Second degree AV block electrocardiogram

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Cafer Zorkun, M.D., Ph.D. [2]

Electrocardiogram

Electrocardiographic Findings

Type I Second Degree AV Block

• Also called the Wenckebach phenomenon or Mobitz type I block
• Intermittent failure of the supraventricular impulse to be conducted to the ventricles, not every P wave is followed by a QRS
• There is progressive prolongation of the PR interval until a P wave is blocked
• Progressive shortening of the RR interval until a P wave is blocked
• The RR interval containing the blocked P wave is shorter than the sum of 2 PP intervals
• The increase in the PR interval is longest in the second conducted beat after the pause
• These rules may not be followed because of fluctuation in vagal tone and secondary to sinus arrhythmia.
• In patients with normal QRS width, the block is usually located in the AV node

• there is progressive prolongation of the AH interval until the blocked P wave occurs

• When it is associated with bundle branch block, the block may occur in the AV node, His bundle or the contralateral bundle branch

• in 75% the block is in the AV node
• in 25% it is infranodal
  

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Shown below is a two lead rhythm strip from a patient in the emergency room. The rhythm is sinus with second degree A/V block. Note the progressive lengthening of the PR interval and that the interval that brackets the blocked P wave is less than twice that of the RR interval. This recording suggests a Mobitz I A/V block, but some care has to be taken as the QRS that ends the pause in at least one case looks like a nodal escape beat.

Copyleft image obtained courtesy of ECGpedia, http://en.ecgpedia.org/wiki/Main_Page

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Shown below is an image of an electrocardiogram showing type I second degree AV block (Wenckebach).

Copyleft image obtained courtesy of ECGpedia,http://en.ecgpedia.org/wiki/Main_Page

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