Background Implantable defibrillators (ICD) are an established primary prevention measure for those at risk of sudden cardiac death. Anti-tachycardia pacing (ATP) in a “fast ventricular tachycardia (VT)” zone can terminate ventricular tachycardia and studies suggest that ATP prevents shocks in ICD patients. Limited data exist regarding ATP in a purely “primary prevention” population. We reviewed outcomes in our primary prevention ICD population according to whether they were programmed “ventricular fibrillation (VF) zone only”, or had both VT and VF zones.
Methods A retrospective study of 139 consecutive patients with primary prevention ICD implanted at our institution between January 2004 and December 2006 was performed. Devices were programmed at implant at the discretion of the operator. Implant reports and ICD clinic notes were reviewed to assess initial programming and events during follow-up. Follow-up ended if devices were re-programmed. Outcomes included all-cause mortality, time to first shock and incidence of one or more shock during follow-up. Groups were compared using Student’s t-test, χ2 test and Kaplan–Meier analysis.
Results Mean follow-up was 30 ± 0.5 months. The ICD of 89 patients were programmed as VF zone only (mean >194 ± 1 bpm), whereas 50 ICD had both VF and VT zones (mean VT zone 171 ± 2 to 205 ± 2 bpm; VF zone >205 ± 2 bpm). 100% of VT zones included ATP and 96% both ATP plus shocks. The two groups were of similar ages (65.9 ± 1 and 65.8 ± 2 years) and had a high incidence of poor LV function (ejection fraction <35%; 68 (78.2%) and 39 (78.0%)). Both groups had similar numbers of biventricular, dual and single chamber devices. Bradycardia pacing modes were also comparable. There was, however, a higher incidence of ischaemic heart disease (IHD) in patients with VT zones (36 (72%) and 45 (51%), p<0.05). During follow-up all-cause mortality (12.4% and 12.0%, respectively, p = NS) and time to first shock (13.6 ± 4 and 9.8 ± 3 months, p = NS) were similar. However, ICD programmed “VF zone only” had a lower incidence of shocks compared with those with VT zones (14.6% vs 32.0%, p = 0.02, see fig). Notably, the incidence of shocks in the subgroup of patients with IHD was similar to that in the whole population (VF zone only: 8 of 46 (17.4%); VT plus VF zones: 11 of 36 (30.5%)). 16 patients with VT zones had shocks. Five of 16 (31.3%) were inappropriate (rapidly conducted supraventricular rhythms in all cases). Inappropriate shocks in those with “VF zone only” were for equipment failure (T wave oversensing and lead failure (n = 2; 15.4% of all VF zone shocks).
Discussion Our study suggests that a VT zone with both ATP and shocks may lead to an increase in the number of shocks in patients with primary prevention ICD. High rate supraventricular tachycardias falling within the VT zone may be a key cause of inappropriate shocks. Compared with previous studies, VT zones in our patients initiated therapy at lower heart rates. This is a trap to avoid when programming primary prevention ICD.
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