2nd Degree Heart Block Dissected

Today we went over the case of an elderly man with episodes of lightheadedness for one month who was found to have 2nd degree AV block on EKG in the ED (with old LBBB as well). Capture

EKG Criteria for AV blocks:

1st degree: PR prolongation (>200msec)

2nd degree Mobitz I: Wenckebach, progressively prolonging PR interval followed by a “dropped beat,” or nonconducted p wave

2nd degree Mobitz II: constantly prolonged PR followed by a “dropped beat,” or nonconducted p wave

3rd degree or complete: complete AV dissociation between p waves and QRS’s

As you move from 1st toward 3rd degree, the block is more clinically severe, more likely to need treatment, and more symptomatic. Anatomically as you move from first to third, the block becomes more distal in the conduction system.

2:1 Second Degree AV Block: Is it Mobitz 1 or Mobitz II?

In the above EKG, we see a 2:1 second degree AV block, which means that every other p wave is conducted. This does not allow us to see the progression of the PR interval. How are we to know if it is prolonging (Mobitz I) or constant (Mobitz II)? There are 4 ways to help answer this question:

  1. Look at the company it keeps. If you see periods of 3:2 in your patient’s telemetry, the 2:1 block you see on your EKG is likely the same type of block as the periods of 3:2.
  2. If the PR is >300msec or the QRS is narrow, your block is more likely to be Mobitz I (higher up the conduction system in origin).
  3. Give atropine! If the AV nodal conduction is enhanced (less frequent nonconducted p waves), your block is likely Mobitz I. If there is no response, your block is likely Mobitz II.
  4. Carotid sinus massage: This increases vagal tone, which will increase the blockade in Mobitz I, but improve conduction in Mobitz II by allowing more time for excitability to return to the bundle of His.


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