First Degree Heart Block

Definition

PR interval > 200ms (five small squares)

• There is delay, without interruption, in conduction from atria to ventricles
• ‘Marked’ first degree heart block is present if PR interval > 300ms

First degree heart block

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Causes of First Degree Heart Block

• Increased vagal tone
• Athletic training
• Inferior MI
• Mitral valve surgery
• Myocarditis (e.g. Lyme disease)
• Electrolyte disturbances (e.g. Hyperkalaemia)
• AV nodal blocking drugs (beta-blockers, calcium channel blockers, digoxin, amiodarone)
• May be a normal variant

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Clinical significance

• As an isolated finding this is a benign entity that does not cause haemodynamic instability
• No specific treatment is required

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ECG Examples

Example 1

• Marked first degree heart block
• PR interval > 300 ms, P waves are buried in the preceding T wave

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Example 2

• Sinus bradycardia with 1st degree AV block
• PR interval > 300 ms

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Example 3

• Normal sinus rhythm with 1st degree AV block
• PR interval 260 ms

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Related Topics

• AV block: 2nd degree, Mobitz I
• AV block: 2nd degree, Mobitz II
• AV block: 2nd degree, “fixed ratio blocks” (2:1, 3:1)
• AV block: 2nd degree, “high grade AV block”
• AV block: 3rd degree (complete heart block)

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Advanced Reading

Online

• Wiesbauer F, Kühn P. ECG Yellow Belt online course: Become an ECG expert. Medmastery
• Wiesbauer F, Kühn P. ECG Blue Belt online course: Learn to diagnose any rhythm problem. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.

Textbooks

• Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
• Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
• Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
• Hampton J. The ECG Made Practical 7e, 2019
• Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
• Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography 1e, 2009
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
• Mattu A, Brady W. ECG’s for the Emergency Physician Part I 1e, 2003 and Part II
• Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004

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LITFL Further Reading

• ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
• ECG A to Z by diagnosis – ECG interpretation in clinical context
• ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
• 100 ECG Quiz – Self-assessment tool for examination practice
• ECG Reference SITES and BOOKS – the best of the rest