This ECG was reported as complete heart block.

What do you think?It sure looks like it, but the ventricular response is irregular.Look closer.

I have now identified the P waves and they follow two patterns:4:3 Wenckebach AV block (solid red with dropped beat stippled)All dropped beats (stippled blue)This rhythm is referred to as alternating Wenckebach AV blockThe features are:

• Each second P wave is conducted in a Wenckebach sequence until the dropped beat (4:3).
• Coupled with this are alternate P waves, which are all dropped
• The result is that the sequence terminates with two dropped beats (highlighted)

How do we explain this?

• Two blocks; AV node and bundle branches. “Multi-level” AV block”
• Can occur with a bundle branch block
• Syncope and sudden death

Here is another example:

This is a 3:2 Wenckebach AV block and the sequences terminate with two dropped beats (highlighted).Alternating Wenckebach AV block can occur with other situations such as atrial tachycardia.

This innocuous looking ECG was reported as sinus rhythm 95 bpm, but the tech thought better! The PR intervals varied.Let us look more closely.

There is a pattern to AV conduction consistent with alternating Wenckebach AV block, but of course this patient doesn’t need a pacemaker. Note again the 2 sequential dropped P waves terminating a sequence (highlighted).When you look for it you find it! This one appeared a few weeks later.

There are other variations of the multi-level AV block phenomenon.

Here are two Wenckebach AV blocks, a few minutes apart. There is a typical Wenckebach sequence below and above is an AV Wenckebach sequence within atrial bigeminy.There are other examples including Wenckebach AV block with sino-atrial Wenckebach and even Wenckebach at the ectopic myocardial junction, but those are different stories.Harry Mond