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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).
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ELECTRONIC FETAL MONITORING
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Internal (Direct) Electronic Monitoring
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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.
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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 ...