Paul Lyons1
(1)
Department of Family Medicine, University of California, Riverside, Riverside, CA, USA
Key Points
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Background
The advent of electronic fetal heart rate monitoring has dramatically changed intrapartum management within the United States. The almost universal presence of such monitoring during the course of most deliveries presents providers with a variety of challenges. Controversy exists concerning the clinical benefit of continuous electronic fetal monitoring. Such controversy, however, does not eliminate the need for obstetrical providers to be familiar with the basics of electronic fetal monitoring, normal and abnormal findings, and appropriate management for abnormal tracings.
Normal Fetal Heart Tracings
Routine fetal heart tracing should be evaluated for baseline heart rate as well as variation from that baseline rate. Normal baseline fetal heart rate during pregnancy is between 120 and 160 bpm. The baseline heart rate may be determined by examining a fetal heart tracing of sufficient length to determine the heart rate to which the tracing consistently returns. It may be helpful to use a ruler or other straight edge along the course of a fetal heart rate tracing to help determine the baseline value. Fetal heart rate tracing should demonstrate a degree of variability over the course of time. A fetal heart rate tracing with little evidence of variability requires careful monitoring and evaluation if persistent.
Once the baseline fetal heart rate has been determined, variation from this baseline should also be noted. Is should be apparent that this variation from baseline may be in either direction. Variation upward (toward higher fetal heart rate) is referred to as acceleration, whereas variation downward (toward lower fetal heart rates) is referred to as deceleration. In addition to the absolute direction of movement, a note should be made of patterns of fetal heart rate activity that may be indicative of careful follow-up or intervention. Nonreassuring fetal heart rate patterns are outlined in Table 25.1.
Table 25.1
Nonreassuring fetal heart rate patterns
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Fetal tachycardia (persistently >160 bpm) |
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Fetal bradycardia (persistently <120 bpm) |
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Variable deceleration (decelerations with onset mid-contraction) |
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Late deceleration with or without short-term variability (decelerations with onset after peak of contraction) |
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Prolonged severe bradycardia (persistently or recurrently <100 bpm) |
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Sinusoidal pattern (smooth, rounded, wavelike pattern) |
bpm beats per minute
Evaluation of Fetal Heart Rate Baseline
The baseline fetal heart rate may, under some circumstances, vary from the normal range of 120–160 bpm. When the baseline is determined to vary from this normal range, a careful review of potential causes should be performed.
Tachycardia
Tachycardia is defined as a baseline at or above 160 bpm. Mild tachycardia is defined as 160–180 bpm. Severe tachycardia is defined as more than 180 bpm. Fetal tachycardia may be associated with fetal hypoxia; maternal fever; drug or medication use; infection; fetal cardiac abnormalities; anemia; and hyperthyroidism. Persistent tachycardia should prompt review of potential causes and intervention if indicated.
Bradycardia
Fetal bradycardia is defined as a baseline at or below 120 bpm. A variety of conditions may produce bradycardia in the range of 100–120 bpm. If variability is good and no other abnormalities are noted, careful monitoring may be sufficient. Prolonged or severe fetal bradycardia may be associated with cord compression or prolapse, anesthesia, uterine tetany, or rapid descent of the fetus through the birth canal.
Evaluation of Fetal Heart Rate Variability
Once the baseline fetal heart rate has been determined, variation from this baseline should also be noted. This variation from baseline may be in either direction. Variation upward (toward higher fetal heart rate) is referred to as acceleration, whereas variation downward (toward lower fetal heart rates) is referred to as deceleration. In addition to the absolute direction of movement, note should be made of patterns of fetal heart rate activity that may be indicative of careful follow-up or intervention.
Acceleration
In contrast to a persistent rise in baseline fetal heart rate (tachycardia), fetal heart rate acceleration is generally a favorable finding and may be associated with fetal stimulation (e.g., with contractions or cervical examinations). Fetal heart rate acceleration following variable deceleration (see below) is a common finding and is generally considered a good prognostic indicator.
Early Deceleration
Early decelerations are defined by a slow onset that coincides with the onset of contractions. The decelerations are thought to correspond to fetal head compression and are considered reassuring. The slow onset is matched by a similarly slow recovery producing a symmetric shape that corresponds with the duration of the contraction.
Variable Deceleration
As implied by its name, the onset, shape, and recovery of variable decelerations is less uniform than for either early or late decelerations. Interpretation of variable deceleration is likewise dependent on the associated clinical factors and the specific findings noted on the tracing. In general, variable decelerations have a relatively rapid onset and recovery with a shape that resembles a “V.” As noted earlier, variable decelerations are often associated with accelerations immediately preceding onset and immediately following recovery, yielding a pattern that resembles shoulders. Variable decelerations are thought to be associated with umbilical cord compression and their interpretation is therefore dependent of the potential causes of such compression. Mild decelerations are of less than 30 s in duration and are no lower than 80 bpm at their nadir. Moderate decelerations last between 30 and 60 s and reach 70–80 bpm at the nadir. Severe variable contractions last longer than 1 min and/or reach less than 70 bpm at the nadir. Several findings on the tracing are considered nonreassuring in the assessment of variable decelerations. These include increasing frequency or severity, delayed recovery, decreased variability, and loss of associated accelerations (“shoulders”).
Late Decelerations
Late decelerations are characterized by an onset at or after the peak of the associated uterine contraction. Distinguishing late decelerations from persistent variable decelerations may be difficult under some circumstances. Late decelerations are thought to be related to uteroplacental insufficiency and are often indicative of fetal hypoxia. Conditions associated with an increased risk for late decelerations include diabetes, hypertension/pre-eclampsia, and postdates pregnancy.