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)
  • Electrolyte disturbances (e.g. Hyperkalaemia)
  • 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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  1. Klaas says

    I think hypokalemia is incorrectly listed as one of the causes of first degree AV block. My ECG handbook says it is hyperkalemia that can cause first degree AV block (and some internet sources confirm this).