Electronic fetal heart monitoring is commonly used for tracking how well the baby is doing within the contracting uterus and for detecting signs of fetal distress.
External fetal heart monitoring is performed by attaching external transducers to the mother’s abdomen with elastic straps. The transducers use Doppler ultrasound to detect fetal heart motion, and the information is sent to the fetal heart monitor which calculates and records the fetal heart rate on a continuous strip of paper. More modern fetal heart monitors have incorporated microprocessors and mathematical procedures to improve the fetal heart rate signal and the accuracy of the recording.
During fetal monitoring, a nurse will evaluate the strip for continuity and adequacy for interpretation, identify the baseline fetal heart rate and presence of variability, determine whether there are accelerations or decelerations from the baseline, identify patterns of uterine contraction, and correlate accelerations and decelerations with the uterine contractions. This will allow the nurse to determine whether the fetal heart rate recording is reassuring, nonreassuring, or ominous. A plan can then be developed for the situation to help deliver the baby in the best possible manner.
Reading The Fetal Monitor Strip
The typical fetal monitor strip consists of two rows of graphs; the upper graph charting the fetal heart rate (in beats per minute) and the lower graph charting the mother’s contractions (in mm of Hg). The normal fetal heart rate range is between 120 and 160 beats per minute. The small up and down fluctuations in the fetal heart rate are called “variability”, and a long term deviation in the fetal heart trate that lasts more than 15 minutes is considered a change in baseline.
Just above the lower graph are notations to the time that corresponds with the progression of the strip. Each small square of the charts represents a span of 10 seconds, equaling 1 minute for every 6 small squares across. Since 2 squares are 1 cm wide, the strip progresses out of the machine at a rate of 3 cm/min. There are usually also numbers just below the upper chart which count an increasing number of panels that have been displayed and a decreasing number of panels left until the roll of paper containing the strip comes to an end.
The electronic fetal heart monitor is primarily used to detect fetal hypoxia (lack of sufficient oxygenation) in hopes of catching it early enough to prevent neurological damage in the unborn baby. Indications of fetal hypoxia are represented by prolonged deviations from a normal baseline pattern of the fetal heart rate. The fetal heart rate is controlled by the autonomic nervous system, with an inhibitory influence coming from the vagus nerve, and an excitatory influence coming from the sympathetic nervous system. Stimulation of the peripheral nerves of the fetus by its own activity (such as movement) or by uterine contractions causes acceleration (rise) of the fetal heart rate. A deceleration (drop) of the fetal heart rate usually indicates that the fetus is under some sort of stress, which may be a good healthy sign if it corresponds with movement or uterine contractions, but may be a bad sign if it happens apart from movement of uterine contractions.
Normal and Reassuring Patterns
The normal fetal heart rate range is between 120 and 160 beats per minute. A constant variation from the baseline (variability) reflects a healthy nervous system, chemoreceptors, baroreceptors and cardiac responsiveness. Beat-to-beat, or short-term, variability is an important indicator of fetal trouble. Loss of this variability may indicate an ominous condition, but it can also indicate healthy rest-activity in the fetus or depression of the central nervous system due to medication. An increase in variability may indicate acute hypoxia or mechanical compression of the umbilical cord.