Mobitz type I second degree atrioventricular (AV) block

Several characteristic features of Wenckebach second degree AV block are noted on this ECG (P waves are marked by arrows): 1) The PR interval after the second P wave is longer than the preceding PR interval; the third P wave is not conducted at all. 2) The PP intervals are constant; however, the RR interval surrounding the completely blocked P wave is longer than the preceding normal RR interval. 3) The fourth P wave (after the blocked beat) is conducted normally; this PR interval is shorter than the PR interval that immediately preceded the pause.

The normal P wave in sinus rhythm is slightly notched since activation of the right atrium precedes that of the left atrium. The P wave is upright in a positive direction in leads I and II. A P wave with a uniform morphology precedes each QRS complex. The rate is between 60 and 100 beats per minute and the cycle length is uniform between sequential P waves and QRS complexes. In addition, the P wave morphology and PR intervals are identical from beat to beat.