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©2013 UpToDate ® Print Email Blocked atrial premature beat Nonconducted or blocked atrial premature beats occur when there is premature activation of the atrial myocardium from an ectopic atrial focus at a time when the atrioventricular node is still refractory due to the previous sinus beat. Since the block is in the atrioventricular node, there is an isolated or nonconducted P wave seen on the ECG. In this case, the P wave is located within T wave of the first beat. 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.