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). The phenomenon of continuous R-to-R wave fetal heart rate computation is known as beat-to-beat variability. The physiological event being counted, however, is not a mechanical event corresponding to a heartbeat but rather an electrical event.
Schematic representation of fetal electrocardiographic signals used to compute continuing beat-to-beat heart rate with scalp electrodes. Time intervals (t1, t2, t3) in milliseconds between successive fetal R waves are used by cardiotachometer to compute instantaneous fetal heart rate. (ECG = electrocardiogram; PAC = premature atrial contraction.)
Electrical cardiac complexes detected by the electrode include those generated by the mother. Although the maternal electrocardiogram (ECG) signal is approximately five times stronger than the fetal ECG, its amplitude is diminished when it is recorded through the fetal scalp electrode. In a live fetus, this low maternal ECG signal is detected but masked by the fetal ECG. If the fetus is dead, the weaker maternal signal will be amplified and displayed as the “fetal” heart rate (Freeman and co-workers, 2003). Shown in Figure 18-3 are simultaneous recordings of maternal chest wall ECG signals and fetal scalp electrode ECG signals. This fetus is experiencing premature atrial contractions, which cause the cardiotachometer to rapidly and erratically seek new heart rates, resulting in the “spiking” shown in the standard fetal monitor tracing. Importantly, when the fetus is dead, the maternal R waves are still detected by the scalp electrode as the next best signal and are counted by the cardiotachometer (Fig. 18-4).
The top tracing shows standard fetal monitor tracing of heart rate using fetal scalp electrode. Spiking of the fetal rate in the monitor tracing is due to premature atrial contractions. The second panel displays accompanying contractions. The bottom two tracings represent cardiac electrical complexes detected from fetal scalp and maternal chest wall electrodes. (ECG = electrocardiogram; F = fetus; M = mother; PAC = fetal premature atrial contraction.)
Placental abruption. In the upper panel, the fetal scalp electrode first detected the heart rate of the dying fetus. After fetal death, the maternal electrocardiogram complex is detected and recorded. The second panel displays an absence of uterine contractions.
External (Indirect) Electronic Monitoring
The necessity for membrane rupture and uterine invasion may be avoided by use of external detectors to monitor fetal heart action and uterine activity. External monitoring, however, does not provide the precision of fetal heart rate measurement or the quantification of uterine pressure afforded by internal monitoring.
The fetal heart rate is detected through the maternal abdominal wall using the ultrasound Doppler principle (Fig. 18-5). Ultrasound waves undergo a shift in frequency ...