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This patient presented with ‘dizziness’. An ECG is handed to you.

What is the diagnosis?

Scroll down for the solution

This ECG shows a Type I Wenckebach block

Lets review using the ECG in 20 Seconds approach:

RATE: 11 x 6 = 66bpm

Are there P waves? and are they inverted in aVR and upright in II? YES
Are the P waves associated with the QRS’s They don’t appear to be. In fact they appear to be disassociated from the QRS….BUT are they? If it were a complete heart block, we would expect a more regular QRS and at probably a slower rate, as it would be coming from the ventricles. Let’s look at the QRS in more detail.

They’re not too tall or too small, too wide or too narrow, nor do they have abnormal morphology. 
They are clumped together however. When that happens always think of a Mobitz type of block.

Constant p-p interval, otherwise it may be premature atrial contraction.
We have a constant p-p interval.
For a Type I second degree block (Wenckebach), there is a PR prolongation until a QRS is dropped. The PR interval right after this is shorter.
For a Type II Mobitz, we need a regular dropping of a QRS and the PR interval stays the same.

I have mapped it out below to show you that this is a Wenckebach ECG 
P waves generate a QRS and some are not conducted. The diagram below this demonstrates the PR prolongation. 

To look at the PR interval becoming more prolonged, look at the diagram below:

1 Comment

  1. […] The Mobitz block is one of the most popular questions in the exam and a sure bet. In fact two ECG’s are a sure bet. Any person with syncope and an ECG, remember the ‘ECG’s of Syncope‘. Beware the hidden p waves.Understand the significance of clumping of QRS’sKnow the importance of the p-p interval. Try some examples  […]

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