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.
