Overview of Fetal Arrhythmias
Julia A. Drose
Bettina F. Cuneo
THE NORMAL CARDIAC CONDUCTION SYSTEM
An understanding of the normal cardiac conduction system is necessary to understand fetal arrhythmia (FIG. 1.1.1). Electrical conduction begins in the sinoatrial (SA) node located in the right atrium (RA). The SA node fires, sending electrical impulses to the left and right atria, causing them to contract. These impulses are then conducted to the atrioventricular (AV) node.
From the AV node, electrical impulses are conducted to the right and left bundle branches, via the bundle of His, then to the Purkinje fibers within the myocardium, resulting in ventricular contraction. If the impulses from the AV node are delayed, 1° block occurs. If the impulses are not conducted due to a structural or functional anomaly of the AV node, 2° or 3° block results and the ventricles beat at an independent and slower rate than the atria. The more distal the damage to the conduction system, the slower the ventricular rate. On the other hand, if electrical impulses from the bundle of His to the Purkinje fibers are accelerated, ventricular or junctional tachycardia occurs.
Atrial depolarization corresponds to the P-wave on the ECG (FIG. 1.1.2), the A-wave on the mitral inflow/aortic outflow spectral Doppler (FIG. 1.1.3A), retrograde flow during atrial systole on the spectral Doppler tracing of a systemic or pulmonary vein (FIG. 1.1.3B and C), and the atrial deflections on simultaneous M-mode tracing of atria and ventricles (FIG. 1.1.3D). Atrial deflections are unusually sharp and clear when the M-mode cursor is positioned simultaneously through the RA and LA in the longitudinal view of the aortic arch (FIG. 1.1.4).
The QRS complex on an ECG (FIG. 1.1.2) corresponds to excitation of the ventricles as seen by the antegrade flow on the spectral Doppler tracing of aorta or pulmonary artery (FIG. 1.1.3A-C) and the ventricular deflections on simultaneous M-Mode tracing of atria and ventricles (FIG. 1.1.3D).
 FIGURE 1.1.1 Normal cardiac conduction system. (Modified from Jones RM. Patient Assessment in Pharmacy Practice. 3rd ed. Philadelphia, PA: Wolters Kluwer; 2015.) |
 FIGURE 1.1.2 Diagram of a normal electrocardiogram (ECG) tracing. The P-wave represents the onset of atrial contraction. The QRS complex represents ventricular contraction. The T-wave represents ventricular repolarization. The distance between the P-wave and the R-wave represents the time interval from atrial to ventricular contraction of the heart (the PR interval). |
 FIGURE 1.1.3 A: Spectral Doppler tracing of mitral valve inflow (top tracing) and aortic valve outflow (bottom tracing). The A-wave of the mitral valve corresponds to the P-wave on an ECG and represents atrial contraction. The aortic outflow waveform represents ventricular contraction and corresponds to the ECG QRS. B: Pulmonary artery and pulmonary vein spectral Doppler showing atrial contraction represented by reverse flow (the A-wave) in the pulmonary vein. The pulmonary artery waveform (arrow) represents ventricular contraction. The time between the two is the AV interval. D, diastole; S, systole. C: Wave forms of the aorta and innominate vein from the three-vessel tracheal view. Atrial contraction is noted as retrograde flow in the innominate vein (IV), and antegrade flow in the transverse aortic arch (Ao) represents ventricular contraction. White circle highlight A-wave of innominate vein. D: Tracing with the M-mode cursor placed simultaneously through the left ventricle and the right atrium to show atrial contractions (a) and ventricular contractions (V). AV, atrioventricular; C, aortic valve clicks; LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle. |
 FIGURE 1.1.3 Cont’d |
 FIGURE 1.1.4 A: Positioning the M-mode cursor through the right atrium (RA) and left atrium (LA) simultaneously in longitudinal view of the aortic arch (arrow) to obtain (B) a sharp M-mode tracing of atrial contractions (a). |
OCCURRENCE OF ARRHYTHMIAS
Fetal arrhythmias are common, occurring in approximately 1% to 3% of all pregnancies.1,2 The most common arrhythmias occurring in utero are isolated ectopic premature atrial contractions (PACs). Although benign, atrial ectopic beats carry a small risk of developing into sustained supraventricular tachyarrhythmias (SVTs) (Part 3, Chapter 2). Approximately 10% of fetal rhythm abnormalities are significant and may require treatment either in utero or immediately following delivery. Of these, tachyarrhythmias (fetal heart rate [FHR] >180 beats/min [bpm]) are more prevalent, accounting for about 7% of cases, while bradyarrhythmias (FHR <the third percentile for gestational age [GA]) occur in around 3%.3 Arrhythmias can occur both with structural or functional cardiac defects and in the normal heart.4,5
EVALUATION OF FETAL HEART RATE AND RHYTHM
Echocardiographic evaluation of FHR and rhythm should follow a systematic approach to include the following (FIG 1.1.5), as well as an assessment of cardiac structure:
1. What is the fetal heart rate (the ventricular rate) and what is the atrial rate?
The ventricular rate can be measured from one ventricular deflection to the next on M-mode or, for more precise measurements, spectral Doppler of the aortic or pulmonary valve clicks (FIG. 1.1.3A). Likewise, the atrial rate can be measured between two atrial contractions on M-mode, the A-waves of tricuspid or mitral inflow spectral Doppler, or reverse flow during atrial contraction in the pulmonary, systemic, hepatic vein, or ductus venosus (FIGS. 1.1.4 and 1.1.6).6
 FIGURE 1.1.5 Step-by-step evaluation of fetal heart rate and rhythm. A, atrial; AV, atrioventricular; BPM, beats per minute; FHR, fetal heart rate; V, ventricular. |
2. What is the atrioventricular (AV) relationship?
The AV relationship is normally 1:1 and can be assessed by simultaneous atrial and ventricular M-mode or color M-mode or spectral Doppler of mitral inflow and aortic outflow or any vein and artery simultaneously.7,8,9 Doppler interrogation is advantageous because the exact time between atrial and ventricular contraction, that is the AV interval (also known as the mechanical PR interval), can be measured. The normal AV interval varies by gestational age but, in general, should be <160 ms.10,11,12 The E- and A-waves are not normally fused; however, if the FHR is >150 bpm or there is severe ventricular dysfunction and the isovolumic contraction time is prolonged, ventricular filling may be monophasic (FIG. 1.1.7). In these situations, an accurate AV interval can be measured by sampling a pulmonary or systemic vein and the pulmonary artery or aorta (FIG. 1.1.3B and C). The normal values for the AV interval differ based on the sampling location and are longer when measured between mitral inflow and aortic outflow tracings.11,12
Quantifying the FHR and the AV relationship is diagnostic in many arrhythmias. Bradycardia can have a 1:1 (FIG. 1.1.8) or ≥1:1 AV relationship (FIGS. 1.1.9 and 1.1.10). Likewise, tachycardia can have a 1:1 (FIG. 1.1.11), >1:1 (FIG. 1.1.12), or <1:1 (FIG. 1.1.13) AV relationship.
3. Is the rhythm regular or irregular?
Normally the fetal heart rhythm is regular, that is the V-V intervals do not vary by more than about 10 ms. The differential diagnosis of an irregular rhythm is atrial or ventricular ectopy (premature atrial or ventricular contractions) (FIG. 1.1.14) and intermittent
AV block (FIG. 1.1.15). One approach to the fetus with an irregular rhythm is to first obtain an atrial M-mode from the aortic arch view (FIG. 1.1.4). If the atrial rate is regular and all atrial deflections appear the same, the diagnosis is most likely AV block. If the atrial rate is regular, but the atrial beats do not all have the same morphology, or if the atrial rate is regular and there are periods of atrial silence, premature ventricular contractions (PVCs) with retrograde conduction to the atrium (FIG. 1.1.16A and B) or PVCs with no retrograde conduction (FIG. 1.1.16C) can be diagnosed.
AV block (FIG. 1.1.15). One approach to the fetus with an irregular rhythm is to first obtain an atrial M-mode from the aortic arch view (FIG. 1.1.4). If the atrial rate is regular and all atrial deflections appear the same, the diagnosis is most likely AV block. If the atrial rate is regular, but the atrial beats do not all have the same morphology, or if the atrial rate is regular and there are periods of atrial silence, premature ventricular contractions (PVCs) with retrograde conduction to the atrium (FIG. 1.1.16A and B) or PVCs with no retrograde conduction (FIG. 1.1.16C) can be diagnosed.
 FIGURE 1.1.6 Atrial contractions as seen as reverse flow in the (A) ductus venosus; (B) pulmonary vein; (C) systemic vein; (D) hepatic vein. A, atrial contractions; D, diastole; S, systole. |
 FIGURE 1.1.7 Spectral Doppler of monophasic (fused E- and A-waves) of mitral inflow (below baseline). The atrioventricular (AV) interval is marked by the solid lines. Comprising part of the AV interval is the isovolumic contraction time (IVCT) which occurs from the end of atrial contraction to the beginning of ventricular contraction and is marked on the figure by the dotted lines. Aortic outflow (Ao) is seen above baseline. |
4. Is the heart structurally normal?
Any structural heart defect that causes an anatomical disruption in the normal conduction system can be associated with an arrhythmia. Therefore, a thorough evaluation of all cardiac structures is always warranted with a fetal arrhythmia diagnosis. For
example, arrhythmias are common in the heterotaxy syndromes and in congenitally corrected transposition of the great vessels (CC-TGV), due to morphological aberrations of the conduction system.13,14,15,16,17,18,19,20 Approximately, 17% to 20% of patients with Ebstein anomaly will have SVT secondary to enlargement of the right atrium and the prevalence of accessory pathways.21 Cardiac tumors, such as a rhabdomyoma or teratoma, may also result in intermittent ectopy or sustained arrhythmias, depending on the size and location.22,23,24
example, arrhythmias are common in the heterotaxy syndromes and in congenitally corrected transposition of the great vessels (CC-TGV), due to morphological aberrations of the conduction system.13,14,15,16,17,18,19,20 Approximately, 17% to 20% of patients with Ebstein anomaly will have SVT secondary to enlargement of the right atrium and the prevalence of accessory pathways.21 Cardiac tumors, such as a rhabdomyoma or teratoma, may also result in intermittent ectopy or sustained arrhythmias, depending on the size and location.22,23,24
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