Diagnosis, Wenckebach Squared?

Answer:

• The presence of a progressively prolonging PR interval that abruptly shortens (i.e. resets) after a pause in the rhythm is strongly suggestive of Wenckebach AV block.

Explanation:

• The underlying process is progressive fatigue of dysfunctional AV node cells. Commonly, this culminates in a non-conducted P wave, as the AV node cells fatigue to the point where they are unable to conduct a further impulse.
• However, anything that interrupts the rhythm (e.g. a sinus pause or ventricular ectopic beat) will give the AV node cells chance to “rest”, following which they will conduct normally.

Answer:

• Notably absent is the lack of a non-conducted P wave. While it is tempting to assume (as I did) that the non-conducted P wave must be hidden in the preceding T wave, there is no evidence of this – the P waves are too tall to be superimposed on the T waves without producing an obvious “bump”.

Explanation:

• As discussed above, the presence of a non-conducted P wave is not required to make the diagnosis of Wenckebach AV block.

Answer:

The frequently dropped P waves occurring in a regular pattern are suggestive of second degree SA block (type I). Features that are supportive of this:

• Grouping of the QRS complexes
• The duration of the each sinus pause is less than double the preceding PP interval

Explanation:

There are two types of cells in the SA node – the inner core of P cells that produce the impulse, and the outer layer of T cells that transmit the impulse out into the atrium. Analogous to Wenckebach AV block, progressive fatigue of the T cells produces second-degree SA exit block (type I), also known as Wenckebach SA block.

This is the most common type of SA block detectable on the surface ECG and is characterized by grouping / clustering of the QRS complexes culminating in a dropped P wave.

See our ECG library page on SA block for further explanation.

Possible causes of this ECG pattern would include conditions that suppress both SA and AV node function, e.g.

• Sick sinus syndrome
• Increased vagal tone
• Inferior AMI
• Myocarditis
• Drugs: digoxin, beta-blockers, Ca channel blockers

So, the final diagnosis:

Wenckebach AV block with Wenckebach SA block!

• Burns E. Diagnosis, Wenckebach?
• Burns E. Block: Mobitz 1 Wenkebach
• Hampton, JR. The ECG In Practice, 6e
• Mattu A, Brady W. ECG’s for the Emergency Physician 1
• Mattu A, Brady W. ECGs for the Emergency Physician 2
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
• Wagner, GS. Marriott’s Practical Electrocardiography 12e

Further Reading

• Wiesbauer F, Kühn P. ECG Mastery: Yellow Belt online course. Understand ECG basics. Medmastery
• Wiesbauer F, Kühn P. ECG Mastery: Blue Belt online course: Become an ECG expert. Medmastery
• Kühn P, Houghton A. ECG Mastery: Black Belt Workshop. Advanced ECG interpretation. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.
• Wiesbauer F. Little Black Book of ECG Secrets. Medmastery PDF