Inappropriate defibrillator discharge despite evident atrial flutter on stored device electrogram having significant RR variability - What is the mechanism?

It is crucial to analyze device stored electrograms (EGMs) to decipher whether the therapies were appropriate or inappropriate. We report a case of an inappropriate defibrillator therapy delivered during an obvious atrial tachyarrhythmia despite the dual chamber SVT discriminators being appropriately programmed for underlying rhythm abnormality. The therapy was delivered even though the tachycardia clearly satisfied V < A rate branch along with significant RR interval variability. The morphology match was misleading possibly due to malalignment of the peaks. On the other hand, the Abbott algorithm decides stability delta based on 2nd shortest and longest RR interval. This delta was <40 ms in spite of a irregular rhythm. The Atrio-Ventricular Association (AVA) algorithm, a second check-post designed for such situations, also got deceived due to variation of AV intervals. These led to fulfilment of stringent 'ALL' criteria programmed to make a diagnosis of VT and delivered therapy inappropriately.