I don’t have clinical information in relation to this month’s ECG but I don’t think it’s absolutely necessary because what I’m interested in is simply a full description of the cardiac rhythm, including any conduction abnormalities. I’m not completely sure of the correct answer, and it may be that there is more than one plausible explanation for what we see, so I’m very interested to find out what you think.

Sorry about the artefact towards the end of the ECG but the V1 rhythm strip is of good quality throughout so analysis should not be hindered.

The Answer

To help me explain what I think is happening here I have drawn a laddergram on which I have labelled all the QRS complexes R1 to R8 and all the P waves P1 to P12. One of these P waves – P7 – cannot be seen clearly but I think its presence and position can be inferred from the attenuated S wave in R5 in lead II. I have also measured in milliseconds all the PR intervals and shown them in red, and all the PP intervals, which are shown in green.

At first sight it may appear that there is no relation between P waves and QRS complexes and, because the ventricular rate is slow, it may be tempting to diagnose complete heart block. However, the ventricular rate is obviously irregular, and this almost rules out complete heart block with an escape rhythm.

Instead, I think there is Wenckebach AV block with some very long PR intervals, so long in fact that it is difficult to see the relationship between the P waves and the QRS complexes. One of the features of this ECG which makes the Wenckebach block particularly difficult to see is that two of the P waves – P4 and P10 – appear shortly before QRS complexes (R3 and R7) but both of these atrial impulses are blocked, not conducted, and it is the previous P waves – P3 and P9 – that represent conducted impulses. This is difficult to appreciate because when a P wave occurs just before a QRS we automatically tend to link the two events in our minds and assume that there is conduction. It is much more difficult to link a P wave to a QRS when they are widely separated with a very long PR interval, as here. The longest conducted PR interval on this ECG is 940 ms, which some may regard as unfeasibly long, but PR intervals of over one second have been reported.

You may wonder why, if this explanation is correct, there is some variation between equivalent PR intervals in the three different complete Wenckebach sequences. I think this is due to slight variation in the atrial rate. Generally, in Wenckebach block, the earlier a P wave occurs in relation to the preceding QRS, the longer is likely to be the PR interval. This is because relatively early atrial impulses are likely to find the AV node in a less advanced state of recovery, causing them to conduct more slowly.  To illustrate how this principle works, let’s compare the first 2 Wenckebach sequences. Each one begins with a PR interval (P2-R2 and P5-R4) of 560 ms. But the 2nd PR interval (P3-R3) of the first sequence is 940 ms, whereas the 2nd PR interval (P6-R5) of the 2nd sequence is only 760 ms. Why the difference? Well, the 1st PP interval (P2-P3) of the 1st sequence, at 720 ms, is less than the 1st PP interval (P5-P6) of the 2nd sequence, at 780 ms. This means that P3 occurs at a slightly shorter interval after the preceding QRS than does P6, with the consequence that AV nodal conduction is slower for P3 than for P6. This phenomenon is known as RP-PR reciprocity.

In summary, this ECG appears to show sinus rhythm with 3:2 Wenckebach AV block and some long PR intervals. Well done to all who got this right, and particularly to Scott Walton, who worked it all out very quickly.

If you think I’ve got it wrong, and especially if you have an alternative explanation, please post your opinion. The discussion doesn’t have to end with my ‘answer’.

I am very grateful to Maxine Guillen for sharing this ECG with me.

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