2:1 atrial capture as the cause of pacemaker alternans.

Two patients with sinus node disease and a physiologic pacemaker presented with 2:1 atrial capture. Both atrial leads were screw-in leads (6957 J, Medtronic), with normal position on X-rays. When atrial capture was absent, paced QRS complexes resulted in retrograde conduction to the atria. The next atrial spike was followed by a P wave and normal AV conduction. With atrial screw-in leads, alternating atrial capture may be caused by an altered ventricular and atrial contraction pattern, when retrograde conduction exists. Its presence may precede permanent exit block by several weeks.