Interruption of cardiac resynchronization therapy triggered by the automatic right-ventricular pacing threshold test

We report on three patients with heart failure and left bundle branch block who received a BIOTRONIK implantable defibrillator with resynchronization therapy which manifested loss of resynchronization only at a specific time of the night. Desynchronization was sudden and repeatedly initiated by the daily automatic right ventricular pacing threshold test. Loss of resynchronization occurred after switching back from the temporary test mode to the permanent biventricular pacing mode due to the reactivation of the left ventricular (LV) control of the timing cycles. LV sensed events prevented the emission of an LV paced event by virtue of a realigned LV upper rate interval, thus inhibiting LV pacing.