Diurnal, respiratory, and other rhythms of fetal heart rate in lambs

In lambs during the last third of gestation fetal heart periods were measured to 0.2 msec. and the variation was calculated with a computer in real time. The rate was subject to low-frequency variations in association with breathing movements, with a 2 to 4 hour rhythm of large amplitude and unknown origin, with the time of day, and with increasing gestational age. The calculated standard deviation of heart period bore a semilogarithmic relation to increasing length of observational epoch. There were also multiple rhythms in heart rate variability. The immeidate effect of fetal hypoxia was to increase heart rate variability despite the arrest of fetal breathing movements. The analysis of heart rate variability is examined critically.