On-line processing of the fetal electrocardiogram. A new direction for fetal monitoring

Digital filtering techniques can be applied to the recovery of the fetal electrocardiogram (ECG) from noise, using optimized digital filters matched to the frequency characteristics of the waveform. By forming a linear model of the fetal ECG, it is possible to measure time constants and characteristics on a real-time basis. The technique provides a powerful tool for the detection of subtle changes in the ECG, but the interpretation of these changes is critical to the application of the technique in fetal monitoring. Shifts in the ST segment and changes in T wave configuration are late indicators of fetal asphyxia, whereas the earliest indicators of stress are shortening of the PR interval and inversion of the normal positive relationship between the PR and RR intervals. The method appears to substantially enhance our ability to predict fetal stress at an early stage.