Did You Know That: There are Two (2) Types of Mobitz I

Second degree AV Block Type I is also known as AV Wenckebach or Mobitz I. Did you know that there are two (2) types of AV Wenckebach? It could either be Typical or Atypical AV Wenckebach. This is how to recognize these ECG Patterns:

Second Degree AV Block Type I (Typical)

ECG Recognition:

• ·       The P wave is normal.
• ·       The PR interval progressively lengthens until a P wave is not followed by a QRS.
• ·       As the PRI lengthens, there is shortening of the RR interval.
• ·       The RR interval containing the dropped P wave is less than 2x of the shortest RR interval.
• ·       The PRI of the first conducted P wave (may be normal or prolonged) is shorter than the PRI of the       last conducted P wave.
• ·       The largest increment in the PRI is usually on the second conducted P wave.
• ·       There is "group-beating" on the ECG.

Figure 1 – Sinus rhythm, second degree AV block type I (Mobitz I/Wenckebach) with 3:2 AV conduction. There are regular sinus P waves at a rate of about 100 bpm. There is group-beating of the QRS with a P to QRS ratio of 3:2. The first conducted P waves are marked with green arrows with a PR interval (PRI) of 0.20 seconds. The next conducted P waves are marked with a blue arrow with a PRI of 0.28 seconds. The non-conducted P waves are marked with red arrows. As can be seen there is prolongation of the PRI interval until a non-conducted P wave.  This is typical AV Wenckebach.

Mobitz I interrupted by a PVC

 What is the interpretation?

Figure 1 - Lead II

Atypical Wenckebach

Vignette: An elderly with pneumonia. What is your ECG diagnosis?

The rhythm is sinus. After QRS # 7, there is a non-conducted P wave. The PRI of the first conducted P wave is shorter compared to the last conducted P wave. There is also a prominent increase in the PRI on the second conducted P wave (QRS # 9). These are all hallmarks of second degree AV block type I (Mobitz I or AV Wenckebach). An interesting feature here is the PRI prior to the dropped beat is seemingly the same (but prolonged). Also, if you measure the PRI in V1, the PRI of the second conducted beat is seemingly the same after that. This is a hallmark of atypical Wenckebach. Thus, this is atypical Wenckebach. When the conduction ratio is more than 5:4 or 6:5, the PRI remains the same but prolonged. 

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Atypical Wenckebach interrupted by bigeminal PAC's

Is this Mobitz II?

Image 1 - Long lead II

The rhythm is sinus at a rate about 94 bpm with a long PRI or first degree AV block. The middle of the strip seemingly shows sudden onset of bradycardia. Remember that the most common cause of sudden onset of bradycardia in premature atrial complex (PAC). However, it cannot be seen in lead II.

Image 2 - Full disclosure

The full disclosure image revealed the PAC in V2 and not on the rest of the leads. There is no preferred lead to look these nibs. You have to train your eyes to spot those distortions.

The second point what this strip would teach us is the existence of atypical Wenckebach. The PRI was measure using an electronic caliper. The PRI remained prolonged (310 ms) then it increased to 320 ms then it was interrupted by a PAC's. The latter PRI are shorter then there is gradual increase in the PRI. Also remember that when the Wenckebach cycle is longer than 5:6, the PRI prolongation becomes unpredictable and does not behave your typical Wenckebach cycle where most student memorize the longer-longer drop. Atypical Wenckebach does not behave that way.

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