Introduction 

• A delay in conduction from the atria to the ventricles resulting in a prolonged PR interval (>0,20 s) with normal morphology and duration of the QRS complex and maintenance of 1 : 1 conduction

• First-degree AV block does not cause bradycardia unless it progresses intermittently to a higher degree block or is associated with sinus node dysfunction.

ECG features:

• PR interval longer than 200 ms
• 1:1 conduction – every P wave is followed by QRS complex
• normal morphology and duration of QRS complex (unless there is another aberrancy in conduction)

Prolonged PR interval - 300 ms

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ECG 1  First degree AV block / AV block type 1

• prolonged PR interval 300 ms
• 1:1 conduction – every P wave is followed by a QRS complex
• sinus rhythm at a rate 78 bpm
  

Causes

• First-degree AV block is generally self-limited and not associated with progression to complete heart block (CHB), but it may be associated with poorer prognosis in those with heart disease
• An increase in vagal tone – causes a reduction in the rate of conduction
• Drugs that slow AV node conduction – beta blockers, digoxin or non-dihydropyridine calcium channel blockers
• Structural abnormalities of the AV node – including dual AV nodal pathways (two sets of PR intervals)
• Myocardial infarction – anterior or inferior wall
• Infiltrative cardiomyopathies (sarcoidosis, etc.)

Management

• Diagnosis is usually easy, based on the surface electrocardiogram (ECG) showing a prolonged PR interval
• First degree AV block is usually asymptomatic and does not require a specific treatment
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• Markedly prolonged PR interval (>300 ms) can lead to symptoms of heart failure:
  - atrial systole occurs too early in diastole, resulting in an ineffective or decreased contribution of the atrial systole to cardiac output
  - As a consequence, the increase of pulmonary capillary wedge pressure may cause dyspnoea and retrograde blood flow in the jugular veins
  - pacemaker syndrome-like symptoms: fatigue, near syncope, shortness of breath, sensation of fullness in the neck and palpitations described as ‘pauses’ or ‘strong beats’

Picture 1 Indications for pacemaker implantation as in ESC guidelines (2013)

Picture 2  First-degree AV block management

Olshansky, B. (2017). First-degree atrioventricular block management [Graph]. In Arrhythmia Essentials (2nd ed., p. 36).

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References

1. Brignole M, Auricchio A, Baron-Esquivias G, Bordachar P, Boriani G, Breithardt OA, Cleland J, Deharo JC, Delgado V, Elliott PM, Gorenek B, Israel CW, Leclercq C, Linde C, Mont L, Padeletti L, Sutton R, Vardas PE; ESC Committee for Practice Guidelines (CPG), Zamorano JL, Achenbach S, Baumgartner H, Bax JJ, Bueno H, Dean V, Deaton C, Erol C, Fagard R, Ferrari R, Hasdai D, Hoes AW, Kirchhof P, Knuuti J, Kolh P, Lancellotti P, Linhart A, Nihoyannopoulos P, Piepoli MF, Ponikowski P, Sirnes PA, Tamargo JL, Tendera M, Torbicki A, Wijns W, Windecker S; Document Reviewers, Kirchhof P, Blomstrom-Lundqvist C, Badano LP, Aliyev F, Bänsch D, Baumgartner H, Bsata W, Buser P, Charron P, Daubert JC, Dobreanu D, Faerestrand S, Hasdai D, Hoes AW, Le Heuzey JY, Mavrakis H, McDonagh T, Merino JL, Nawar MM, Nielsen JC, Pieske B, Poposka L, Ruschitzka F, Tendera M, Van Gelder IC, Wilson CM. 2013 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy: the Task Force on cardiac pacing and resynchronization therapy of the European Society of Cardiology (ESC). Developed in collaboration with the European Heart Rhythm Association (EHRA). Eur Heart J. 2013 Aug;34(29):2281-329. doi: 10.1093/eurheartj/eht150. Epub 2013 Jun 24. PMID: 23801822.
2. Brian Olshansky, Mina K. Chung, Steven M. Pogwizd, Nora Goldschlager, Chapter 2 - Bradyarrhythmias—Conduction System Abnormalities, Editor(s): Brian Olshansky, Mina K. Chung, Steven M. Pogwizd, Nora Goldschlager, Arrhythmia Essentials (Second Edition), Elsevier, 2017, Pages 28-86, ISBN 9780323399685, https://doi.org/10.1016/B978-0-323-39968-5.00002-0.
3. CAMM, A. J., LÜSCHER, T. F., & SERRUYS, P. W. (2009). The ESC textbook of cardiovascular medicine. Oxford, Oxford University Press
4. STRAUSS, David G., et al. Marriott's Practical Electrocardiography. Wolters Kluwer, 2021

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