A 65-year-old man is referred to the Emergency Room by his family physician because of several episodes of faintness during the past 2 days.

• Sinus rhythm with 3:2 Mobitz II second degree AV Block
• Ventricular rate 56/min
• Alternating right bundle branch block and left bundle branch block pattern

Comment: This ECG demonstrates that the anatomic site of Mobitz II second degree AV block is at the level of the His-Purkinje system. The PR interval is 130 msec and constant. The conduction through the AV node is normal. There is second degree AV block. As the right bundle has a longer refractory period than the left bundle, after the longer pause it is still refractory, while the left bundle has recovered. The AV conduction is therefore through the left bundle (right bundle branch block). The next AV conduction is through the right bundle which has recovered while the left bundle is now refractory (the QRS has a LBBB pattern). Finally, both bundles are refractory and there is AV block. This pattern repeats itself through the tracing. Alternating bundle branch block often progresses to complete heart block and is a Class I indication for pacemaker implantation.