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
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
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
Type II Second-Degree AV Block: Mobitz Type II Block
- There are intermittent 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