Following earlier work by Hon (1958), continuous electronic fetal monitoring (EFM) was introduced into obstetrical practice in the late 1960s. No longer were intrapartum fetal surveillance and the suspicion of fetal distress based upon periodic auscultation with a fetoscope. Instead, the continuous graph-paper portrayal of the fetal heart rate was potentially diagnostic in assessing pathophysiological events affecting the fetus. Indeed, there were great expectations that:
- Electronic fetal heart rate monitoring provided accurate information
- The information was of value in diagnosing fetal distress
- It would be possible to intervene to prevent fetal death or morbidity
- Continuous electronic fetal heart rate monitoring was superior to intermittent methods.
When first introduced, electronic fetal heart rate monitoring was used primarily in complicated pregnancies, but gradually became used in most pregnancies. By 1978, it was estimated that nearly two thirds of American women were being monitored electronically during labor (Banta and Thacker, 1979). In 2002, approximately 3.4 million American women, comprising 85 percent of all live births, underwent electronic fetal monitoring (Martin and colleagues, 2003). Indeed, fetal monitoring has become the most prevalent obstetrical procedure in the United States (American College of Obstetricians and Gynecologists, 2005).
Internal Electronic Monitoring
The fetal heart rate may be measured by attaching a bipolar spiral electrode directly to the fetus (Fig. 18-1). The wire electrode penetrates the fetal scalp, and the second pole is a metal wing on the electrode. Vaginal body fluids create a saline electrical bridge that completes the circuit and permits measurement of the voltage differences between the two poles. The two wires of the bipolar electrode are attached to a reference electrode on the maternal thigh to eliminate electrical interference. The electrical fetal cardiac signal—P wave, QRS complex, and T wave—is amplified and fed into a cardiotachometer for heart rate calculation. The peak R-wave voltage is the portion of the fetal electrocardiogram most reliably detected.
Internal electronic fetal monitoring. A. Scalp electrode penetrates the fetal scalp by means of a coiled electrode. B. Schematic representation of a bipolar electrode attached to the fetal scalp for detection of fetal QRS complexes (F). Also shown is the maternal heart and corresponding electrical complex (M) that is detected.
An example of the method of fetal heart rate processing employed when a scalp electrode is used is shown in Figure 18-2. Time (t) in milliseconds between fetal R waves is fed into a cardiotachometer, where a new fetal heart rate is set with the arrival of each new R wave. As also shown in Figure 18-2, a premature atrial contraction is computed as a heart rate acceleration because the interval (t2) is shorter than the preceding one (t1...