Electronic fetal monitoring (EFM) involves the use of an electronic fetal heart rate (FHR) monitor to record the baby's heart rate. Elastic belts are used to hold sensors against the pregnant woman's abdomen. The sensors are connected to the monitor and detect the baby's heart rate as well as the uterine contractions. The monitor then records the FHR and the contractions as a pattern on a strip of paper. Electronic fetal monitoring is performed late in pregnancy or continuously during labor to ensure normal delivery of a healthy baby. EFM can be utilized either externally or internally in the womb.
Purpose
All electronic fetal monitors detect the FHR and maternal uterine activity (UA), and both are displayed for interpretation since the pattern of the baby's heartbeat during labor often reflects the baby's condition. During contractions, the normal pattern is for the FHR to slow somewhat, picking up again as the contraction ends. The EFM continuously prints out a record of both the FHR and the duration and frequency of the uterine contractions, so that deviations from normal patterns can be identified. Certain variations in this pattern, such as precipitous drops in the FHR at the end of a contraction can constitute a true life or death situation requiring emergency delivery of the baby. Prior to the use of EFMs, nurses and doctors periodically monitored the baby's heartbeat manually by placing a stethoscope on
the mother's abdomen. It is important to note that the EFM is a screening tool and not diagnostic of any particular disorder.
Fetal asphyxia (an impaired exchange of oxygen and carbon dioxide) is recognized as an important cause of stillbirth and neonatal death. Asphyxia has also been implicated as a cause of cerebral palsy, although many cases of cerebral palsy have occurred without evidence of birth asphyxia. Most fetuses, however, tolerate intrauterine hypoxia during labor and are delivered without complications. If the interruption to the supply of oxygen is short, the baby may recover without any damage. If the time is longer, there may be some injury that is reversible. If the time period without oxygen is especially long, there may be permanent injury to one or more organs of the body.
Fetal monitoring can be helpful in a variety of different situations. During pregnancy, fetal monitoring can be used as a part of antepartum testing. If the practitioner feels that a baby may be at risk for problems during pregnancy, non-stress tests, biophysical profiles, or even contraction stress tests are performed twice a week to monitor fetal well-being. In this test, changes in the baby's heart rate are noted with the fetus's own movements. The heart rate of a healthy baby should go up whenever she or he moves.
Description
Using the external fetal monitor is simple and painless. Two belts are placed around the pregnant woman's abdomen. One is to hold the transducer that picks up the FHR and the other is to hold a tocodynanometer, which picks up uterine activity or contractions. External monitoring of the fetus is accomplished by means of a transducer that emits continuous sound waves (ultrasound). A water-soluble gel is placed on the underside of the transducer to permit the conduction of fetal heart sounds. When the transducer is placed correctly on the maternal abdomen, the sound waves bounce off the fetal heart and are picked up by the electronic monitor. The actual moment-by-moment FHR can be simultaneously viewed on a screen while being printed on graph paper. Incorrect placement of the transducer may detect a pulsating maternal vessel with a resultant swooshing sound (uterine soufflé), and the rate will be the same as the maternal pulse. Maternal uterine activity is noted and recorded when the pressure of a contraction pushes on a sensor, which is on the underside of a tocodynanometer. Once again, incorrect placement may not completely detect contractions. The sensor on the tocodynanometer must be placed on that part of the uterus that can be palpated easily. If it is too high or too low, the contractions may not be detected.
If it becomes difficult to detect the FHR with the external monitor or if there are subtle signs of a developing problem, the practitioner may recommend the use of an internal monitor. The measurement of fetal heart activity is performed most accurately by attaching an electrode directly to the fetal scalp. This is an invasive procedure that requires the rupture of membranes (amniotomy) and is associated with occasional complications.
An internal monitor may also be used to determine the actual strength of the contraction as well as the resting tone of the uterus. A woman may appear to be having strong contractions but not be progressing in labor. Progress in labor is determined by cervical dilation. The insertion of an intrauterine pressure catheter (IUPC) permits the determination of the strength of the contractions in millimeters of Hg, a measurement used for pressure. A good labor pattern that facilitates cervical dilation can be calculated by taking the difference in pressure between the peak of the contraction and the resting tone and adding them up over a ten-minute period. The unit of measurement for this calculation is called a Montevideo unit, and ideally the sum total of the pressures should be between 150 and 250 Montevideo units to achieve cervical dilation. If the calculation is in this range and the woman's cervix is not changing, then and only then can a diagnosis of failure to progress be made. The IUPC also provides an accurate measurement of the resting tone of the uterus. It is important that the uterus relax between contractions in order for the baby to receive oxygen. If the uterus is not relaxing or if the resting tone is rising, this can be an indication of a placental abruption (the tearing away of the placenta from the wall of the uterus).
Another use of an IUPC is for amnioinfusion. This is a procedure in which a physiologic solution (such as normal saline) is infused into the uterine cavity to replace the amniotic fluid. It is used to relieve cord compression, reduce fetal distress caused by meconium staining, and as a correction of decreased amniotic fluid.
Preparation
There are no special preparations needed for fetal monitoring. An explanation of the procedure and its risks should be provided by the healthcare provider and a consent form may be signed for the procedure.
Risks
Besides the risk of an unnecessary cesarean section, other risks posed to the mother by EFM include her immobilization in bed. Immobilization simultaneously limits changing positions for comfort and causes changes in blood circulation, which decreases the oxygen supply to the fetus and can lead to abnormal changes in the FHR on the EFM that was applied to detect these changes. Another problem with the use of the EFM is that practitioners have a tendency to focus on it instead of the laboring woman. For these and other reasons, the United States Preventive Services Task Force states that there is some evidence that using EFM on low-risk women in labor might not be indicated. EFM, however, has become an accepted standard of care in many settings in the United States for management of labor. Interestingly, there has not been a reported reduction in perinatal morbidity in the United States with the use of EFM. There is a benefit to using EFM in women with complicated labors, such as those induced or augmented with oxytocin, prolonged labors, vaginal birth after having a cesarean section, abnormal presentation, and twin pregnancy.
Generally the insertion of a fetal scalp electrode is a safe procedure, but it may occasionally cause umbilical cord prolapse or infection due to early amniotomy. Problems could also occur if the electrode or IUPC causes trauma to the eye, fetal vessels, umbilical cord, or placenta. Scalp infections with the herpes virus or group B streptococcus are possible, and concern has been raised regarding the potential for enhancing transmission of the human immunodeficiency virus (HIV). As with any procedure, the potential benefit of EFM must be weighed against the potential risks.
Normal results
The average fetal heart rate is in the range of 110 to 160 beats per minute (bpm). A baby who is receiving sufficient oxygen through the placenta moves around and the monitor strip will show the baby's heart rate rising briefly as he or she moves. The monitor strip is considered to be reactive when the baby's heart rate elevates at least 15 bpm above the baseline heart rate for at least 15 seconds twice in a 20-minute period. Other indicators of fetal well-being include short term variability (STV), which constitutes changes in the FHR from one beat to another, and long term variability (LTV), which is changes in the FHR over a long period of time.
KEY TERMS
Amnioinfusion—A procedure whereby a physiologic solution such as normal saline or lactated ringer's solution is infused through a lumen in an intrauterine pressure catheter into the uterus to alleviate cord compression and to help dilute meconium staining.
Amniotic fluid—The liquid in the amniotic sac that cushions the fetus and regulates temperature in the placental environment. Amniotic fluid also contains fetal cells.
Amniotomy—Rupturing or breaking the amniotic sac (bag of waters) to permit the release of fluid.
Asphyxia—Lack of oxygen.
Deceleration—A decrease in the fetal heart rate that can indicate inadequate blood flow through the placenta.
Hypoxia—A condition characterized by insufficient oxygen in the cells of the body
Meconium—A greenish fecal material that forms the first bowel movement of an infant.
Perinatal—Referring to the period of time surrounding an infant's birth, from the last two months of pregnancy through the first 28 days of life.
If the baby's heart rate drops very low or rises very high, this can signal a serious problem if it occurs for longer than a ten-minute period. During a contraction, the flow of oxygen (from the mother) through the placenta (to the baby) is temporarily blocked. The baby should be capable of withstanding this condition since it
is receiving sufficient oxygen between contractions. The first sign that a baby is not getting enough oxygen between contractions is often a drop in the baby's heart rate after a contraction, called a late deceleration. The baby's heart rate recovers to a normal level between contractions, only to drop again after the next contraction. This is a more subtle sign of distress. These babies will do fine if they are delivered in a short period of time following the onset of the late decelerations. Sometimes, these signs develop long before delivery is expected. In that case, a c-section may be necessary.
One of the worst indications of fetal distress, however, is a tracing that shows no variability at all. It is a flat tracing and indicates that the baby has sustained a severe assault on its central nervous system. It is not capable of responding to stimuli in its environment. The mother may report that she has experienced decreased fetal movement as the baby has only enough oxygen to keep the heart beating. It is for this reason that all pregnant women should be taught to keep track of fetal movement every day and to report any significant changes.
