BackgroundTo establish the chronic stability of defibrillation thresholds (DFTs) in a transvenous cardioverter/defibrillator (TCD) system, we studied 37 consecutive patients with TCD systems implanted for >6 months.Methods and ResultsDFT was measured by a step-down method at implant and 2 and 6 months later. The mean ejection fraction was 34.5±14.3%. Coronary artery disease with previous myocardial infarction was present in 31 patients. The mean DFT rose from 13.3 ±4.3 J at implant to 16.5 ±4.7 J at 2 months (P< .001) and 17.6±5.4 J at 6 months (P< .0001). ANOVA revealed a statistically significant rise in DFT over time (P< .0005). At 2 months, 25 patients had a rise in DFT, and 14 had a rise ≥5 J. The observed rise at 2 months persisted in 19 patients. A chronic rise, defined as ≥5 J rise at 6 months, occurred in 17 patients. Univariate analysis of clinical as well as implant variables revealed no predictors of a rise in DFT in this group.ConclusionsWe conclude that there is a significant rise in DFT at 2 and 6 months in this TCD system. Although the chronic threshold remained well within the available energy range of the pulse generator, this observation has important implications for implantation guidelines, programming, and future pulse generator development for TCD patients.