This 56-year-old man reports that he had been well prior to one week ago, when he had an episode of syncope that lasted 15-20 seconds. Since then he reports fatigue and dyspnea on moderate exertion:

• Sinus rhythm
• Complete heart block with junctional escape rhythm (ventricular rate 43/min)

The sinus rate is 70/min, and the ventricular rate is 43/min. The rhythm of the P waves and the rhythm of the QRS complexes are regular, but the P waves have no relationship to the QRS complexes. The PR varies. The atrial impulses do not reach the ventricles: there is complete AV block (third degree AV block). The ventricles are activated by an escape pacemaker, distal to the site of the block. The fact that the QRS complexes are narrow indicates that the escape pacemaker is in the region of the AV node.