Sinoatrial exit block

Failure of impulse conduction from the sinus node into the atrium results in sinoatrial block. This arrhythmia is characterized by an absent P wave and a prolongation of the RR cycle length (measured as P to P length in this tracing), usually twice the underlying sinus P-P interval. This is a distinguishing feature from sinus pause in which the beat after the pause has a P-P interval less than two times the underlying sinus P-P interval.

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.