Electronic fetal monitoring eclipsed periodic fetoscopic auscultation of the fetal heartbeat in the late 1960s (Hon, 1958). It was hoped that the continuous graph-paper portrayal of the fetal heart rate (FHR), termed cardiotocography, would reflect pathophysiological events affecting the fetus. Initially, electronic FHR monitoring was used primarily in complicated pregnancies. Now, more than 85 percent of all live births in the United States undergo electronic FHR monitoring (Ananth, 2013).
ELECTRONIC FETAL MONITORING
Internal (Direct) Electronic Monitoring
Direct FHR measurement is accomplished by attaching a bipolar spiral electrode directly to the fetus (Fig. 24-1). The wire electrode penetrates the fetal scalp, and the second pole is a metal wing on the electrode. The P wave, QRS complex, and T wave of the electrical fetal cardiac signal are amplified and fed into a cardiotachometer for heart rate calculation. The peak R-wave voltage is the most reliably detected portion of the fetal electrocardiogram (ECG). Time (t) in milliseconds between fetal R waves is fed into a cardiotachometer, and a new FHR is set with the arrival of each new R wave (Fig. 24-2). The phenomenon of continuous R-to-R wave FHR computation was known as beat-to-beat variability, and is now called baseline variability. If present, fetal premature atrial contractions (PACs) cause the cardiotachometer to rapidly and erratically seek new heart rates and create the “spiking” shown Figure 24-3.
With internal electronic fetal monitoring a bipolar electrode is attached to the fetal scalp for detection of fetal QRS complexes (F). At times, the maternal heart and its electrical complex (M) also may be detected.
Fetal electrocardiographic signals from the scalp electrode are used to compute continuous beat-to-beat heart rate. Time intervals (t1, t2, t3) in milliseconds between successive fetal R waves are used by a cardiotachometer to compute instantaneous fetal heart rate. In this example, the t2 interval has complexes that are close together, which indicates a slighted higher heart rate. This is reflected in the vertical rise on the cardiotachometer graph. ECG = electrocardiogram; PAC = premature atrial contraction.
Standard fetal monitor tracing of heart rate using a fetal scalp electrode. Spiking of the fetal rate reflects premature atrial contractions.
Electrical cardiac complexes detected by the fetal electrode include those generated by the mother. However, the amplitude of the maternal ECG signal is diminished when recorded through the fetal scalp electrode. Thus, maternal cardiac signals do not appear on the FHR tracing. However, if the fetus is dead, maternal R waves can be still detected by the scalp electrode as the ...