Home time? Or Pacemaker implantation?

Answer: This is an example of 2:1 AV block during sinus rhythm.

We cannot differentiate 2:1 block to diagnose Mobitz 1 or Mobitz II without further information.

• If this represents asymptomatic Mobitz I, it is usually benign and can be safely observed.

 

• If this is Mobitz II, the patient is at high risk of progression to complete heart block and will very likely require permanent pacemaker implantation.

 

 

A Brief Overview of Mobitz 1 and Mobitz II:

Mobitz 1:

• A normal Physiological behaviour of the AV node.

 

• Every person’s AV node is capable of producing this rhythm under the right conditions, and in an EP study, we routinely induce this rhythm in every single patient on the EP table, even when they do not have any conduction system disease.

 

Causes & Consequences:

• Majority of the time, due to high vagal tone, when patient is rested.

 

• Normalises when patient is exercising/during high sympathetic drive.

 

• Benign, no cause for concern.

 

Occurs primarily within the AV node, because the AV node is “mostly” the only tissue that displays this progressive Conduction Velocity slowing, especially when influenced by vagal tone. Known as “decrementation”

 

Mobitz II:

• Abrupt failure of the His-Bundle or Bilateral Bundle Branches to conduct (Often a fixed 2:1 – 4:1 (A:V) conduction), without prior PR lengthening.

 

• Each conducted P wave will have a fixed PR interval.

 

Causes

• Due to fixed and progressive His-Purkinjie system disease. Essentially scar build up.

 

• Ischaemic heart disease causing ischaemic injury & permanent scarring and Purkinjie-myocyte dysfunction.

 

• Non-ischaemic heart disease.

 

Sudden loss of CO = risk of Syncope. Unlike Mobitz 1, it will WORSEN with Exercise and usually gets worse over time

 

How can we differentiate them?

Dynamic manoeuvres:

Ask them to perform some straight leg raise whilst in bed. The aim is to safely increase sympathetic drive, raising the SA nodal rate and theoretically improving the AV nodal conduction in a healthy AV node.

• If 2:1 becomes a more classic “Wenckebach” pattern with periods of 1:1 conduction, then the 2:1 AV block was likely Mobitz 1, occurring within the AV node.

 

• If conduction worsens and becomes 3:1, then this is likely Mobitz II and fixed His-purkinjie disease requiring PPM treatment.

 

Indicators of His-Purkinjie disease (predicting Mobitz II) are:

• Bundle Branch Blocks (especially LBBB)
• Fascicular Blocks
• Periods of 3:1 AV conduction (or worse)
• Older age
• History of anterior MI
• History of Syncope

 

Indicators of Mobitz I are:

• Younger age
• No cardiac history
• Fit and high volume endurance training (high vagal tone)
• Inferior MI presentation (often associated with transient vagal mediated arrhythmias)

 

My two Cents:

• 2:1 AV Block is an ECG feature, and NOT a diagnosis. It provides no prognostic information. Commonly seen in inferior MI presentation. 

 

• 2:1 AV block can be an example of a fairly benign “Short-Cycle Wenckebach” or the more sinister Mobitz II 2nd degree AV block due to His Purkinjie disease.

 

• If AV block worsens with increasing sympathetic drive, then fixed His-Purkinjie disease is a likely cause and Mobitz II is a more likely diagnosis.

 

• Other ECG features which predict a Mobitz II diagnosis includes pre-existing bundle branch/fascicular blocks or evidence of 3:1 AV Block or worse

 

• Don’t jump too quickly to diagnosis in the context of 2:1 AV block as the two differentials have very different prognostic and management implications.

 

Thanks for tuning in :)
Cheers
Mitch & CPiP Team.

These were predominantly based on the ECG in Practice course lectures on differentiating infrahisian and suprahisian AV blocks.