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©2013 UpToDate ® Print Email Atrial premature beat with aberration The atrial premature beat in this tracing is superimposed upon the T wave of the preceding sinus beat. Since the atrial premature beat occurs early, before the right bundle has fully recovered, •the QRS complex has a complete right bundle branch block morphology (RSR'), reflecting the inability of the right bundle branch to conduct the impulse since it is still refractory. Sinus rhythm 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.