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

Premature atrial contractions are the most common cause in cases of irregular fetal cardiac rhythm. When PACS are multiple and non-conducted they can slow the fetal hear rate below 100 bpm resulting in bradycardia¹. Such cases should be differentiated from complete heart block which has normal regular atrial rate with ventricular escape rhythm (40-80 bpm) ¹. Premature atrial contractions are self-limited in the majority of cases except for a minority where a re-entry mechanism can lead to supraventricular tachycardia; this risk is higher if the PACs are multiple and blocked leading to slowing in the ventricular rate². 

Case report

A 23-year old primigravida patient was referred to our unit due to an irregularity in the fetal heart rate observed during routine ultrasound in antenatal care. Our ultrasound examination revealed the presence of irregularity in heart rate during the time of examination (40 minutes scanning). This irregular heart rate was associated with slow fetal heart rate (97 bpm). M-mode and pulsed wave Doppler techniques were used to demonstrate the atrial and the ventricular contractions and the chronological relation between both of them. They revealed that each ventricular contraction was preceded by an atrial contraction. There were multiple PACs with the vast majority of them blocked and not conducted to the ventricles resulting in a slow heart rate. Very few PACs were conducted to the ventricles which were less premature than their blocked homologues. Follow-up was recommended due to higher risk in such cases of multiple blocked PACs of developing re-entrant mechanism resulting in supraventricular tachycardia. Follow-up revealed persistence of the rhythm disorder till the end of pregnancy. Follow up after one month of delivery revealed complete resolution of the arrhythmia. 

The differential diagnosis of this case includes all causes of fetal bradycardia and specifically complete heart block in which there is dissociation between the atrial and the ventricular contractions with slow ventricular escape rate (40-80 bpm) and carries a different prognosis. 

Video 1: Four chamber view obtained to show irregular heart rate with periods of compensatory pause.

Image 1, 2, 3 and 4: M-mode images with the M line passing through the right atrium and the left ventricle. Note that each ventricular contraction is preceded by an atrial contraction (premature atrial contractions were pointed to using arrows of different colors in image 1 and 2, white arrows in image 3 and a blue arrow in image 4). Note that in the first three images obtained in different scan times there were 5 ventricular contractions per three second denoting that the FHR is around 100 bpm while in image four, we found 6 ventricular contractions denoting FHR around 120 bpm but this was exceptional in comparison to the rest of the study³.

Image 5: pulsed-wave simultaneous insonation of both aorta/SVC demonstrating the prematurity of the premature atrial contractions (white arrows) which are blocked and not followed by ventricular contractions.

Image 6: Pulsed-wave of aorta/SVC showing a PAC (white arrow) which was conducted to the ventricle. Note that the ventricular contraction (blue arrow) following the PAC is shorter than the rest of the ventricular contractions.

Image 7: pulsed-wave of LV inflow/outflow showing two premature atrial contractions (white arrows). The first one is blocked while the second one is conducted.

Image 8: pulsed-wave simultaneous insonation of both pulmonary artery and vein showing the hemodynamic changes in forward flow inside pulmonary vessels during normal (white arrows) and premature atrial contractions (blue arrow). A reduced diastolic pulmonary venous filling was noted during the premature atrial beat (blue arrow) compared to normal diastolic filling of pulmonary veins during normal atrial beats (white arrows).

References:

1. Weber R, Stambach D, Jaeggi E. Diagnosis and management of common fetal arrhythmias. Journal of the Saudi Heart Association 2011; 23: 61-66.

2. Simpson JM. Fetal arrythmias. Ultrasound Obstet Gynecol 2006; 27:599-606.

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