This a patient that was implanted a few week before with a dual chamber pacer for sick sinus syndrome. A case could have been made for an AAI pacer. (see paper on pacemaker)

The pacer is not working properly as the atrial and the ventricular leads both pace the ventricle. This was proved by placing the pacer in an AAI mode which paced the ventricle and not the atrium.

The chest X-ray at first looks good but the PA chest x-ray seems to show that the atrial J wire is slightly lower than expected.

The most likely explanation is that the atrial J wire is caught on the rim of the tricuspid valve. Curiously this region of the valve is ventricular and it is therefore conceivable that the lead is in the atrium but still pacing the ventricle!

For this patient the lead was repositioned and the pacer is now working well.