This 52-year-old man with history of hypertension and episodes of paroxysmal atrial fibrillation is seen in the cardiac clinic for follow-up assessment:

• Sinus bradycardia with sinus arrhythmia – Atrial rate 48, min
• AV dissociation, with junctional escape rhythm and occasional AV capture – Ventricular rate 63/min
• T wave abnormality, consider lateral ischemia

Comment: The sinus rate is 48/min, the rate of the escaping junctional pacemaker 55/min. There is intermittent  AV dissociation and occasional AV capture (with first degree AV block, except for the PAC). The P waves fail to conduct, as the AV node is still refractory because of the preceding junctional discharge (the P wave is too close to the preceding QRS). As the junctional pacemaker is faster than the sinus node, the P wave eventually reaches the AV junction when it is no longer refractory and AV capture occurs. The AV capture occurs with a longer PR interval when the P follows closely the preceding junctional complex (the AV node has not completely recovered and conducts slowly). When the P wave occurs through the fully recovered AV node the PR interval is normal.

Rule of the thumb: In AV dissociation, a diagnosis of complete heart block can be difficult to make when the atrial rate is slower than the escape rate as in this case.  A diagnosis of complete heart block typically requires an atrial rate faster than the escape rate, although exceptions can occur.