Study population

Between June 2003 and August 2006, 239 survivors of life-threatening ventricular tachyarrhythmias were prospectively included at our institution. All patients received an ICD for secondary prophylaxis indications. Inclusion criteria were patients who had survived an aborted cardiac arrest or a symptomatic episode of spontaneous sustained VT, and patients with left ventricular structural heart disease. Exclusion criteria were ventricular tachyarrhythmias with a transient reversible cause (acute myocardial infarction, drugs, electrolyte abnormalities); structurally normal heart or isolated right ventricular heart disease without left ventricular involvement; unexplained syncope with left ventricular dysfunction and inducible VT; and age < 18 years. Written informed consent was obtained from all enrolled patients.

Data collection and follow-up

Clinical and paraclinical characteristics of the study population were collated at baseline. A diagnosis of ischaemic cardiomyopathy was made in patients with previous myocardial infarction and/or previous coronary revascularization and/or ≥ 50% coronary stenosis by angiography. The presenting arrhythmia (VT/VF) was assessed in each patient, as well as whether it was tolerated (cardiac arrest or not). Sustained VT was defined as VT lasting ≥ 30 seconds or terminated by cardioversion before that time. Symptomatic VT was defined as VT that caused haemodynamic compromise requiring prompt termination (hypotension, presyncope or syncope, angina, heart failure symptoms, dyspnoea). The medication regimen was recorded at hospital discharge. All patients underwent ICD implantation before hospital discharge, with prepectoral cans and endocardial bipolar leads. Single-chamber, dual-chamber or biventricular ICDs were implanted, as appropriate. All devices were programmed to store far-field electrograms of detected episodes; they were programmed with three detection zones (two VT zones and one VF zone), and algorithms for the discrimination of supraventricular tachycardias were activated in the VT zones. In the VT zones, antitachycardia pacing (ATP) was the first-line therapy in all patients. The VF zone was set at ≥ 240 beats/min, and the fast VT zone was set in the range 200–240 beats/min. The lower limit of the slow VT zone was set at 150 beats/min in patients who had VF, and was set to be 10 beats/min below the slowest episode of VT in those who had VT.

Follow-up started at the time of ICD implantation (June 2003 to August 2006), and ended in December 2014. Initially, ambulatory follow-up was scheduled, with routine consultations twice a year. Beginning in 2007, a substantial proportion of the study population was also followed up with remote monitoring. All stored electrograms were reviewed to determine whether ICD therapies were delivered appropriately or inappropriately. The occurrence of electrical storms, defined as at least three episodes of ventricular tachyarrhythmia over a 24-hour period terminated by ATP and/or shock, was also noted. Finally, the survival status of each patient was collected at the end of the follow-up period.

Endpoints

The primary endpoint was the occurrence of the first appropriate ICD therapy. Secondary endpoints were the occurrence of electrical storms and all-cause mortality.

Statistical analysis

Qualitative variables are expressed as numbers and percentages. Continuous variables are reported as means ± standard deviations or as medians and interquartile ranges (IQRs) in case of non-Gaussian distribution. The normality of continuous variables was checked graphically and by using the Shapiro-Wilk test. We estimated cumulative incidences of appropriate device therapies and electrical storms using a competing risk method, treating death as the competing risk. Cumulative incidences of appropriate device therapies and electrical storms were estimated using the approach of Kalbfleisch and Prentice . Univariate predictors of appropriate device therapies and electrical storms were identified using Fine-Gray regression models. Variables with values of P < 0.20 in bivariate analyses were entered into a multivariable Fine-Gray regression analysis. To assess the effect of electrical storm occurrence on all-cause mortality, we used a time-varying Cox regression analysis. We estimated overall survival using the Kaplan–Meier method. Univariate analyses were performed using Cox proportional hazard regression models. Variables with values of P < 0.20 in bivariate analyses were entered into a multivariable Cox proportional hazard regression analysis. For both the Fine-Gray and Cox models, the proportional hazards assumption for each potential predictor was assessed by plotting the Schoenfeld residuals. We derived from Fine-Gray and Cox models, sub-hazard ratios (SHRs) and hazard ratios (HRs) with their 95% confidence intervals (CIs).

All statistical tests were performed at the two-tailed α level of 0.05. Data were analysed using SAS software, version 9.4 (SAS Institute Inc., Cary, NC, USA).

Study population

The study population consisted of 239 patients, prospectively included from June 2003 to August 2006. Baseline characteristics of the study population are presented in Table 1 . The main characteristics were as follows: mean age 64 ± 12 years (range, 24–82 years); 215 men (90%); and mean LVEF 37 ± 12%, with 128 (54%) patients having an LVEF ≤ 35%. A history of atrial fibrillation was noted in 74 patients (31%). The main underlying cardiac condition was ischaemic heart disease ( n = 172, 72%), the other underlying cardiomyopathies being idiopathic dilated cardiomyopathy ( n = 53, 22%), valvular heart disease ( n = 7, 3%), arrhythmogenic right ventricular cardiomyopathy ( n = 3, 1%), hypertrophic cardiomyopathy ( n = 2, 1%), congenital heart disease ( n = 2, 1%) and cardiac sarcoidosis ( n = 1, 0.5%). Presenting ventricular tachyarrhythmias were VT in 187 patients (78%) and VF in the 52 remaining patients (22%). Among patients with VT, 183 (98%) had monomorphic VT and 4 (2%) had polymorphic VT. Sixty-six patients (28%) presented with aborted sudden cardiac death (SCD) requiring cardiopulmonary resuscitation. At hospital discharge, the majority of patients were receiving β-blockers ( n = 229, 96%) and angiotensin converting enzyme inhibitors or angiotensin receptor blockers ( n = 221, 93%). Cardiac resynchronization therapy (CRT) was associated with the ICD in 33 patients (14%).

The median overall follow-up period was 7.8 years (range, 3.5–9.3 years; mean 6.6 ± 3.4 years), the longest being 11.4 years after ICD implantation. During follow-up, appropriate device therapy (ATP and/or shocks) occurred in 139 (58.2%) patients, electrical storm in 73 (30.5%), and death in 141 (59%). Inappropriate device therapies occurred in 42 (17.6%) patients. Five (2%) patients underwent heart transplantation.

Appropriate device therapies

The cumulative incidences of appropriate therapies were 33%, 46%, 54% and 57% at 1, 2, 5 and 8 years, respectively. As shown in Fig. 1 , the risk of appropriate device therapy was the highest in the early period of time after ICD implantation, and persisted over long-term follow-up. The median period between ICD implantation and the first appropriate ICD discharge was 5.3 months (IQR 4–23 months; range, 1 day to 9.7 years). The first appropriate therapy was delivered during the first year after ICD implantation in 78 out of the 139 patients (56%) who received appropriate ICD discharges. Among these 139 patients, the first treated arrhythmia was VT in 127 patients (91%) and VF in the remaining 12 patients (9%). In patients who received an ICD for VT, the first arrhythmia recurrence terminated by the ICD was VT in almost all cases (96%), whereas in those with VF as the index arrhythmia, the first arrhythmia recurrence was VT in 71% of cases and VF in the remaining cases (29%). Shock therapy was triggered in nearly one of three cases ( n = 43, 30.9%), specifically the 12 patients with VF plus 31 patients with VT (12 patients with very fast VTs detected in the VF zone and 19 patients with slower VTs but failure of ATP). Among the 127 patients in whom the first appropriate therapy was delivered to terminate an episode of VT, the median VT rate was 180 beats/min (IQR 166–203 beats/min). Table 2 shows the type of ICD therapies according to the main characteristics of the patients.

Cumulative incidence rates for appropriate device therapies. Note that the risk of appropriate therapies is high during the first 2 years after implantable cardioverter–defibrillator implantation (line “A”); this risk is lower but persistent over long-term follow-up (line “B”).

Table 2

Implantable cardioverter–defibrillator therapies according to the main characteristics of the study population.

	Only ATP	ATP + shocks	Only shocks
	( n = 96)	( n = 19)	( n = 24)
Index arrhythmia
Ventricular tachycardia	84 (87.5)	18 (94.7)	13 (54.2)
Ventricular fibrillation	12 (12.5)	1 (5.3)	11 (45.8)
Cardiopathy
Ischaemic	69 (71.9)	16 (84.2)	17 (70.8)
Non-ischaemic	27 (28.1)	3 (15.8)	7 (29.2)
LVEF
≤ 35%	48 (50.0)	13 (68.4)	13 (54.2)
> 35%	48 (50.0)	6 (31.6)	11 (45.8)
NYHA
I–II	80 (83.3)	15 (79.0)	22 (91.7)
III–IV	16 (16.7)	4 (21.0)	2 (8.3)
Age
< 65 years	42 (43.8)	8 (42.1)	14 (58.3)
≥ 65 years	54 (56.2)	11 (57.9)	10 (41.7)

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