2:1 AV Block

Report:

Sinus tachycardia 112/min

2:1 AV block

Comment:

The term "advanced" or "high-grade" AV block is used to describe 50% or less conduction. It is not a useful term except for the fact that it is used! Marriott is right in suggesting the term be used for <50% conduction only (not, therefore, for 2:1 block).

It is not possible to assign Möbitz 1 or Möbitz 2 label to a 2:1 block: there is no opportunity to observe two consecutive conducted sinus impulses required for the diagnosis. This patient in fact had AV nodal block with its expected Möbitz 1 mechanism at other times (Fig 2a below). The narrow QRS complex and the first-degree block in the conducted beats suggest an AV nodal mechanism even in 2:1 conduction. It is far commoner than Möbitz 2, anyway.

The underlying tachycardia is often not appreciated in the presence of relatively slow ventricular rate; it has, however, almost the same significance as a fully conducted sinus tachycardia. Why no bradycardia? Because the sinus rate is too fast: 112/2 = 56.

In aortic endocarditis, the presence of AV block is a rather sinister sign, implying a ring abscess and a poor prognosis.

Figure 2a. Möbitz 1 (Wenckebach) conduction revealed a few minutes later, with several 3:2 sequences.

Fig 3. Some complexes are different. Why?