Normalized spectral power of fetal heart rate variability is associated with fetal scalp blood pH

Background Spectral power of fetal heart rate variability is related to fetal condition. Previous studies found an increased normalized low frequency power in case of severe fetal acidosis. Aims To analyze whether absolute or normalized low or high frequency power of fetal heart rate variability is associated with fetal scalp blood pH. Study design Prospective cohort study, performed in an obstetric unit of a tertiary care teaching hospital. Subjects Consecutive singleton term fetuses in cephalic presentation that underwent one or more scalp blood samples, monitored during labour using ST-analysis of the fetal electrocardiogram. Ten-minute continuous beat-to-beat fetal heart rate segments, preceding the scalp blood measurement were used. Outcome measures Absolute and normalized spectral power in the low frequency band (0.04–0.15 Hz) and in the high frequency band (0.4–1.5 Hz). Results In total 39 fetal blood samples from 30 patients were studied. We found that normalized low frequency and normalized high frequency power of fetal heart rate variability is associated with fetal scalp blood pH. The estimated ß of normalized low frequency power was -0.37 (95% confidence interval -0.68 to -0.06) and the relative risk was 0.69 (95% confidence interval 0.51–0.94). The estimated ß of normalized high frequency power was 0.33 (95% confidence interval 0.01–0.65) and the relative risk was 1.39 (95% confidence interval 1.01–1.92). Conclusions Normalized low and normalized high frequency power of fetal heart rate variability is associated with fetal scalp blood pH.