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©2013 UpToDate ® Print Email Atrial premature beats The third beat is an atrial premature beat that is normally conducted. The interval between the second sinus beat and the ectopic beat is shorter than the interval between the first two sinus beats. The P wave morphology differs from that of sinus rhythm. Since the RR cycle length is shorter, there is a decrease in the rate of conduction of the ectopic beat through the atrioventricular node. The PR interval is therefore longer than that of the sinus beat. Activation of the ventricular myocardium occurs in a normal fashion; as a result, the QRS complex is unchanged from that of sinus rhythm. 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.