Cardiology In Critical Care
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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