Diurnal rhythms of fetal and maternal heart rate in the baboon

To investigate the organization of diurnal rhythmicity during gestation, th e relationship between daily cycles of maternal and fetal heart rate were m easured in long-term studies of healthy chronically instrumented pregnant b aboons. In each of six pregnancies, hourly mean values over a 168 h time se ries were obtained during a 7 to 10 day interval between 135 and 160 days o f gestation. Data were modeled by a least squares fit to a cosine function with a period of 24 h. Hourly mean heart rate in the fetus ranged from 161 to 172 bpm (167.9+/-0.6 bpm), and the mother from 105 to 125 bpm (107.9+/-1 .4 bpm). The amplitude of the daily fluctuations were 15 to 25 bpm for the fetuses and 25 to 60 bpm for the mothers. The relation between time series data and model estimates were significant (P < 0.001) in all cases with agg regate r(2) = 0.747 for fetuses and 0.737 for the mothers. On average the t ime of day of the peak in fetal heart rate (15:05+/-0.3 h) was about 45 min after the maternal peak (14:21+/-0.4 h). This phase delay was significant (t = 2.63, P < 0.05). There was significant (P < 0.01) diurnal periodicity for each of six parameters used to assess different aspects of fetal heart rate variability with peak variability at night (23:00 to 2:00). Thus, duri ng the latter third of pregnancy in both the maternal and fetal baboon 24 h periodicities of heart rate are present with peak rates in the midafternoo n. The daily rhythms in fetal heart rate are linked with periodicities in m aternal heart rate with a phase delay in the majority of cases. The synchro ny of 24 h fluctuations in rate with parameters of rate variability is cons istent with diurnal input into the fetal autonomic nervous system. (C) 1999 Published by Elsevier Science Ireland Ltd. All rights reserved.