Fetal Monitors

When electronic fetal monitoring (EFM) was introduced in 1968, it was not widely used because medical insurance wouldn’t cover it. By the end of the 1970s, EFM had become both a standard and controversial practice.

 Electrodes strapped to the mother’s abdomen detect uterine contractions and fetal heart rate.

Before EFM, maternal and fetal heart rates were checked by stethoscope several times per hour. With this method, it was often difficult to distinguish the maternal from the fetal heart rate. Some larger hospitals would use a fetal Doppler to more easily separate the two. Both methods required a nurse or physician to make the checks.

Edward Hon, MD, and Orvan W. Hess, MD, are generally considered the creators of the modern fetal monitoring system. The first workable system was fielded in 1957, and the first electronic system was introduced in 1968. In an early publication, Hon identified six factors that could adversely interfere with normal fetal heart rate, thus making electronic fetal monitoring most valuable. Those factors are: 1) intrinsic fetal disease, 2) placental disease, 3) cord compression, 4) maternal disease, 5) drug administration for analgesia or anesthesia, and 6) maternal hypotension.

One of the prime benefits of electronic fetal monitoring has been the reduction of fetal and maternal infections attributed to frequent vaginal exams during labor. One of the prime drawbacks of electronic fetal monitoring has been the increase in cesarean deliveries. Fetal monitoring has not reduced, to any great extent, incidents of injuries to the fetus during labor or infant respiratory problems, as some of the early literature has suggested.

The Birth of EFM
Some of the early fetal monitors could only pick up the fetal heart rate using a scalp electrode. The scalp electrode has a “corkscrew” on the end of it and is inserted up the birth canal and attached to the fetus’ scalp. Sometimes this electrode would injure the fetus or the mother. Plus, the amniotic membrane had to have already ruptured (ie, the woman’s water had to have broken) or be broken for the electrode to be placed.

The next generation of fetal monitors used surface electrodes to pick up the fetal heart rate. The placement of these electrodes was more of an art than a science, and as labor progressed, signal loss was common. A major problem with the surface electrode was the interaction with the maternal heart rate. This was addressed by using a “phase-lock-loop” circuit where the maternal heart rate could be blocked from the counter. Unfortunately, if the fetal heart rate was a multiple of the mother’s, errors could occur.

The current generation of electronic fetal monitors uses ultrasound to detect the fetal heart rate. This method has mostly eliminated the maternal heart rate as an error source.

As with surface electrodes, the ultrasound transducer has to be repositioned as labor progresses. Another benefit of the ultrasound method is that the amniotic membranes do not have to be ruptured, so patients can be monitored and tested at various times during labor.

Contraction Measuring
One of the key items that clinical personnel look for is the fetal heart rate during a contraction and its recovery to the previous rate after the contraction. The measurement of contractions has not changed much over the years. A pressure transducer (called the toco transducer) is placed on the mother’s abdomen using a belt and measures the length and intensity of contractions. As labor progresses toward delivery, the transducer has to be repositioned several times for best results. The processed data is displayed digitally and printed out as a graphic (chart recorder). Sometimes, telemetry systems are used to send information from the mother to the fetal monitor. Using telemetry allows the mother to be mobile. Lack of mobility can increase time in labor.

Early fetal heart rate decelerations that occur at the same time as uterine contractions are usually due to fetal head compression. If the fetus’s heart rate remains normal until the peak of the contraction and then dips and doesn’t really recover until about 30 seconds or more after the contraction, this is a late deceleration and is indicative of fetal distress. Variable decelerations can be a sign of hypoxia and cord compression. Prolonged decelerations may be caused by maternal hypotension, uterine rupture, or rapid fetal descent.

Many physicians have written articles against the use of EFM unless there is evidence to suggest an other-than-normal pregnancy. They claim there appears to be little evidence that fetal monitoring has any measurable effect on death or illness of infants or mothers. However, electronic fetal monitoring does appear to be associated with a higher rate of cesarean deliveries, which increase surgical risks to mothers. Many hospitals routinely use fetal monitoring as a way to keep a record of events for legal reasons.

For biomedical technicians, there are a number of companies that manufacture electronic fetal monitoring simulators to check their accuracy and performance. Most problems arise from transducer heads and the cables that connect them to the monitor and chart recorder thermal heads. Also, the chart paper is vendor specific and generally cannot be used on another vendor’s recorder. The chart speed is 3 cm per minute, with an option of 1 cm per minute on some units.

Many hospitals have central systems where reviewable electronic records are kept on patients. The Health Insurance Portability and Accountability Act of 1996 (HIPAA) and other legal requirements demand that this data be well protected.

Review Questions

1) In fetal monitoring, a toco is used to measure ________________________.
    a. fetal heart rate
     b. length of contractions
     c. intensity of contractions
     d. b and c

2) The chart speed on a fetal monitor is generally _______________________.
     a. 2 mm per minute
     b. 3 cm per minute
     c. 2.5 cm per second
    d. 10 cm per minute

Answers: 1-d, 2-b

David Harrington, PhD, director of staff development and training at Technology in Medicine (TiM), Holliston, Mass, is a member of 24×7’s editorial advisory board.

Mark Pilar is a TiM biomed assigned to Anna Jacques Hospital in Newburyport, Mass.