First Degree Heart Block


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


First degree heart block (PR interval > 200 ms)

1st degree heart block

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

severe first degree heart block


  • 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

Further Reading

Author Credits


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