Fetal Bradycardia

Bradycardia is a persistant fetal heart rate of less than 110 bpm (1).

O36.3- Maternal care for signs of fetal hypoxia
I49.5- Sick sinus syndrome/ Tachycardia-bradycardia syndrome
I49.4-Other and unspecified premature depolarizationl ectopic beats
I44- Atrioventricular and left bundle-branch block
I44.0- Atrioventricular block, first degree
I44.1- Atrioventricular block, second degree: Atrioventricular block, type I and II, Möbitz block, type I and II, Second-degree block, type I and II, Wenckebach block
I44.2- Atrioventricular block, complete/Complete heart block 
 

CHB 1:15 000 to 1: 22 000 live births. CHB occurs in 2% of newborns of women with positive anti- Ro/La antibodies in CTD-connective tissue disease. (2)
 

Multiple blocked atrial ectopic beats - multiple non-conducted atrial extrasystoles (1)
Blocked atrial bigeminy - atrial ectopy
Sinus bradycardia - When detected before 15 weeks’ gestation it is highly associated with chromosomal anomaly but other associations include ectopic atrial pacemaker, -sinus node dysfunction (including immune mediated or infection-related),- channelopathies (including LQTS-long-QT syndrome), and secondary causes (including maternal medications, maternal hypothyroidim, fetal hypoxia/distress or fetal CNS-central nervous system abnormalities)(7, 8) 
AV-Atrioventricular block  -immune mediated (SSA/SSB antibody),-developmental abnormality of the AV node,-channelopathies (including NKX2.5, LQTS) (3)

Multiple blocked atrial ectopic beats - could be a marker of a re-entry pathway with the potential for a re-entrant supraventricular tachycardia
Sinus bradycardia - structural cardiac abnormalities, l-looped ventricles (cc-TGA), visceral heterotaxy/isomerism of the left or right atrial appendages, in long QT syndrome
Atrioventricular block  -in immune-mediated (anti-Ro/La), in isomerism of the left atrial appendages or discordance of the atrioventricular connections (3). 

In immune AVB the risk of recurrence in the next pregnancy is 19% (2)

Sinus bradycardia - 1:1 A:V interval on M-mode evaluation (it is not possible to differentiate sinus from ectopic atrial bradycardia)
Atrioventricular block - 1st degree: prolongation of the mechanical PR interval >~170ms,- 2nd degree: fixed ratio of atrial to ventricular contractions e.g. 2:1 or 3:1 or progressive prolongation of the PR interval until an atrial beat is non-conducted (Wenkebach),- complete: complete dissociation between atrial and ventricular contractions (1). The PR interval measurement by echocardiography is a valuable tool for identification of early and potentially reversible conduction abnormalities in fetuses at risk for more advanced and permanent forms of CHB(4,5). 
Channelopathies - diagnosis can be made in utero by fMCG-fetal magnetocardiogram (3) and/or genetic testing.

Ectopic atrial pacemaker can be seen with heterotaxy syndromes (right and left atrial isomerism)
Sinus node dysfunction - test for maternal anti-SSA/SSB antibodies, maternal IgG/IgM for TORCH diseases and Parvovirus 
Channelopathies - ventricular tachycardia and second-degree AV block could be seen in< 25%

Fetal echocardiography is to ascertain mechanism of bradycardia, and if persistent weekly heart rate monitoring until resolved should be recommended (3). AV block in structurally normal heart may have concomitant EFE- endocardial fibroelastosis or myocardial or valvular dysfunction if anti-Ro antibodies are present (3)

In SSA+ pregnancies it is appropriate to perform weekly monitoring at least between 16-24 wks in AVB. A "biomarker" of reversible injury, such as the PR interval prolongation above 150-170 ms, is a useful tool also to begin an early treatment (4).

Bradycardia/CHB – survival in affected newborns without structural cardiac lesions in about 85%, with many requiring pacemaker implantation (3)
Multiple blocked atrial ectopic beats - self-limiting, 10% risk of fetal SVT in blocked atrial bigeminy (3), much lower for less frequent ectopy
Atrioventricular block - in structural cardiac anomalies prognosis is very poor (1).
Grade III CHB is associated with a 19% fetal mortality risk, 70% of children with CHB require a pacemaker (2)

Sinus bradycardia: Ectopic atrial pacemaker-rule out fetal distress as a cause for bradycardia
Sinus node dysfunction (including immune mediated or infection)-close surveillance until 
bradycardia resolves or delivery
Channelopathies (including LQTS)-surveillance for VT and second-degree AV block/ avoid QT-prolonging drugs 
Secondary causes (including maternal medications, maternal hypothyroidism, fetal distress or fetal CNS abnormalities-treat underlying cause of bradycardia- treat underlying cause of bradycardia (3)
Blocked atrial bigeminy: Atrial ectopy-observe/reduce maternal stimulants
AV block Immune mediated (SSA/SSB antibody) - Daily fetal rhythm home monitoring (2), weekly fetal echocardiography (2), Dexamethasone: 《 24 weeks of gestation 8mg/dosę (6) for second-degree block or first-degree block with findings of cardiac inflammation or/and for CHB as prevention for death or cardiomyopathy (3), Hydroxychloroquine initiated from prior to 10th weeks' gestation 400 mg daily (6),-experimental one dose of IVIG (1 g/kg of maternal weight max dose 70g at diagnosis of 2nd AVB Within 12 hours of detection by mother via home monitoring and within 6 hours of confirmation by echocardiogram) (2),-sympathomimetics for rate < 55 bpm or higher rates with associated CHD, cardiac dysfunction or hydrops (3). 
Developmental - abnormality of the AV node-observation, sympathomimetics for rate < 55 bpm or higher rates with associated CHD, cardiac dysfunction or hydrops (3). 
SSA/SSB antibody negative block - only observation, dexamethasone not recommended   

Channelopathies (including NKX2.5, LQTS)-observaion, avoid QT-prolonging drugs, surveillance for VT (3). (See VISUOG chapter on Congenital Heart Block for further discussion)

1.    Simpson, J.M. (2006), Fetal arrhythmias. Ultrasound Obstet Gynecol, 27: 599-606. https://doi.org/10.1002/uog.2819
2.    www.stopbloq.org
3.    Donofrio MT, Moon-Grady AJ, Hornberger LK, Copel JA, Sklansky MS, Abuhamad A, Cuneo BF, Huhta JC, Jonas RA, Krishnan A, Lacey S, Lee W, Michelfelder EC Sr, Rempel GR, Silverman NH, Spray TL, Strasburger JF, Tworetzky W, Rychik J; American Heart Association Adults With Congenital Heart Disease Joint Committee of the Council on Cardiovascular Disease in the Young and Council on Clinical Cardiology, Council on Cardiovascular Surgery and Anesthesia, and Council on Cardiovascular and Stroke Nursing. Diagnosis and treatment of fetal cardiac disease: a scientific statement from the American Heart Association. Circulation. 2014 May 27;129(21):2183-242. doi: 10.1161/01.cir.0000437597.44550.5d. Epub 2014 Apr 24. Erratum in: Circulation. 2014 May 27;129(21):e512. PMID: 24763516.
4.    De Carolis S, Garufi C, Garufi E, De Carolis MP, Botta A, Tabacco S, Salvi S. Autoimmune Congenital Heart Block: A Review of Biomarkers and Management of Pregnancy. Front Pediatr. 2020 Dec 22;8:607515. doi: 10.3389/fped.2020.607515. PMID: 33415090; PMCID: PMC7784711.
5.    Murlewska J, Preis K, Słodki M, Strzelecka I, Smolewska E, Respondek-Liberska M. Management in maternal autoantibody-mediated clinical foetal myocardial disease. Prenatal Cardiology. 2019;(1):5-11. doi:10.5114/pcard.2019.91001.
6.    Saito M, Silverman E, Golding F, Guerra V, Hiraki L, Thakur V, Jaeggi E. Effects of Transplacental Dexamethasone Therapy on Fetal Immune-Mediated Complete Heart Block. Fetal Diagn Ther. 2021;48(3):183-188. doi: 10.1159/000513202. Epub 2021 Feb 16. PMID: 33592603
7.    Benson DW, Wang DW, Dyment M, Knilans TK, Fish FA, Strieper MJ, Rhodes TH, George AL Jr. Congenital sick sinus syndrome caused by recessive mutations in the cardiac sodium channel gene (SCN5A). J Clin Invest. 2003 Oct;112(7):1019-28. doi: 10.1172/JCI18062. PMID: 14523039; PMCID: PMC198523.Benson DW, et al.  J Clin Invest 2003
8.    Milanesi R, Baruscotti M, Gnecchi-Ruscone T, DiFrancesco D; Familial sinus bradycardia associated with a mutation in the cardiac pacemaker channel January 12, 2006 N Engl J Med 2006; 354:151-157
DOI: 10.1056/NEJMoa052475 

Please note that the maximum upload size is 5MB, and larger images and video clips can be sent to [email protected].