Abnormal Beats

Fetal arrhythmias are broadly categorized into irregular rhythm, bradycardia and tachycardia. Irregular beats include mainly premature atrial contractions and premature ventricular contractions. It is worth noting that type I second degree heart block can be added to this group and, in contrast, blocked atrial bigeminy belongs to the group of bradycardia since it produces a regular rhythm and a slow rate.

Fetal arrhythmias affect between 1-3 % of all pregnancies and represent around 10-20% of the cases referred for fetal cardiology; the vast majority of those cases belong to the group of irregular rhythm 1, 2. Premature atrial contractions are by far the most common, not only among the group with irregular rhythm,  but also among the category of fetal arrhythmias in general. In a study by Jean-Claude Fouron that covered 5 years in Saint-Justine Hospital in Montreal, Canada, 356  cases of sustained arrhythmias were reported; 307 of them were irregular rhythm with 303  cases  of PACs and only 4 cases of PVCs, thus emphasizing the predominance of irregular rhythms among fetal arrhythmias owing to the  predominance of PACs3.

The fetal conductive system is now believed to be derived from cardiomyocytes by a process of differentiation as opposed to the previous prevailing concept that it is a derivative of migrating ectodermal neural crest cells 4. The fetal conductive system begins to function at around 16 weeks of gestation and under normal conditions the sino-atrial node is the pacemaker of the heart5. Atrial ectopic beats develop when an atrial cell fires before the sino-atrial node and thus resets it. Depending on their prematurity they may be conducted to the ventricles and manifest as an extra beat, or be blocked within the atrioventricular node and manifest as a missed beat 6. If the ectopic beat is premature enough to arrive at the atrioventricular node while it is in the refractory period, when it is resistant to depolarization, it will be blocked within the atrioventricular node and will not be conducted to the ventricle 2,3,5. 

There are three types of irregular rhythm
1.    Premature atrial contractions
2.    Premature ventricular contractions
3.    Type I second degree heart block
Although the vast majority of publications in the antenatal literature mentions only premature atrial and ventricular contractions as the sole types of irregular rhythms, it should be noted that type I second degree heart block can also result in irregular rhythm. In type I second degree heart block there is a progressive increase in the mechanical PR until a beat drops 7. Ectopic beats are generally benign and rarely cause any hemodynamic compromise except in uncommon situations such as development of sustained supraventricular tachycardia, which manifests in about 2-5% of cases 5. The prognosis is generally excellent and the majority of ectopic beats resolve spontaneously even before birth 5,8. 

The vast majority of ectopic beats occur in structurally normal hearts; the presence of structural cardiac malformations in cases of ectopic beats is estimated to be around 1% 9. An  association with a redundant flap of fossa ovalis has been frequently reported and has been suggested as a trigger mechanism of ectopic beats 2,3. The development of supraventricular tachycardia should be carefully considered and occurs in around 0.5 to 1 %.  This risk increases significantly  to around 10% if the ectopic beats occur in couplets or triplets, or in cases of blocked atrial bigeminy 10.

Ultrasound is the prevailing technique to diagnose fetal arrhythmias, driven by the limited availability of magnetocardiography and technical difficulties of recording the fetal electrical atrial activity by electrocardiography 11. Mechanical events recorded by different ultrasound techniques reflect the fetal cardiac electrical events,  such techniques mainly include m-mode, pulsed Doppler and tissue Doppler 12, 13. Any modality that can record the atrial and ventricular mechanical events or blood flows simultaneously can be utilized. In general diagnosis of ectopic beats is more frequent in the third trimester 5,14. 
Atrial ectopic beats may be conducted to the ventricle or blocked (non-conducted) if they reached the ventricles in the refractory period. Thus they will be auscultated as an extra beat if conducted,  or  a missed beat if blocked 15. Ectopic beats can occur in isolation or in a  rhythmic pattern, such as bigeminy (if the extra beat occurs every  sinus beat),  trigeminy (if the extra beat occurs every third beat), quadrigeminy (every forth beat) or it may may occur in couplets or triplets pattern (two or three ectopic beats  occuring  sequentially) 8,12. Blocked atrial bigeminy is classified under the bradycardia group of arrhythmia as it produces a regular slow fetal heart rate , ranging from 70-100 beat per minute16.Blocked atrial bigeminy has to be carefully differentiated from heart block and failure to achieve that may result in unnecessary emergency delivery,  taking into consideration the completely different prognosis for each condition 5.
Premature ventricular contractions are ten times less common than premature atrial contractions 2,8; . Although differentiating them from premature atrial contractions may not be easy . In cases of premature ventricular contractions, a regular aatrial contraction may be confirmed,  and the absence of a preceding atrial contraction before the ventricular ectopy can be used for such differentiation 3,8. 

The vast majority of premature atrial contractions are benign and resolve spontaneously. Follow up is warranted, since a minority of these cases (~ 1%) may develop supraventricular tachycardia, mostly by a triggering a re-enterant mechanism in an accessory pathway; such accessory pathways are frequent in fetal life 3,10,15,16,17. Spontaneous resolution is observed in the vast majority of premature ventricular contractions,  however a follow up is also recommended for  the possible development of paroxysms of ventricular tachycarida 1.

In the vast majority of ectopic beats ,whether they are atrial or ventricular in origin, no treatment is required 9. However, in premature atrial contractions, monitoring recommended for possibility of development of supraventricular tachycardia 9,10. Although premature ventricular contractions are also benign, a follow up is recommended, due to an association with myocardidtis and long QT syndrome 6.

1.    Yuan SM, Xu ZY. Fetal arrhythmias: prenatal evaluation and intrauterine therapeutics. Ital J Pediatr. 2020;46(1):21.

2.    Strasburger JF, Cheulkar B, Wichman HJ. Perinatal arrhythmias: diagnosis and management. Clin Perinatol. 2007;34(4): 627-52, vii-viii.

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

4.    Mirzoyev S, McLeod CJ, Asirvatham SJ. Embryology of the conduction system for the electrophysiologist. Indian Pacing Electrophysiol J. 2010;10(8):329-338. 

5.    Stott D, Pandya PP, Attilakos G, Lang J, Wolfenden J, Yates R. The diagnosis and management of fetal cardiac arrhythmias.The Obstetrician & Gynaecologist 2022;24:119–30.

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

7.    Hansahiranwadee W. Diagnosis and Management of Fetal Autoimmune Atrioventricular Block. Int J Womens Health. 2020;12:633-639. 

8.    Bravo-Valenzuela NJ, Rocha LA, Machado Nardozza LM, Araujo Júnior E. Fetal cardiac arrhythmias: Current evidence [published correction appears in Ann Pediatr Cardiol. 2018;11(3):332]. Ann Pediatr Cardiol. 2018;11(2):148-163.

9.    Batra AS, Balaji S. Fetal arrhythmias: Diagnosis and management. Indian Pacing Electrophysiol J. 2019;19(3):104-109. 

10.    Wacker-Gussmann A, Strasburger JF, Cuneo BF, Wakai RT. Diagnosis and treatment of fetal arrhythmia. Am J Perinatol. 2014;31(7):617-28.

11.    Simpson JM. Fetal arrhythmias. Ultrasound Obstet Gynecol. 2006;27(6):599-606. 

12.    Carvalho JS. Fetal dysrhythmias. Best Pract Res Clin Obstet Gynaecol. 2019;58:28-41.

13.    Strasburger JF, Wakai RT. Fetal cardiac arrhythmia detection and in utero therapy. Nat Rev Cardiol. 2010;7(5):277-290. 

14.    Hornberger LK, Sahn DJ. Rhythm abnormalities of the fetus. Heart. 2007;93(10):1294-1300. 

15.    Api O, Carvalho JS. Fetal dysrhythmias. Best Pract Res Clin Obstet Gynaecol. 2008;22(1):31-48.

16.    Yuan SM. Fetal arrhythmias: Surveillance and management. Hellenic J Cardiol. 2019;60(2):72-81

17.    Simpson JM, Yates RW, Sharland GK. Irregular heart rate in the fetus—not always benign. Cardiology in the Young. 1996;6(1):28-31.

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