External Fetal Heart Rate Monitoring
External fetal heart rate monitoring involves a small, hand-sized device called a transducer, which is placed on the mother's abdomen and connected by wires to the fetal heart rate monitor. Although it appears simple, the transducer is a sophisticated ultrasound device that directs ultrasound beams into the mother's abdomen and then detects those beams reflected back to it from the baby's heartbeats. To monitor the fetal heart, the transducer must be precisely placed to allow its narrow range of ultrasound beams to pass through the fetal heart. If the transducer beam is directed through a large, maternal artery rather than the fetal heart, the mother's heart rate may be mistakenly detected, because the transducer is designed to detect and count anything moving rhythmically. The external monitor is perfectly sufficient for most deliveries.
External monitoring systems also include a device called a tocodynamometer to detect uterine contractions. Like the heart rate monitor, this device is held in place on the mother's abdomen with a soft strap. When the uterine muscle contracts, it raises the abdominal wall, depressing the button and sending an electronic signal to the fetal monitor displayed as a uterine contraction wave form; this allows the doctor or nurse to know the contraction's beginning, its frequency, and its end. However, the device does not detect the precise strength of the contractions. To obtain this information, the doctor or nurse needs to use an internal monitoring device or feel the mother's abdomen.
Although the external fetal monitor has no side effects, it does have some disadvantages. If the baby or mother moves, the transducer's beam may no longer go through the fetal heart, losing the signal.. In addition, the signal quality may be decreased in a mother who is overweight, or if the baby (and thus its heart) is very small, providing a tiny target for the ultrasound beams. When there is more than one baby being monitored, it can require constant attention to maintain the signal and to be sure each baby is being accurately traced.
Internal Fetal Heart Rate Monitoring
Internal fetal heart rate monitoring does not pose signaling problems. However, the cervix has to be dilated and the membranes ruptured before this device can be used Internal monitoring involves placement of a small plastic device about the size of a pencil eraser through the cervix. A spiral wire called the fetal scalp electrode is placed just beneath the skin of the baby's scalp.. The fetal scalp electrode then transmits direct information about the fetus's heart rate through a wire to the fetal monitor that prints out this information. Because the internal fetal monitor is attached directly to the baby, the fetal heart rate signal is sometimes much clearer and more consistent than with an external monitoring device. This may be very useful if the mother and baby are moving around, if the baby is quite small, if the mother is obese, or if there is more than one baby. An internal monitor decreases the chance that the monitoring system will mistakenly detect the mother's heart rate.
The internal pressure transducer is the second part of the internal monitoring system. This is a small device that is placed through the dilated cervix into the amniotic fluid cavity alongside the baby. A specialized electronic pressure gauge on the tip of this device detects the internal pressures generated by the uterus as it contracts and transmits that information through a wire to the fetal monitor. The main advantage of the internal pressure transducer is it measures the frequency, duration, and intensity of contractions. In some situations this additional information may be helpful in managing the labor and delivery.