Overview of fetal arrhythmias

Abstract

Purpose of review: Though fetal arrhythmias account for a small proportion of referrals to a fetal cardiologist, they may be associated with significant morbidity and mortality. The present review outlines the current literature with regard to the diagnosis and, in brief, some management strategies in fetal arrhythmias.

Recent findings: Advances in echocardiography have resulted in significant improvements in our ability to elucidate the mechanism of arrhythmia at the bedside. At the same time, magnetocardiography is broadening our understanding of mechanisms of arrhythmia especially as it pertains to ventricular arrhythmias and congenital heart block. It provides a unique window to study electrical properties of the fetal heart, unlike what has been available to date. Recent reports of bedside use of fetal ECG make it a promising new technology. Fetal magnetocardiography is also developing. The underlying mechanisms resulting in immune-mediated complete heart block in a small subset of 'at-risk' fetuses is under investigation.

Summary: There have been great strides in noninvasive diagnosis of fetal arrhythmias. However, we still need to improve our knowledge of the electromechanical properties of the fetal heart as well as the mechanisms of arrhythmia to further improve outcomes. Multiinstitutional collaborative studies are needed to help answer some of the questions regarding patient, drug selection and management algorithms.

Figure 1. M-Mode and pulsed Doppler evaluation of fetal arrhythmias

(a–c) M-mode recordings with representative SVC–Ao tracings from respective patients shown in (d–f). ‘A’ indicates atrial events and ‘V’ ventricular events. (a) M-mode recording in sinus rhythm. 1: 1 AV relationship is noted at a heart rate (HR) of 136 bpm (HR not shown). (b) M-mode recording in SVT with 1: 1 AV relation, fetal HR of 200 bpm is seen. (c) Color M-mode recording in atrial flutter with an atrial rate of 420 bpm and ventricular rate of 210 bpm indicating 2: 1 block. Here, flow in the aorta seen on color flow evaluation marked ‘V’ represents ventricular ejection. (d) SVC–Ao tracing in sinus rhythm. Parallel lines denote the mechanical PR interval, measured from beginning of atrial flow to the beginning of ventricular ejection. (e) SVC–Ao tracing in SVT. Parallel lines show the long ventriculoatrial interval of 170 ms (AV interval 130 ms). Gradual increase in HRs in tachycardia (not shown) indicated likely atrial ectopic tachycardia. (f) SVC–Ao tracing in atrial flutter showing 2: 1 block. Note: prominent ‘A’ waves. bpm, beats per minute; HR, heart rate; SVC–Ao, superior vena cava–aorta; SVT, supraventricular tachycardia.

Figure 2. Measurement of mechanical PR interval from simultaneous inflow-outflow Doppler obtained from the left ventricular outflow tract

The mechanical PR interval, indicated by parallel lines (B), is calculated from the beginning of the mitral valve ‘A’ signal to the beginning of the aortic flow signal. In this example, it measures 0.11 s with a fetal heart rate of 146 beats per minute (A).

Figure 3. Fetus with intermittent supraventricular tachycardia and preexcitation noted on magnetocardiogram

(a) Heart rate trend (top) and actogram (bottom). Intermittent supraventricular tachycardia at rates of approximately 300 beats per minute is seen. (b) Averaged ECG showing short PR and delta wave. (c) Real-time tracing obtained from magnetocardiogram. Line 4 represents a composite of maternal and fetal signals. Maternal signals have been averaged out in 1, 2 and 3. Arrowheads point to ectopic beats with a different morphology from QRS in sinus rhythm. In line 1, the ectopic QRS is isoelectric, revealing the hidden ‘P’ (*) wave buried in the QRS, indicating ventricular or aberrantly conducted junctional ectopic beats. (d) Prenatal M-mode with premature ventricular ‘V’ beats and a regular atrial rate (A). In this scenario, the possibility of ventricular tachycardia with 1: 1 conduction becomes difficult to rule out by M-mode analysis. (e) Postnatal rhythm strip with preexcitation and ventricular ectopy.

Figure 4. Algorithm for evaluation of mechanism of tachyarrhythmia based on Doppler and relationship of atrial and ventricular events

Cases of junctional tachycardia and ventricular tachycardia with retrograde conduction may present with 1: 1 AV relationship, but there is near-simultaneous depolarization of the ventricles and atria resulting in a very short VA interval and VA<