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PubMed This is a summary of 19 peer-reviewed journal articles Updated
Cardiology

Causes and Degrees of Heart Block

At a Glance

Congenital heart block (CHB) occurs when a baby's heart electrical signals are disrupted, usually by maternal autoantibodies or structural birth defects. While 1st and 2nd-degree blocks may be reversible, 3rd-degree block is permanent and typically requires a pacemaker.

To understand why your baby has heart block, it helps to look at the heart’s “wiring.” Congenital heart block (CHB) occurs when the electrical signal that coordinates the heartbeat is disrupted. There are two primary reasons this happen: the signal is being blocked by a physical barrier (structural) or the “wires” have been damaged by inflammation (autoimmune).

Two Pathways to Heart Block

1. The Autoimmune Pathway (Neonatal Lupus)

In most cases of CHB, the heart itself is built perfectly, but the “wiring” system is damaged. This happens when maternal autoantibodies—specifically anti-Ro/SSA and anti-La/SSB—cross the placenta [1][2].

  • The Attack: These antibodies, meant to protect the mother, mistakenly target the baby’s developing heart cells. They interfere with how calcium moves in the cells and trigger an inflammatory response [3][4].
  • The Result: This inflammation can lead to fibrosis (scarring) of the atrioventricular (AV) node, which is the heart’s natural relay station. Once the tissue is scarred, the electrical signal can no longer pass through normally [5][3].

2. The Structural Pathway

In other cases, the heart block is caused by the way the heart was built. Certain structural birth defects create a physical “roadblock” for the electrical signal.

  • Common Defects: The most frequent structural causes are ccTGA (congenitally corrected transposition of the great arteries) and heterotaxy syndromes, where the heart or internal organs are not in their typical positions [6][7].
  • The Result: Because the heart’s anatomy is different, the electrical path may be physically interrupted or the relay station may be missing or misplaced [7].

The Three Degrees of Heart Block

Doctors use “degrees” to describe how much of the electrical signal is getting through. Think of it like a communication system between two rooms: the top of the heart (atria) and the bottom (ventricles).

Degree Description Analogy Reversibility
1st Degree The signal is delayed but still reaches the bottom chambers. A Weak Signal: Like a slow internet connection; the message gets through, but it takes longer than it should [8]. Potentially reversible with treatment [9].
2nd Degree Some signals get through, but others are “dropped” or lost. Dropped Calls: Sometimes the bottom chambers hear the signal to beat, and sometimes they don’t [10]. Potentially reversible or may progress [9].
3rd Degree (Complete) No signals pass from the top to the bottom. Cut Wires: The top and bottom chambers are completely disconnected and must beat independently [11]. Irreversible: This stage is permanent but can be managed with a pacemaker [12][13].

How Doctors Measure the “Delay”

To determine the degree of block in a fetus, doctors use fetal echocardiography to measure the mechanical PR (mPR) interval [14][15]. This measures the exact time (in milliseconds) it takes for the top of the heart to squeeze and the bottom to respond.

Because babies are different sizes, doctors often use a Z-score [16]. Think of a Z-score as a “growth-adjusted calculator.” It tells the doctor if your baby’s signal delay is normal for their specific size or if it is long enough to qualify as heart block (typically a Z-score above 3) [8][17].

Knowing the degree is critical because 1st and 2nd-degree blocks can sometimes progress to 3rd-degree very quickly—occasionally within 24 hours—making close monitoring essential [18][19].

Common questions in this guide

What causes congenital heart block?
Congenital heart block is typically caused by either an autoimmune response, where maternal antibodies trigger inflammation in the baby's heart, or by structural birth defects that physically block the heart's electrical signals.
Can congenital heart block in a fetus be reversed?
First and second-degree heart block can sometimes be reversed with prompt treatment. However, third-degree or complete heart block involves permanent scarring of the heart tissue and is irreversible, typically requiring a pacemaker to manage.
How do doctors measure heart block in a fetus?
Doctors use a fetal echocardiogram to measure the mechanical PR (mPR) interval. This ultrasound measures the exact time in milliseconds it takes for the top of the heart to squeeze and the bottom to respond, indicating how delayed the electrical signal is.
What does a Z-score mean on my baby's fetal echocardiogram?
A Z-score is a growth-adjusted calculator used during a fetal echo. It compares your baby's electrical signal delay to what is normal for a fetus of their specific size. A Z-score above 3 typically indicates that heart block is present.
What are anti-Ro and anti-La antibodies?
Anti-Ro and anti-La are maternal autoantibodies that can cross the placenta during pregnancy. In the autoimmune pathway of heart block, these antibodies mistakenly attack the developing baby's heart cells, leading to inflammation and scarring of the heart's electrical pathways.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What is the current mechanical PR (mPR) interval measurement for my baby, and what is its Z-score?
  2. 2.Is there any evidence of structural heart defects, such as heterotaxy, that could be causing this?
  3. 3.Is the block currently at a stage (1st or 2nd degree) where treatments might still be an option?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (19)
  1. 1

    Congenital Atrioventricular Block: Comprehensive Review of Pathophysiology, Management, and Future Therapeutic Directions.

    Schechter N, Macanian J, Frishman WH

    Cardiology in review 2025; doi:10.1097/CRD.0000000000001077.

    PMID: 41166688
  2. 2

    Autoimmune congenital heart block: a case report and review of the literature related to pathogenesis and pregnancy management.

    Huang Y, Deng J, Liu J, et al.

    Arthritis research & therapy 2024; (26(1)):8 doi:10.1186/s13075-023-03246-w.

    PMID: 38167489
  3. 3

    Interferons and innate immune activation in autoimmune congenital heart block.

    Ambrosi A, Thorlacius GE, Sonesson SE, Wahren-Herlenius M

    Scandinavian journal of immunology 2021; (93(1)):e12995 doi:10.1111/sji.12995.

    PMID: 33188653
  4. 4

    Advanced Atrioventricular Block in Athletes: Prevalence and Role of Anti-Ro/Sjögren Syndrome-Related Antigen A Antibodies.

    Lazzerini PE, Bertolozzi I, Cartocci A, et al.

    Journal of the American Heart Association 2024; (13(12)):e034893 doi:10.1161/JAHA.124.034893.

    PMID: 38879447
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    Siglec-1 Macrophages and the Contribution of IFN to the Development of Autoimmune Congenital Heart Block.

    Clancy RM, Halushka M, Rasmussen SE, et al.

    Journal of immunology (Baltimore, Md. : 1950) 2019; (202(1)):48-55 doi:10.4049/jimmunol.1800357.

    PMID: 30518570
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    Arrhythmias in congenitally corrected transposition of the great arteries: an international study.

    Sellal JM, Dib N, Karakachoff M, et al.

    European heart journal 2026; (47(7)):831-838 doi:10.1093/eurheartj/ehaf466.

    PMID: 40679844
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    Complete heart block in a young adult with non-isolated congenitally corrected transposition of the great arteries: Case report.

    Faraj R, Bachar A, Sidaty O, et al.

    Annals of medicine and surgery (2012) 2022; (76()):103500 doi:10.1016/j.amsu.2022.103500.

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    Assessment of atrioventricular conduction by echocardiography and magnetocardiography in normal and anti-Ro/SSA-antibody-positive pregnancies.

    Cuneo BF, Bitant S, Strasburger JF, et al.

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    Autoimmune-mediated congenital heart block.

    Wainwright B, Bhan R, Trad C, et al.

    Best practice & research. Clinical obstetrics & gynaecology 2020; (64()):41-51 doi:10.1016/j.bpobgyn.2019.09.001.

    PMID: 31685414
  10. 10

    Type I Wenckebach second-degree AV block: A matter of definition.

    Barold SS

    Clinical cardiology 2018; (41(3)):282-284 doi:10.1002/clc.22874.

    PMID: 29460961
  11. 11

    Transient fetal atrioventricular block: A series of four cases and approach to management.

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    Journal of cardiovascular electrophysiology 2022; (33(10)):2228-2232 doi:10.1111/jce.15642.

    PMID: 35924469
  12. 12

    Novel approaches to the surveillance and management of fetuses at risk for anti-Ro/SSA mediated atrioventricular block.

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    Seminars in perinatology 2022; (46(4)):151585 doi:10.1016/j.semperi.2022.151585.

    PMID: 35410713
  13. 13

    Long term clinical outcomes in patients requiring cardiac pacing due to congenital complete heart block.

    Seitler S, Rafiq I, Behar JM

    International journal of cardiology. Congenital heart disease 2022; (9()):100337 doi:10.1016/j.ijcchd.2022.100337.

    PMID: 39713551
  14. 14

    [Fetal ECG and arrhythmias].

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    Autoimmune Congenital Heart Block: A Review of Biomarkers and Management of Pregnancy.

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    Aorta (Stamford, Conn.) 2016; (4(4)):124-130 doi:10.12945/j.aorta.2016.16.014.

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    Development of a Z-score equation for atrioventricular interval measurement by two-dimensional pulsed Doppler echocardiography in normal fetuses between 16 and 33+6 weeks of gestation.

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    Home Monitoring for Fetal Heart Rhythm During Anti-Ro Pregnancies.

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This page provides educational information about fetal heart block causes and degrees. It is not a substitute for professional medical advice from your pediatric cardiologist or maternal-fetal medicine specialist.

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