Cardiology In Critical Care

Arrhythmia Recognition

Second Degree AV Block Type I (Wenckebach)

Rhythmic impulses originating in the SA node are conducted through the AV junction at progressively slower (decremental) speed.  P waves are positive in L2 and negative in AVR and occur regularly at a rate 60-100.  A blocked P wave occurs after 2-5 conducted P waves and this so-called Wenckebach cycle or period is repeated.  The P:QRS ratio of 3:2, 4:3, 5:4 may be constant in all cycles or may vary from cycle to cycle.  The PR interval immediately following a dropped QRS complex is the shortest and may be normal or prolonged; with each successive beat the PR interval becomes progressively longer until a P wave is blocked.  QRS complexes may be normal or wide and bizarre if BBB is present.

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