated. For each fetus, heart rate decreased with gestational age (mean+/-SD, r = -0.530+/-0.324, P <0.05). In contrast, there were increases with age in markers of various components of autonomic control of fetal R-wave to R-wave interval (RRi) variability as reflected in a positive correlation with age for all fetuses of SD RRi (r = 0.656+/-0.347, P < 0.01), root mean squared differences in RRi (r = 0.686+/-0.223, P <0.05), and power at low frequency in the RRi spectrum (r = 0.800+/-0.161 P < 0.01). In each of the seven fetuses, scatter plots of RRi as a function of the prior RRi (Poincare plots) had increased dispersion around the median with gestational age (0.605+/-0.371, P<0.05). Additional measures of variability evaluated changes in RRi from one interval to the next (deltaRRi). The incidence of sustained deltaRRi changes, either decelerations or accelerations, rose with gestation (r = 0.920+/-0.057, P < 0.001) while the incidence of no detected deltaRRi changes (<+/-1 ms) diminished (r = - 0.649+/-0.364, P <0.05). Sequential decreases in fetal heart rate, increases in RRi variability and increases in changes in RRi and deltaRRi with age imply an overall maturation in autonomic cardio-regulatory control processes. Increases with gestation in measures of high frequency components of variability are compatible with enhanced parasympathetic modulation of fetal heart rate.