Sick sinus syndrome

Tracing of sick sinus syndrome (tachycardia-bradycardia syndrome). The initial three beats show atrial fibrillation (AF) which terminates abruptly and is followed after a long offset pause by a junctional escape beat (J) and then a sinus beat (S).

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.