8.1 Fetal Arrhythmias: PACs, SVT & Heart Block

Introduction to Fetal Arrhythmias

Fetal arrhythmias account for a significant proportion of referrals for fetal echocardiography. The normal fetal heart rate ranges from 110 to 160 beats per minute (bpm). Any sustained rhythm outside this range, or any irregularity, warrants detailed evaluation. Fetal arrhythmias can be broadly classified into irregular rhythms (ectopy), tachyarrhythmias, and bradyarrhythmias. The evaluation of these rhythms relies heavily on understanding the mechanical manifestations of electrical events, as standard electrocardiography (ECG) is not readily available for the fetus. Instead, fetal echocardiographers utilize M-mode, pulsed wave (PW) Doppler, and tissue Doppler imaging to analyze the sequence and timing of atrial and ventricular contractions. Accurate diagnosis is paramount because some arrhythmias, if left untreated, can lead to hemodynamic compromise, hydrops fetalis, and fetal demise. The approach to a suspected arrhythmia involves determining the atrial rate, the ventricular rate, and the relationship between the two (A-V conduction). Furthermore, the structural integrity of the heart must be assessed, as certain arrhythmias are highly associated with congenital heart defects (e.g., heart block with left isomerism).

Premature Atrial Contractions (PACs)

Premature atrial contractions (PACs) are the most frequently encountered fetal arrhythmia, comprising approximately 80% to 90% of all irregular fetal rhythms. They are typically benign and often resolve spontaneously as the pregnancy progresses or shortly after birth. PACs occur when an ectopic focus in the atrium fires before the next expected sinus node impulse. Sonographically, a PAC is identified as an early atrial contraction. Its effect on the ventricle depends on the state of the atrioventricular (AV) node.

If the AV node has recovered from the previous beat, the PAC is conducted to the ventricles, resulting in a premature ventricular contraction followed by a compensatory pause. This is known as a conducted PAC. If the early impulse reaches the AV node while it is still in its absolute refractory period, the impulse is not transmitted to the ventricles. This is a blocked PAC or non-conducted PAC. In M-mode or Doppler interrogation (such as simultaneous superior vena cava/ascending aorta Doppler or mitral/aortic inflow-outflow Doppler), a blocked PAC presents as an early atrial wave (A wave) without a corresponding ventricular wave (V wave), followed by a pause until the next sinus beat.

While usually benign, frequent PACs require monitoring because a small percentage (1-2%) may trigger a sustained tachyarrhythmia, such as supraventricular tachycardia (SVT). Therefore, fetuses with frequent PACs are typically followed with serial echocardiograms. Parents can often be reassured, but they should be counseled about the potential need for reassessment. Avoidance of maternal stimulants (like caffeine) is sometimes recommended, although evidence for its efficacy is limited. Structural heart disease is rarely associated with isolated PACs.

Supraventricular Tachycardia (SVT)

Fetal supraventricular tachycardia (SVT) is a critical condition that requires prompt identification and, frequently, intervention. SVT is the most common fetal tachyarrhythmia, typically presenting with a sustained fetal heart rate between 180 and 240 bpm. The mechanism is usually an atrioventricular reentrant tachycardia (AVRT) utilizing an accessory pathway.

Echocardiographically, SVT is characterized by a 1:1 AV relationship, meaning every atrial contraction is followed by a ventricular contraction. The rapid rate significantly compromises diastolic filling time, leading to reduced stroke volume and cardiac output. If the tachycardia is sustained for a prolonged period, it can lead to elevated central venous pressure, right heart dilation, tricuspid regurgitation, and eventually hydrops fetalis. The development of hydrops is a dire prognostic sign, significantly increasing perinatal mortality.

The diagnostic approach involves utilizing short axis M-mode through the atrium and ventricle to document the 1:1 conduction and measure the rates. Pulsed wave Doppler of the mitral and aortic valves can also demonstrate the short interval between atrial systole and ventricular ejection. Furthermore, the PR interval analog (mechanical AV interval) is often evaluated. Treatment typically involves administering transplacental anti-arrhythmic medications to the mother. Digoxin is often the first-line agent for SVT without hydrops. However, if hydrops is present, transplacental transfer of digoxin is poor, and agents like flecainide or sotalol are often preferred. Close monitoring of maternal ECG (for QTc prolongation, especially with sotalol) is mandatory.

Atrial Flutter

Atrial flutter is another significant fetal tachyarrhythmia, characterized by an exceptionally rapid atrial rate, typically ranging from 300 to 480 bpm. Unlike SVT, the AV node cannot conduct impulses at this rate due to its refractory period, resulting in a variable degree of atrioventricular block. Most commonly, there is a 2:1 or 3:1 AV block, meaning the ventricular rate might be 150-240 bpm. Occasionally, variable block (e.g., alternating 2:1 and 3:1) can produce an irregular ventricular rhythm, which can be misdiagnosed as PACs if the rapid atrial rate is not carefully documented.

The mechanism is usually an intra-atrial macro-reentrant circuit. Diagnosis is established by demonstrating the rapid, regular atrial contractions with a slower ventricular rate. M-mode across the atrial and ventricular walls clearly shows multiple atrial excursions for every ventricular excursion. Doppler interrogation of the pulmonary vein or SVC can also reveal the rapid "flutter" waves.

Management of atrial flutter is similar to SVT, relying on transplacental anti-arrhythmic therapy. Sotalol and digoxin are commonly used. Conversion to sinus rhythm in utero significantly improves the prognosis. If delivery occurs before conversion, the neonate will require direct treatment, which may include electrical cardioversion or pharmacological therapy. Structural cardiac anomalies are present in a small percentage of fetuses with atrial flutter, so a detailed anatomical survey is necessary.

Congenital Heart Block

Bradyarrhythmias in the fetus are primarily due to varying degrees of atrioventricular (AV) block. A sustained fetal heart rate below 110 bpm is abnormal, and rates below 55 bpm carry a severe prognosis. Congenital heart block (CHB) can be classified into first, second, and third-degree (complete) heart block.

First-degree AV block involves a prolongation of the PR interval without dropped ventricular beats. In the fetus, the PR interval is measured mechanically using Doppler techniques, such as the time from the onset of the mitral A wave (atrial systole) to the onset of aortic ejection. While often benign, it can be a precursor to more advanced block in the presence of maternal autoantibodies.

Second-degree AV block is characterized by intermittent failure of AV conduction. It can present as Mobitz Type I (Wenckebach), with progressive prolongation of the PR interval until a beat is dropped, or Mobitz Type II, with sudden dropped beats without preceding PR prolongation. Second-degree block can cause an irregular fetal heart rate and requires close monitoring.

Third-degree or Complete Heart Block is the most severe form, characterized by complete dissociation between the atria and ventricles. The atrial rate is typically normal, while the ventricular rate is governed by a slow escape pacemaker (often <55 bpm). Complete AV block is most commonly associated with either major structural heart disease (especially left atrial isomerism/polysplenia syndrome and congenitally corrected transposition of the great arteries) or maternal autoimmune disease.

Maternal autoantibodies, specifically anti-Ro (SSA) and anti-La (SSB), can cross the placenta and cause inflammatory destruction of the fetal AV node, leading to irreversible complete heart block. This immune-mediated block typically develops between 18 and 24 weeks of gestation. Fetuses of mothers with these antibodies are screened serially with PR interval measurements in an attempt to detect early stages of heart block.

The prognosis of complete heart block depends heavily on the presence of structural heart disease, the ventricular rate, and the development of hydrops. A ventricular rate below 50-55 bpm, a rapidly declining rate, or the presence of hydrops indicates a poor prognosis. Management may include maternal administration of beta-sympathomimetics (like terbutaline or salbutamol) to increase the fetal ventricular escape rate, although efficacy varies. Corticosteroids (like fluorinated dexamethasone) have been used to reduce myocardial inflammation in immune-mediated block, particularly for 1st or 2nd-degree block or associated endocardial fibroelastosis (EFE), but they do not reverse established complete heart block. Postnatal pacemaker placement is almost invariably required.

Echocardiographic Techniques for Arrhythmia Evaluation

Accurate diagnosis of fetal arrhythmias requires specialized echocardiographic techniques designed to document the timing of atrial and ventricular events simultaneously.

M-mode Echocardiography: The M-mode cursor is placed to intersect both an atrial wall and a ventricular wall (typically right atrium and left ventricle) simultaneously. This allows visual tracking of atrial wall motion (A wave) and ventricular wall motion (V wave). The temporal relationship between the A and V waves defines the rhythm. M-mode is particularly useful for demonstrating the 1:1 relationship in SVT, the multiple atrial beats in flutter, and the AV dissociation in complete heart block.

Pulsed Wave (PW) Doppler: Several PW Doppler techniques are employed.

1. Simultaneous Mitral/Aortic Inflow-Outflow: The sample volume is placed to encompass both the mitral inflow and aortic outflow tracts. This records the atrial kick (A wave) from mitral inflow and the ventricular ejection wave. The time interval between the onset of the A wave and the onset of ventricular ejection is the mechanical PR interval.
2. Superior Vena Cava (SVC) and Ascending Aorta (AAo): The sample volume is placed at the junction of the SVC and AAo. This records the retrograde flow in the SVC during atrial systole (A wave) and the forward flow in the aorta during ventricular systole (V wave).

Tissue Doppler Imaging (TDI): TDI can record myocardial wall velocities. By placing the sample volume at the AV annulus or the atrial and ventricular walls, the specific timing of myocardial contraction can be recorded, providing precise evaluation of conduction intervals.

These techniques require skill in obtaining optimal imaging planes and interpreting the spectral tracings, emphasizing the importance of specialized training in fetal echocardiography for managing these complex cases.