A 68-year-old is admitted for surgical treatment of a small bowel carcinoma. Recently she has complained of episodes of weakness and faintness:

• Sinus rhythm with Mobitz II second degree AV block
• Left bundle branch block

Comment: Symptoms of faintness along with evidence of Mobitz II block and existing LBBB are diagnostic for severe His Purkinje disease and are an indication for pacemaker implantation.

Second ECG is recorded 1 day later. Notice the changes of the QRS pattern:

• Sinus rhythm with Mobitz II second degree AV block (3:2 and 2:1 conduction)
• The QRS pattern now varies. After the longer RR interval there is normal or RBBB pattern (in ECG 1 all the QRS complexes had a LBBB pattern)
• Alternating bundle branch block

Comment: Disease in both bundle branches with differing refractory periods and conductive properties allows for varying degrees of recovery and is the cause of this alternating bundle branch block pattern; these patients will typically progress to complete heart block and pacemaker implantation is warranted.