Distinguishing Block
- Fri, 11/13/09 - 1:49pm
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What are your tips for distinguishing between second-degree atrioventricular block type 1 and 2?
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My tip for distinguishing between second-degree AV block, Mobitz type 1 and 2 on the surface electrogram is to pay close attention to the PR interval.
If the PR interval varies with an occasional dropped QRS, it is type 1 or commonly referred to as Wenckebach. Specifically, the PR interval lengthens with each beat until it is blocked and does not conduct through to the ventricle. This is shown on the surface electrogram as a dropped beat. After the dropped QRS complex, the PR interval resets and the cycle repeats.
In second-degree, type 2 there will be more P waves than QRS complexes. However, the PR interval for those beats that a QRS follows a P wave, the PR interval will remain constant and unchanged. The ratio of P waves (atrial activation) to QRS complexes (ventricular activation) is expressed as a ratio. This ratio may be constant or variable. Example: If a surface electrogram displays two P waves for every QRS complex and the PR interval is always the same, the rhythm would be second-degree AV block type 2 with 2:1 conduction.
— D. Passey, RCIS, RCES
Reply to this comment »It really is pretty simple. With Mobitz I (WBB), there is a gradual (sometimes fairly subtle) prolongation of the PR interval until a QRS is "dropped". There is a short PR interval after the dropped beat as the cycle starts over again.
In Mobitz II, there is no change in the PR interval; a QRS is randomly dropped (although it is often in a fairly regular pattern, such as 2:1 block). The important thing to remember is that in Mobitz II the PR interval is consistent in conducted beats.
Also, you really have to have a 12 lead or at least a 6 or 12 second strip to make the diagnosis of heart blocks. In WBB, the PR may prolong over 3 or 4 beats, and in Mobitz II you really need to be able to see the pattern of the "dropped QRS.
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