I don’t have any clinical history to go with this ECG, but how best should we describe the rhythm?  I have numbered the beats to facilitate discussion.

Analysis & continuing discussion below

I agree with Scott’s analysis that this is second-degree AV block in which beats 1, 3, 5 and 7 are conducted beats and beats 2, 4, 6 and 8 are junctional escapes. There is an underlying sinus arrhythmia in which two of the P waves cannot be visualised because they occur simultaneously with QRS complexes (beats 2 and 4).

Once all the P waves have been identified it can be seen that only 1 in 3 of them are conducted. This may lead some to describe this as an example of high-grade block, but only one of each pair of non-conducted P waves can truly be described as being blocked – failing to conduct when they would be expected to – because the other non-conducted P wave in each pair occurs at such a time – almost simultaneous with the junctional discharge – that it cannot possibly conduct to the ventricles.

As there are never two consecutively conducted P waves, this cannot be described as either Mobitz I or Mobitz II AV block. Instead, it is a variety of second degree block that doesn’t fit comfortably into the Mobitz classification. The phenomenon we see here in which conducted beats alternate with escape beats is often described as an escape-capture bigeminy. I have tried to illustrate all of this with a laddergram, which now accompanies the 12-lead ECG.

One question: why do the PR intervals of the conducted beats vary slightly?

Continue the discussion in the comments and share your thoughts.

Dr Dave Richley, 
Associate Lecturer
Newcastle University

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