Over the Top
Report:
Sinus rhythm
First degree AV block (shortest PR 0.48â)[!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 0.60â and the QRS complexes are initiated not by the P waves in front of them, but by the ones before! The âskippedâ 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 âskippingâ 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.
Fig 50a. Blocked P waves âskippedâ during 3:2 Wenckebach conduction with unusually long PR intervals. Fig 51. Drowsy 83 year old lady in Casualty.
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