A 57 year old women with sick sinus syndrome had been implanted with a Biotronik Pikos LP VVI pacemaker. Sinus rhythm was successfully restored by antiarrhythmic drugs three times during the past year. Because of recurrence of atrial fibrillation she was on permanent anticoagulant treatment. A recent attack of atrial fibrillation did not respond to antiarrhythmic treatment, therefore we planned a direct current cardioversion. The pacemaker was programmed to VOO mode with 50 beats/min frequency. The figure shows atrial fibrillation with high ventricular rate, as well as asynchronous pacing. No ventricular capture could be seen, because the pacemaker spikes were falling on the refractory period. The third pacemaker spike—which coincided with the beginning of the T wave—triggered the synchronisation of the cardioverter, resulting in ventricular fibrillation; she underwent immediate defibrillation.

The VOO mode or use of a magnet are recommended for protective purposes during certain interventions, like lithotripsy or electrocautery. Be cautious, however, as the VOO mode should be avoided during direct current cardioversion—as with our patient, the false synchronisation of the cardioverter may result in ventricular fibrillation.

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