Undersensing by an ICD Due to Alternans of the Ventricular Electrogram


  • No conflicts of interest.

Address for correspondence: Frederic Van Heuverswyn, M.D., Heart Center, University Hospital Ghent, De Pintelaan 185, 9000 Ghent, Belgium; Fax: +32/9/332.34.66; Email: frederic.vanheuverswyn@uzgent.be


Alternans of the ventricular electrogram (VEGM) during ventricular tachycardia (VT) is a rare cause of ventricular undersensing by an implantable cardioverter-defibrillator (ICD). This report describes a patient with a St. Jude ICD who exhibited sustained monomorphic VT associated with surface QRS alternans, alternating cycle lengths, alternans of the VEGM causing intermittent undersensing of the smaller component, and intermittent 2:1 counting of ventricular intervals during 1:1 sensing in response to the ICD detection algorithm. VEGM undersensing was corrected noninvasively simply by programming the threshold start from 62.5% to 50% which increased the sensitivity based on the amplitude of the VEGM. This maneuver did not affect the satisfactory and stable defibrillation threshold.