Ontogeny and control of the heart rate power spectrum in the last third of gestation in fetal sheep

Abstract

Power spectral analysis of fetal heart rate variability has been proposed to provide a non-invasive estimate of autonomic balance. However, there are few systematic data before birth. We therefore examined developmental changes in the frequency power spectrum at very low (0-0.04 Hz), low (0.04-0.15 Hz) and high (0.15-0.4 Hz) frequencies, and the ratio of low- to high-frequency power (LF/HF) in chronically catheterised, healthy fetal sheep at 0.6 (n = 8), 0.7 (n = 7) and 0.8 gestation age (ga, n = 11). In a second study, 0.8 ga fetuses received either atropine (4.8 mg bolus, then 4.8 mg/h for 30 minutes, n = 6) or 6-hydroxydopamine (6-OHDA, 20 mg/ml at 2.5 ml/h for 3 h, n = 9). Data were analysed by sleep state defined by low voltage-high frequency (LV) or high voltage-low frequency (HV) EEG. Total spectral power increased with gestational age (P < 0.05), while LF/HF decreased from 0.6 to 0.7 ga. At 0.8 ga, heart rate and LF/HF were significantly higher during HV than LV sleep (P < 0.05). Consistent with this, although total spectral power was not significantly greater during HV sleep, there was a significant interaction between sleep state and frequency band (P = 0.02). Both atropine (P = 0.05) and 6-OHDA (P < 0.05) were associated with an overall reduction in spectral power but no significant effect on the LF/HF ratio. This study does not support substantial, consistent differences between the frequencies of sympathetic and parasympathetic activity in late gestation fetal sheep.