* Fetal medicine unit, Cairo University, Egypt;
** Radiology department, Kasr Alainy teaching hospitals, Cairo University, Egypt.

Fetal rhythm disorders are common findings during routine fetal scanning. They are classified into irregular, brady or tachy arrhythmias. Premature atrial contractions are the cause of irregular rhythms in majority of cases and is much more common that their ventricular homologues (303 PACs in contrast to 4 premature ventricular contractions in a series of 307 cases of irregular rhythms in the review of Saint-Justine Hospital). They can be either conducted or blocked depending on the degree of prematurity. When PACs are multiple and blocked they can cause brady arrhythmias.

Most of PACs are self-limited and in rare instances may trigger re-entrant phenomenon via an accessory pathway resulting in supra ventricular tachycardia. 

Case Report

A 28-year old (G1P0) patient was referred to our institution due to an abnormality in rhythm that was detected during auscultation of fetal heart tones. Our ultrasound examination revealed the presence of blocked PACs in a trigeminy pattern. Every three atrial contractions, there was a PAC which was not conducted to the ventricles causing regular irregularity of the ventricular rhythm that can be easily heard by fetal heart sound auscultation. Follow up of the case revealed persistence of the arrhythmia after delivery and follow-up was recommended, waiting for maturity of the conduction system.

The differential diagnosis of this case is composed mainly of premature ventricular contractions in ventricular bigeminy pattern which in most cases are associated with regular atrial rhythm while the second important one is complete heart block in which regular atrial rate with slow and regular ventricular escape rhythm are present.

Images 1 and 2: M-mode image with the M line passing through the RA and LV. It can be easily visualized that every three atrial contractions, a PAC is present which is not conducted to the ventricle resulting in the regular irregularity of the ventricular rhythm. Note that during three seconds there are five ventricular contractions in all the M-mode and pulsed wave images except for figure 2 (six ventricular contractions ¨V¨) indicating that the ventricular rhythm is somewhere slightly above 100 bpm (Reference 1).

Image 3: M line is passing through both atria. The premature atrial contraction occurring every three atrial beats can be easily detected and is most obvious in the left atrial wall.

Images 4 and 5: Pulsed wave image obtained by simultaneous insonation of  both aorta and SVC demonstrating the relationship between the atrial and ventricular contractions and is consistent with the M-mode tracing. Note that the PAC waveform in SVC (A wave) is occurring during the ventricular contraction (V wave) and is taller than the previous normal atrial contractions. This can not be an artifact caused by the movements of the walls of both aorta and SVC as it occurs in synchrony with the reduced venous velocities in SVC tracing.

Image 6: Color M-mode tracing with the M line passing through the tricuspid valve and LVOT showing the relationship between atrial events (coded in blue) and the ventricular events (coded in red). The PAC is evident.

Video 1: Magnified four-chamber view showing clearly the atrial and ventricular contractions.

References:

1- Jeanty P , Chaoui R, Romero R, Pilu G, Goncalves L. Fetal echocardiography: The normal examination. thefetus.net.

2- Weber R, Stamback D, Jaeggi E. Diagnosis and management of common fetal arrythmias. J Saudi Heart Assoc. 2011; 23(2): 61–66.

3- Fouron JC. Fetal arrythmias: the Saint-Justine hospital experience. Prenat Diagn. 2004; 24(13):1068-80.