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ECG Findings

  • Progressive PR-interval prolongation throughout the cardiac cycle until a P wave occurs without a QRS complex (“dropped” beat).

  • After the dropped QRS complex, the cycle continues again with the PR interval of the 1st beat in the cycle always shorter than the PR interval of the last beat in the previous cycle.

  • P wave may be hidden by the preceding T wave.


  1. The number of P-QRS complexes prior to the “dropped” beat may vary.

  2. A clue to the diagnosis of Mobitz type I AV block can be found in the appearance of grouped QRS complexes.

  3. This type of block is often asymptomatic and may be seen in athletes or others with high vagal tone.

  4. These patients have low risk of progression to complete heart block and usually do not require a pacemaker. However, Mobitz type I heart block may be caused by inferior MI or drugs (digoxin, amiodarone, β-blockers, calcium channel blockers).


Second-Degree AV Block (Mobitz I, Wenckebach). (ECG contributor: James Paul Brewer, MD.)


The PR interval gradually increases (double arrows) until a P wave is not followed by a QRS and a beat is “dropped” (brackets). The process then recurs. P waves occur at regular intervals, although they may be hidden by T waves.

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