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 babo ons were monitored under steady state conditions between 120 and 165 days g estation (term approximate to 175 d). Forty records of 24 h duration (5-7 r ecords/fetus) were evaluated. For each fetus, heart rate decreased with ges tational 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 contr ol of fetal R-wave to R-wave interval (RRi) variability as reflected in a p ositive 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 < 0.0 5), 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 t he prior RRi (Poincare plots) had increased dispersion around the median wi th gestational age (0.605+/-0.371, P < 0.05). Additional measures of variab ility evaluated changes in RRi from one interval to the next (Delta RRi). T he incidence of sustained Delta RRi changes, either decelerations or accele rations, rose with gestation (r = 0.920+/-0.057, P < 0.001) while the incid ence of no detected Delta RRi changes (< +/-1 ms) diminished (r = -0.649+/- 0.364, P < 0.05). Sequential decreases in fetal heart rate, increases in RR i variability and increases in changes in RRi and Delta RRi with age imply an overall maturation in autonomic cardio-regulatory control processes. Inc reases with gestation in measures of high frequency components of variabili ty are compatible with enhanced parasympathetic modulation of fetal heart r ate. (C) 1999 Published by Elsevier Science Ireland Ltd. All rights reserve d.