Power spectrum analysis of heart rate and blood flow velocity variability measured in the umbilical and uterine arteries in early pregnancy: a comparative study

Objective To compare power spectral derived variability parameters from the fetal side of the placental circulation with those from the maternal side of the placental circulation, during early pregnancy. Methods Doppler velocity waveforms were obtained from both the umbilical an d the uterine arteries in a study group of 40 pregnant women between 10 and 14 (n = 25) and 15 and 20 (n = 15) weeks of gestation. The coefficient of variation of both the beat-to-beat heart rate variability and the blued flo w velocity variability was determined. The ratio of the integrated low-freq uency components (< 0.2 Hz) and the integrated high-frequency components (> 0.2 Hz) from normalized power spectrum analysis (LH-ratio) was established . to reflect sympathovagal balance. Results The coefficient of variation and LH-ratio of fetal heart rate varia bility constitute only a fraction of the same maternal heart rate variabili ty parameters. Nevertheless a highly significant increase (P < 0.001) in LH -ratio was demonstrated with advancing gestational uge. The coefficient of variation and LH-ratio of blood flow velocity variability were significantl y lower in the fetal umbilical artery only in the 10-14-weeks' gestation gr oup. Due to a decrease of the maternal uterine blood flow velocity variabil ity parameters with advancing gestational age, statistically equal fetal an d maternal values for coefficient of variation and LH-ratio were found in t he 15-20 weeks' gestation group. Conclusions The increase in LH-ratio of fetal heart rate variability indica tes functional development of the fetal autonomic nervous system at 15-20 w eeks' gestation. The umbilical blood flow velocity variability may be secon dary to maternal uterine arterial flow variability rather than due to prima ry changes in fetal cardiovascular function.