Discrimination of shockable tachycardia by morphologic stability of ventricular electrogram

Most tachycardia episodes detected by implantable defibrillators as VF do not require shock therapy. Pace terminable VT represents >80% of appropriately detected VF episodes. Rapidly conducted AF accounts for ~75% of inappropriately shocked VF detections. An algorithm for discriminating VT and AF from VF on the basis of morphologic stability has been devised. Morphologic similarity is determined using a template-matching algorithm. Morphologic stability was evaluated by comparing morphology of the current beat to that of a single previous beat. We compared the morphology of the current beat to that of beats preceding the current beat by 1, 4 and 7 beats. As the delay between compared beats increased, the algorithm did better at recognizing unstable morphology, but classified more non-shockable episodes as unstable. Morphologic stability can reduce inappropriate shocks for VT by 95% and inappropriate shocks for AF by 81%