A young man with lightheadedness and bradycardia, and an impatient AV node.

A healthy 20 y.o. man presented with lightheadedness. 

The symptoms began about 2 weeks prior and were exertional. He stated that he plays on a college basketball team and he noticed over the previous 2 weeks that every time he exercised with the team he felt lightheaded. 

There was no actual history of syncope. He had had no associated chest pain, shortness of breath or palpitations. He had had no symptoms at rest or associated with positional changes. No history of similar symptoms previously. No history of heart or lung disease. There was no family history heart problems, sudden death, drowning, deafness. He did not take any medications.

Here is his ECG:

What do you think?
The treating physicians diagnosed complete AV block.

There is a narrow complex bradycardia at a rate of about 42.  It is hard to determine if there is a P-wave before the first complex, as that complex is at the edge of the tracing.  
The 2nd complex definitely does NOT have a P-wave in front, nor does the 3rd or 4th.  However, the 5th has a P-wave which is followed very shortly (at less than 120 ms) by a QRS.  The 6th has a slightly longer PR interval, and the 7th and 8th longer still.  
The longest of the PR intervals is the 7th.
What is this rhythm?  Is there AV block?  
No!  At least we see no evidence of block here.  There is no P-wave which does not conduct.  This is AV dissociation.  But not all AV dissociation is due to AV block.  In this case, it is “Isorhythmic Dissociation.”  The sinus node and the AV node just happen to be discharging at the same rate, and also coincidentally are happening at about the exact same time.  
The AV node is too impatient to wait for the sinus beat to conduct.
Let’s look at it again with annotation:
The P-wave in complex 7 probably conducts (red line is PR interval).
But I cannot prove this!
The black lines in complexes 6 and 8 are exactly the same length as the red line in the 7th.
You can see that the QRS initiates before the end of the black line in 6 and 8.
Thus, the AV node is firing before the impulse from the sinus node had a chance to arrive.
So the AV node was too impatient to wait for AV conduction.
For complexes 2, 3, and 4, the P-wave is hidden in the QRS.
Complex 5 has a preceding P-wave, but the very short PR interval makes it obvious that the QRS fired before that sinus impulse had a chance to conduct.
Beyond rhythm, the ECG is completely normal for a young man, with early repolarization (see classic J-waves in II, aVF, V4-V6)
Could there be AV block?  Yes, it is possible, and we cannot disprove AV block based on this ECG. But we have no reason to think there is AV block.
If the AV node is firing, why are there no retrograde P-waves?  Because the sinus node fires before the impulse from below can reach the atrium.  The ascending impulse from the AV node meets the descending impulse from the sinus node and they block each other.
How could we demonstrate absence of AV block?    Just have the patient do a bit of exercise to increase his sinus rate to a rate faster than the AV node rate.  
Another ECG was recorded later in the ED:
There is a slightly faster sinus rate now, almost 50, and now all P-waves are conducting.
This shows that the J-waves were indeed J-waves, not hidden P-waves

Clinical course:
The patient was admitted because of concern for intermittent complete AV block.
A walk test showed appropriate responsiveness of the sinus node with good AV conduction.
An echo was normal.

Learning Point:

1. Complete AV block is only one etiology of AV dissociation.  Isorhythmic dissociation is another.

2. Isorhythmic Dissociation is a benign condition.

Here is a very nice article on AV dissociation:

Here is a nice article on Isorhythmic Dissociation:http://circ.ahajournals.org/content/circulationaha/42/4/689.full.pdf

Here is a nice example of Isorhythmic Dissociation with a Laddergram:

Here are other examples of Isorhythmic Dissociation:

Sudden weakness with bradycardia and bizarre T-waves

Here are other posts on AV dissociation and AV block

AV Dissociation Lecture by K. Wang (28 minutes)

A Mystery Rhythm, Explained by K. Wang’s Ladder Diagram.

Atrial Flutter. What else?? (AV dissociation with block)

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