Fetal arrhythmias, accounting for ˜10% to 20% of referrals to fetal cardiologists, can be accurately diagnosed in the prenatal period.¹,²,³,⁴ Clinical detection and diagnosis of fetal rhythm abnormalities is important as some may cause fetal hemodynamic compromise and increase the risk of in utero or postnatal demise of the affected fetus, while others are benign. Because direct examination of fetal cardiac electrical activity remains a challenge, M-mode and Doppler echocardiography are the most widely used tools to evaluate fetal rhythm and define the arrhythmia mechanism. These techniques, which are described in detail later, are the mechanical surrogates of fetal electrophysiological cardiac events.

M-mode imaging was one of the first echocardiographic modalities used to assess fetal arrhythmias. Because of its high temporal resolution and ability to visualize atrial and ventricular events simultaneously, M-mode is excellent for assessing the atrial and ventricular beat rates and the AV relationship. M-mode is not as helpful in determining ejection, filling, and isovolumic contraction times or duration of the atrioventricular (AV) interval. Electrical events are inferred from the motion of the cardiac chambers.⁵,⁶,⁷,⁸,⁹ The M-mode cursor can be positioned to evaluate only atrial or ventricular activity, or it can be positioned so that both atrial and ventricular activity can be recorded simultaneously (FIG. 2.2.1A-C). M-mode imaging is dependent on favorable fetal position and good image quality and is limited by poor image resolution or a hypocontractile myocardium. Atrial contractions can be difficult to visualize, especially with simultaneous interrogation of atrium and ventricles, thus simultaneous right and left atrial M-mode obtained from the sagittal view of the aortic arch can give very clear imaging of atrial contraction.

Simultaneous atrial and ventricular M-mode can differentiate AV block from blocked atrial bigeminy in the bradycardic fetus (FIG. 2.2.2A and B). The nonconducted (“blocked”) atrial beat (a′) is seen closely following the conducted atrial beat (a), in comparison to the regular atrial contractions and AV dissociation seen with AV block. Similarly, two common causes of nonsinus tachycardia can be differentiated by M-mode: the rapid atrial contractions of atrial flutter with 2:1 or 3:1 block and the 1:1 AV
relationship from reentrant supraventricular tachycardia (FIG. 2.2.2C and D). Lastly, isolated atrial M-Mode can give valuable clues to the diagnosis of an irregular (ventricular) fetal rhythm. If the atrial rate is normal and the atrial defections are regular, the fetus most likely has AV block (FIG. 2.2.2E). In contrast, premature beats can be easily visualized as two atrial defections (premature atrial contractions) close together (a and a′) or a gap in the atrial contractions when the ventricle beats prematurely (FIG. 2.2.2F and G).

Another helpful technique to better assess simultaneous atrial and ventricular contractions is using the so-called anatomic M-mode feature, which is available on most ultrasound platforms and can improve atrial image quality.

The simultaneous display of atrial and ventricular mechanical activities and the timing and duration of these events is best seen using spectral Doppler (SD).³,¹⁰,¹¹,¹²,¹³ There are several fetal cardiac sites where SD can be performed: the goal is to measure atrial and ventricular contraction in the same cycle. Conventionally, SD has been performed in the apex up or down in the left ventricular five-chamber view with the cursor parallel to the mitral and aortic valves. Displayed in this view is the mitral a-wave (atrial systole) and the aortic outflow (ventricular systole) (FIG. 2.2.3A-C). Notably, SD sampling in the left ventricular chamber is primarily useful in fetal arrhythmias with a 1:1 AV relationship. When the atrial rate exceeds the ventricular rate, such as atrial flutter or AV block, simultaneous SD of a vein and an artery may better define atrial and ventricular contractions. In fetuses where the atrial rate exceeds the ventricular rate, atrial contraction may occur against a closed mitral valve, interrupting flow from the atrium to the ventricle and limiting atrial Doppler information reaching the left ventricle.