Effects of neuromuscular blockade on fetal heart rate variability: a power spectrum analysis

Spectral analysis of fetal heart rate variability allows quantitative determination of the main components that affect this variability. The physiological significance of these components is unclear; however, movements appear to contribute to variability. We studied six fetuses in which immobility required for in utero magnetic resonance or invasive fetal procedures was achieved by fetal intravascular injection of curare between 32 and 36 amenorrhea weeks. For each fetus, we compared spectral density parameters of heart rate variability. After curare administration, mean spectrum power was halved. We did not observe a larger significant decrease in any (very low, low, or high) frequency band. The other parameters of spectral analysis of variability were unaltered. Fetal movements accounted for a significant proportion of human fetal heart rate variability but did not constitute a unique frequency component.