Electronic Monitoring
Rachel L. Rodel
Henry L. Galan
INTRODUCTION
The two most common obstetrical means to assess fetal well-being are electronic fetal heart rate (FHR) monitoring and the biophysical profile (BPP). Electronic FHR monitoring is the cornerstone of fetal well-being assessment before birth and during labor but is unreliable and uninterpretable in the presence of many arrhythmias. On the other hand, the BPP, an ultrasound evaluation of amniotic fluid, fetal breathing, movement, and tone, most accurately assesses well-being of the fetus with arrhythmia, especially in the late preterm fetus when survival is generally high and morbidity is relatively low.
While FHR monitoring is of limited use to assess well-being in the fetus with arrhythmia, FHR monitoring can characterize the frequency and duration of the arrhythmia and some of its characteristics, which, in turn, guide management. Is the arrhythmia intermittent or incessant? Does it start and stop suddenly or gradually? With treatment, are there periods of normal FHR and how long do they last? Does treatment restore FHR variability or just change the rate?
This chapter will first review the current recommended terminology and the standard practices for assessing fetal well-being by electronic FHR monitoring and the BPP score. Second, it will review how these practices can be applied in the face of fetal arrhythmias and review FHR patterns in various arrhythmias.
STANDARD PRACTICES
Electronic FHR Monitoring
FHR testing, including the nonstress test (NST), is routinely used in the antepartum and intrapartum periods.1,2,3,4 The normal baseline FHR tracing is 110 to 160 beats per minute (bpm) with a beat to beat variability of 6 to 25 bpm. FHR tracings are classified as either “reactive” or “nonreactive.” Reactive tracings demonstrate moderate FHR variability with a normal baseline FHR and at least two FHR accelerations ≥15 bpm above the baseline lasting ≥15 seconds within a 20-minute period. The two accelerations in the 20-minute period must occur within a 40-minute window of time. If the fetus is < 32 weeks, acceptable accelerations are ≥10 bpm above the baseline for ≥10
seconds. A NST not meeting the criteria for reactivity is defined as nonreactive and typically requires additional evaluation of the fetus: either additional time for the NST to become reactive or performing a BPP. FIG. 2.1.1 demonstrates a reactive and normal fetal NST, and reflects a normal acid-base balance in the fetus.
seconds. A NST not meeting the criteria for reactivity is defined as nonreactive and typically requires additional evaluation of the fetus: either additional time for the NST to become reactive or performing a BPP. FIG. 2.1.1 demonstrates a reactive and normal fetal NST, and reflects a normal acid-base balance in the fetus.
 FIGURE 2.1.1 A reactive nonstress test (NST) is demonstrated with a normal baseline fetal heart rate (FHR), moderate variability, accelerations, and no decelerations. The maternal and FHR tracings are shown in pink and aqua, respectively. |
Certain fetal arrhythmias may undermine the overall utility of electronic fetal monitoring. These arrhythmias include very short lived tachycardias and bradycardias, arrhythmias with beat to beat irregularities such as premature atrial and ventricular contractions (FIG. 2.1.2), and tachycardias that exceed the upper and lower rate limits of the monitoring. For example, if the monitor records a consistent FHR of 60 to 120 bpm
with absent variability, the rhythm is probably a supraventricular tachycardia (SVT) at a rate of 240 bpm (FIG. 2.1.3). The true FHR, as well as the atrioventricular (AV) relationship, can be determined by ultrasound examination.
with absent variability, the rhythm is probably a supraventricular tachycardia (SVT) at a rate of 240 bpm (FIG. 2.1.3). The true FHR, as well as the atrioventricular (AV) relationship, can be determined by ultrasound examination.
 FIGURE 2.1.2 This tracing demonstrates premature atrial contractions in a term laboring patient. The maternal and FHR tracings are shown in pink and aqua, respectively. Uterine monitoring is shown as a green line in the bottom panel. The FHR tracing begins at a normal baseline (in yellow box) but then becomes uninterpretable. A return to a normal baseline with moderate variability was captured in the second yellow box, and afterward, the ectopy resolved and the fetus was able to be monitored adequately for a safe vaginal delivery. |
 FIGURE 2.1.3 This is a classic tracing of a fetus with supraventricular tachycardia. The FHR tracing is shown in aqua. Uterine monitoring is shown as a green line in the bottom panel. On the right is a nonreactive FHR tracing at a rate of 120 bpm (in reality the monitor has cut the rate in half and FHR is 240 bpm). In the yellow box, the fetus has converted abruptly to sinus rhythm, as seen by the normal FHR variability. The abrupt transition suggests a reentrant mechanism of tachycardia. |
Biophysical Profile
The premature and late preterm (34-36 6/7 weeks) fetus with arrhythmia benefits from delayed delivery as long as fetal well-being can be assured. In the presence of an arrhythmia, frequent BPPs—modified to exclude FHR parameters because they will not reflect fetal state—are the most reliable method to indicate fetal distress necessitating delivery. In the presence of an arrhythmia, the BPP has a maximum score of 8, with 0 to 2 points for each of four components5 (TABLE 2.1.1). Both the 10-point (including FHR) and the 8-point BPP perform similarly in predicting fetal well-being.5 A BPP score of 0 to 4 typically indicates fetal distress and delivery is recommended. For example, in a case series of fetuses with anti-Ro/SSA-mediated AV block, two fetuses with 2° AV block and ventricular rates in the 70s were delivered in the late preterm period because of an abnormal BPP and did well.6
TABLE 2.1.1 COMPONENTS OF THE 8-POINT BIOPHYSICAL PROFILE SCORE | ||||||||||||||||||
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In addition to helping with the timing of delivery, BPP of the tachycardic fetus can help direct therapy. For example, transplacental antiarrhythmic therapy alone would be appropriate for a hydropic preterm fetus with tachycardia and a BPP score of 8/8, but for the same fetus with a score of 4/8, escalation to include direct intramuscular or intracordal therapy may be indicated for a more rapid conversion to sinus rhythm.7,8
IRREGULAR CARDIAC RHYTHMS
Premature Atrial Contractions and Premature Ventricular Contractions
The most common fetal arrhythmias are premature atrial or ventricular contractions (PACs or PVCs; also known as ectopic beats or extra-systoles) (FIG. 2.1.2).9 These rhythms do not benefit from FHR monitoring as the results are uninterpretable since ectopy presents as “dropped” or “skipped” beats (TABLE 2.1.2), unless the ectopy is very intermittent. Blocked (or nonconducted) atrial bigeminy (BAB) presents as an intermittent or sustained bradycardia of 60 to 80 bpm. If sustained, it is impossible to differentiate BAB from AV block by monitoring alone. Structural congenital heart disease is seen uncommonly (0.3%-2.0%) in fetuses with atrial or ventricular ectopy. In general, limiting maternal stimulants such as excessive caffeine or sugar and listening to the FHR each week Doppler until the ectopy is resolved is recommended. Antepartum testing with NST or BPP is not required. A detailed discussion of ectopy is found in Part 3, Chapter 1.
TABLE 2.1.2 MONITORING CHARACTERISTICS OF ARRHYTHMIAS | ||||||||||||||||||||||||||||||||||||
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