Second degree AV block electrocardiogram

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Cafer Zorkun, M.D., Ph.D. [2] Syed Musadiq Ali M.B.B.S.[3]

• Also called the Wenckebach phenomenon or Mobitz type I block^[1][2]
• Intermittent failure of the supraventricular impulse to be conducted to the ventricles, not every P wave is followed by a QRS
• There is a 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^[3]
• 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.^[4]
• In patients with normal QRS width, the block is usually located in the AV node

• There is a 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

Shown below is a two-lead rhythm strip from a patient in the emergency room.

• The rhythm is sinus with second-degree AV 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 AV 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

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

• There are intermittently blocked P waves.
• In the conducted beats, the PR intervals remain constant.
• The PR is fairly constant except that slight shortening may occur in the first beat after the blocked cycle.
• This is the result of improved conduction following the block.
• Most patients with type II second-degree AV block have associated bundle branch block.
• In these instances the block is usually located distal to the His bundle, in approximately 27 to 35% of patients however, the lesion is located in the His bundle itself, and a narrow complex may be inscribed.
  
• 2:1 AV Block:

• Impossible to determine whether the second-degree AV block is type I or type II.
• A long rhythm strip is helpful to document any change in the behavior of the conduction ratio
• When the atrial rate is increased by exercise or by atropine, the AV block in type I tends to decrease, and that in type II tends to increase.

Shown below is an electrocardiogram of a 12 lead EKG with a 2:1 AV block.

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

Shown below is an electrocardiogram of a type II second degree AV block (Mobitz type II).

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

For more EKG examples of Second Degree AV Block click here

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