Atrioventricular block electrocardiogram

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Atrioventricular block Microchapters


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Differentiating Atrioventricular block from other Diseases

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Edzel Lorraine Co, DMD, MD[2]



The main diagnostic modality used in determining whether a person has heart block, is the electrocardiogram.

  • First degree heart block consists of a prolonged PR interval of more than >200msec.
  • Second degree heart block consists of Mobitz type I and Mobitz type II heart block.
    • Mobitz I or Wenckebach block will show a progressive prolongation of the PR interval, until a ventricular beat is missed.
    • Mobitz II AV block consists of a constant PR interval with intermittent missed beats.
  • Complete heart block or third degree heart block will be depicted by a complete disassociation of atrial and ventricular beats.

EKG Examples

First Degree AV Block EKG Examples

Shown below is an EKG image showing sinus rhythm with a prolonged PR interval (>200ms.) which is first degree AV block. There is also a left axis deviation (axis between -30 and -90 degrees) with r waves in the inferior leads. This axis deviation is consistent with a left anterior fasicular block.


Copyleft image obtained courtesy of ECGpedia,

Second Degree AV Block EKG Examples

Mobitz I Second Degree AV Block EKG Examples

Shown below is an EKG image of ventriculophasic reflex during second degree AV block Mobitz I. The PP interval where the blocked sinus beat occurs, is prolonged.

Ventriculophasic Reflex.jpg

Copyleft image obtained courtesy of ECGpedia,

Shown below are two rhythm strips showing Mobitz I A/V block with a gradual increase in the PR interval before the dropped p wave. Note the 2:1 block in the lower strip, and that one can not use this to determine if the block is Mobitz I or II as more than one conducted P wave is required to do this.

Mobitz type I block.jpg

Copyleft image obtained courtesy of ECGpedia,

Mobitz II Second Degree AV Block EKG Examples

Shown below is an EKG image of two-to-one AV block, which can represent benign block within the AV node or disease of the His-Purkinje system. Certain electrocardiographic features and maneuvers can help in distinguishing where the location of block exists. A long PR interval with a narrow QRS suggests an intranodal block. A short PR interval with intraventricular conduction delay or bundle branch block suggests disease below the node. Responses to atropine, exercise and carotid sinus massage can be helpful in diagnosis. Atropine will improve AV nodal conduction but will worsen block within diseased His-Purkinje fibers. Exercise has a similar effect, improving conduction in cases where block exists only in the node, but worsening when block is subnodal. Alternatively, Carotid Sinus Massage will slow conduction when block occurs in the AV node, but will improve conduction in diseased His-Purkinje tissue by allowing for refractoriness to recover.


Copyleft image obtained courtesy of ECGpedia,

For more EKG examples of First Degree AV Block click here


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