Diurnal rhythms in cardiorespiratory function of the fetal baboon

Diurnal periodicities of cardiorespiratory function were monitored between 144 and 156 days of gestation (term = 175 days) in six chronically instrumented fetal baboons. For each fetus, 5-11 days of electrocardiographic and tracheal fluid pressure data were summarized as hourly means of fetal heart rate (FHR), standard deviation of FHR, breath-to-breath interval (B-Bi) and percent time spent in fetal breathing activity (PFB). Summaries were evaluated by cosinor analysis to determine the least squares fit to a 24-h cycle. For all fetuses, FHR had a significant (P < 0.001) diurnal rhythm with peak to nadir fluctuations of 17.4 beats/min around a 24-h mean of 163.2 beats/min. The time of the peak FHR was similar across animals occurring in the mid-day between 10:49 h and 14:45 h. For each fetus, standard deviation of FHR also had a significant (P < 0.01) diurnal periodicity with highest values at night between 20:15 h and 02:04 h. The times of the acrophase for these heart rate parameters were correlated (R = 0.88, P < 0.02) across fetuses. Significant (P < 0.001) 24-h rhythms were found in four of six fetuses for B-Bi and five of six for PFB. These PFB rhythms accounted for a fluctuation of 14.4% around a mean of 36.9 +/- 4.5%. In contrast to heart rate, the acrophases of fetal breathing parameters were distributed throughout the entire 24-h cycle and not significantly correlated across fetuses. It is concluded that diurnal rhythms of fetal heart rate, which are synchronized with light/dark conditions in the environment, are evidence for a passive response or entrainment of fetal systems to maternal circadian influences. Alternately, the absence of synchronization across fetuses in daily rhythms of fetal breathing activity provides evidence for a functioning fetal pacemaker, and not simply the imposition of maternal rhythms on her fetus. This differential in the cardiac and breathing activity of the developing primate indicates that pathways for entrainment of fetal pacemaker function are subject to important maturational influences during late gestation.