Chapter 18 - The effect of noise on the accuracy of the determination of fetal heart rate variability

Abstract

This chapter discusses the effect of noise on the accuracy of the determination of the fetal heart rate (FHR) variability. It is often assumed that the variability of the FHR trace produced from the fetal electrocardiogram (FECG) by commercially available electronic fetal monitors (EFM) accurately measures a physiological effect. The accuracy with which an EFM measures the intervals between successive FECG complexes and displays this information on a chart-recorder may affect the appearance and clinical interpretation of the FHR trace. Because 16–20% of the interval differences of the human FHR are less than 1 ms, the ideal EFM should be able to resolve the interval to less than 1 ms. The unwanted noise signal present on the FECG recording is the combination of electromyographic signals from the mother and fetus, electrode interface effects, noise from the electronic amplifiers, and electromagnetic interference from external sources such as electric motors and 50 Hz mains interference. EFMs usually detect the prominent QRS complex of the FECG waveforms to calculate the FHR from the interval between successive QRS complexes. The accuracy with which the same point in each cardiac cycle is detected has a direct effect on the precision of the calculated FHR. The major difficulty in measuring the beat-to-beat variation in the human FHR is the restricted precision with which the fetal R-wave trigger point can be determined.