2:1 2nd degree block

2nd Degree 2:1 Block (2:1 heart block)

2nd degree AV Block 2:1 is a fairly rare form of 2nd degree AV Block. It can either be
2nd degree Mobitz I (Wenkebach) or 2nd degree Mobitz II. The term "2:1 block" simply means
there are two P-waves for every QRS. The first P-wave has no QRS because it is nonconduted
(a dropped beat). The second P-wave is conducted having a QRS. This 2:1sequence is a fixed
pattern. Just by looking at the 2nd degree rhythm (see above), you would not be able to
determine the origin of the block, meaning if it is a Mobitz I (Wenkebach) or Mobitz II.
It is crucial to be able to determine which type of block it is, understanding that Mobitz II
puts the patient at a high risk of going into 3rd degree block, having a stroke or sudden death,
and would be in great need of a pacemaker placement. To determine the origin of the block
(Mobitz I or Mobitz II), there are a few indicators (see below) that will give you a high
probability of determination. Without any identifying factors or previous EKGs, and just
observing a 2:1 pattern, a cardiologist may need to do a electrophysiology study to determine
the type of block. There are a few other techniques using meds etc. to identify which block.
In the past, the term "Pseudo" 2nd degree type II was used in some studies because the
2nd degree type I (wenkebach) was masquerading around as a Mobitz II in disguise.

As a general rule, If the 2:1 Block is a 2nd degree Mobitz I (Wenkebach):

The 2:1 block would be preceded by a widening PR interval
The QRS complex would also need to be narrow.(see below)
If a wide QRS complex (bundle branch block)is present, there would be a good
chance of being a Mobitz II even if a widened PR interval precedes the 2:1 block.

This ekg strip shows a sinus rhythm with a widening PR going into 2nd degree 2:1 block
with a narrow QRS. If the 2:1 AV block continues on for any duration, all of the 2nd degree
block is Mobitz I (Winkebach). This particular strip was determined to be 2nd degree type I.

Remember that second degree type I (wenkebach) is an "AV nodal block", which is an issue in
the AV node itself. While second degree Mobitz II AV block is "infranodal" (below the AV node),
a conduction disease of the His-Purkinje system.

In order to distinguish between the two potential rhythms when an ECG reveals 2:1 AV nodal block,
a couple different maneuvers can be employed:

Carotid sinus massage or adenosine: This slows the sinus rate allowing the AV node more time to
recover, thus reducing the block from 2:1 to 3:2 and unmasking any progressing prolonging PR intervals
that would indicate second degree type I AV nodal block.
Atropine administration: This enhances AV nodal conduction and could eliminate second degree
type I AV nodal block since it is due to slowed AV nodal conduction)
Exercise ECG testing (enhances AV nodal conduction and could eliminate second degree type I
AV nodal block since it is due to slowed AV nodal conduction)
A type of atrioventricular (AV) block caused by an abnormality below the AV node, either in
the bundle of His or in both bundle branches. Infranodal block has more serious clinical implications
than a block at the level of the AV node. The condition is often the result of arteriosclerosis,
degenerative diseases, a defect in the conduction system, or a tumor

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Information was used from following websites
Wikipedia and Emedicine Medscapes

Info and graphics were used from, WWW>healio.com