Fainting episode in a pacemaker dependent patient with two implanted devices. Too much of a good thing!

A 73 year old man with CRT-D system and pacemaker dependence was admitted to the emergency department due to a syncopal episode. The device interrogation was performed. An RV ventricular sensing test was executed in VVI mode at 50 b/m. During testing the patient developed transient loss of consciousness with full recovery after stopping the sensing test. Applying asynchronous pacing mode, a pacing spike occurred at a faster rate of 65 b/m without evoking any ventricular capture. Looking at the chest X-ray, we found an abandoned right sided, single chamber VVI pacemaker in a submuscular pocket. The CRT-D system was positioned on the left side. The abandoned pacemaker was in ERI and it switched to VVI 65 b/m resulting in inhibition CRTD device during sensing test. This troubleshooting highlights the possible interference between two devices.