First Degree Heart Block

Definition

• PR interval > 200ms (five small squares)
  
• ‘Marked’ first degree block if PR interval > 300ms

Examples

First degree heart block (PR interval > 200 ms)

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

Causes

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

Clinical significance

• Does not cause haemodynamic disturbance
• No specific treatment is required

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)

Further Reading

• ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
• ECG CLINICAL CASES — Your favourite ECG’s placed in clinical context with a challenging Q&A approach
• ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
• ECG Exam Template — a framework for the FACEM part 2 exam.
• ECG Reference Sites on the WEB — the best of the rest

Author Credits

References

• Hampton, JR. The ECG in Practice (5th edition), Churchill Livingstone 2008.
• Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.