Chronic intermittent atrial standstill with intraatrial block and split atrial potentials.

A 24-year-old male with chronic atrial standstill underwent an electrophysiologic study. No atrial activity was recorded from the right atrium which did not respond to stimulation with up to 10 volts. After the administration of atropine, non-localized split atrial potentials appeared, and intraatrial phase 4 block and atrial flutter with intraatrial 2:1 block were observed. The atrium then responded to electrical stimulation (less than 2 volts). Persistent sinus node activity with intraatrial block was found during post-pacing atrial pauses.