* 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.
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.