AV block: 2nd degree, “high-grade” AV block


  • Second degree heart block with a P:QRS ratio of 3:1 or higher, producing an extremely slow ventricular rate.
  • Unlike 3rd degree heart block there is still some relationship between the P waves and the QRS complexes.
  • High-grade AV block may result from either Mobitz I or Mobitz II AV block.


high grade AV block

  • High-grade AV block (4:1 conduction ratio).
  • Atrial rate is approximately 140 bpm.
  • Ventricular rate is approximately 35 bpm.
  • Broad QRS complexes suggest that this may be due to Mobitz II block (see “fixed ratio blocks” for a discussion of this concept).

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.

Print Friendly


  1. ECGnaive says

    This is probably a stupid question..but how do you differentiate between this and atrial flutter with block..it is merely the atrial rate of 140 vs usu 300? At 140 there is atrial tachycardia still…is this in an attempt by the body to improve chronotropy by increasing heart rate at sinus node which obviously does not work as most P’s non conducted? Thanks!