Over the Top
Report:
Sinus rhythm
First degree AV block (shortest PR 0.48”)[!xe "AV block:first degree:skipped P waves" \b!]
Second degree AV block, Möbitz 1 (Wenckebach)
Skipped P waves (over the top conduction)[!xe "P wave:skipped" \b!]
Comment:
This is quite a rare phenomenon32. To cut the long story short, the longest PR intervals in Wenckebach periods exceed the P-P interval of approximately 0.60” and the QRS complexes are initiated not by the P waves in front of them, but by the ones before! The ‘skipped’ P waves, while still in front of the last QRS in each sequence, are in fact blocked.
Below (Fig 50a) is a bigeminal rhythm engendered by 3:2 conduction; the “skipping” is easier to see. The blocked P wave is just in front (rather than behind) the QRS complex before the pause. The longest reported PR interval33 is 1 second!
Over the top (another apt term learnt from Marriott) conduction occurs regularly in atrial flutter and is not rare in atrial tachycardia. In sinus rhythm, it remains extremely rare.
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