Troubleshooting implantable cardioverter-defibrillator sensing problems II.

Abstract

Accurate atrial sensing during tachycardia is essential for reliable determination of atrial rate, which is a critical component of interval-based dual-chamber supraventricular tachycardia (SVT)–VT discrimination algorithms.1 Thus, atrial sensing in dual-chamber implantable cardioverter-defibrillators (ICDs) must be reliable at fast ventricular rates, a technically challenging requirement. Determining the atrial rate during rapidly conducted atrial flutter (AF) requires accurate sensing of low-amplitude signals (Figure I in the Data Supplement). The required high atrial sensitivity increases the risk of atrial oversensing, especially of far-field R waves. Pacemakers rely on a long postventricular atrial blanking period (PVAB) to prevent oversensing of far-field R waves, but ICDs cannot: for a fixed PVAB, the blanked fraction of the cardiac cycle increases with the ventricular rate. For example, a 100-ms PVAB blanks 10% of the cycle at a bradycardia pacing cycle length of 1000 ms, but 33% during rapidly conducted AF with ventricular cycle length of 300 ms. PVABs longer than ≈80 ms produce clinically significant, functional atrial undersensing in rapidly conducted AF,2 and any PVAB may cause undersensing of alternate atrial electrograms in AF with 2:1 atrioventricular conduction. The inherent trade-off between atrial oversensing and undersensing is problematic. Both cause errors in dual-chamber SVT–VT discrimination algorithms.2–6 In contemporary ICDs, ventricular electrogram morphology algorithms mitigate this problem but do not eliminate it. ### Determinants of Far-Field R Waves #### General Considerations In the absence of atrial lead dislodgement, the amplitude of far-field R wave depends on 4 variables. The first is the amplitude of the ventricular electrogram in ventricular myocardium. Thus, there is a correlation between far-field R–wave oversensing and left ventricular hypertrophy.7 The second is interelectrode spacing.8 Closely spaced electrodes (1.1 mm) minimize far-field R waves in comparison with conventional 10-mm spacing, but they also reduce the amplitude of atrial electrograms during short-term follow-up9,10 and …