Eligibility of Pacemaker Patients for Subcutaneous Implantable Cardioverter Defibrillators.

INTRODUCTION: The subcutaneous implantable cardioverter defibrillator (ICD) has emerged as a viable therapeutic option for patients who are deemed high risk for sudden cardiac death. Previous studies have shown that 7-15% of patients are not candidates for the S-ICD based on their intrinsic QRS/T-wave morphology. Presently, it is not known if the S-ICD can be considered as supplementary therapy in patients who are ventricularly paced. We sought to determine the proportion of ventricularly paced patients who would qualify for an S-ICD. METHODS AND RESULTS: We evaluated 100 patients with transvenous pacemakers/ICDs, including 25 biventricular devices to determine S-ICD candidacy during right ventricular (RV) pacing and biventricular pacing based on the recommended QRS:T-wave ratio screening template. Fifty-eight percent of patients qualified for an S-ICD based on their QRS morphology during ventricular pacing. More patients during biventricular pacing met criteria compared to during RV pacing alone (80% vs. 46%, P <0.01). Patients that were paced from the RV septum were more likely to qualify compared to those paced from the RV apex (67% vs. 37%, respectively, P <0.01). CONCLUSION: While S-ICD implantation may be considered as supplemental therapy in select patients with preexisting transvenous devices, relatively fewer candidates who are paced from the RV apex qualify. QRS morphologies generated from biventricular pacing as well as from septal RV pacing are more likely to screen in based on the recommended S-ICD template.

Effect of diurnal variations in the QRS complex and T waves on the eligibility for subcutaneous implantable cardioverter-defibrillators.

BACKGROUND: Subcutaneous implantable cardioverter-defibrillators (S-ICDs) are an established therapy for preventing sudden cardiac death. However, a considerable number of patients still undergo inappropriate shocks even after conventional preimplantation electrocardiographic (ECG) screening. OBJECTIVE: This study aimed to elucidate the additional effect of diurnal variations in the QRS complex and T waves of 24-hour Holter screening on S-ICD eligibility. METHODS: Patients with transvenous ICDs who did not need pacing were selected for the study. The ECG was recorded by placing the electrodes to simulate the 3 sensing vectors of the S-ICD, with the patient in the standing and supine positions (conventional), during exercise, and during 24-hour Holter screening. We investigated the additional discrimination of diurnal variations in patients ineligible for S-ICDs as well as characteristics of those patients. RESULTS: Of the 86 patients (82% men; mean age 54±16 years) analyzed by all 3 screenings, 2 (2.3%) and 3 (3.4%) were considered ineligible by conventional and exercise screening, respectively. An additional 21 patients (24.4%) were found ineligible through Holter screening. A multivariate logistic regression analysis demonstrated that Brugada syndrome and an increased QRS duration per millisecond were associated with ineligibility (odds ratio 5.74; 95% confidence interval 1.74-20.2; P = .003 and odds ratio 1.04; 95% confidence interval 1.01-1.07; P = .007, respectively). T-wave oversensing was mostly observed during 0-6 AM, but no significant diurnal variations were observed in the incorrect QRS profiles. CONCLUSION: The detection of diurnal variations through Holter monitoring in addition to conventional screening is expected to be useful for determining S-ICD eligibility.

Prevalence of subcutaneous implantable cardioverter-defibrillator candidacy based on template ECG screening in patients with hypertrophic cardiomyopathy.

BACKGROUND: Subcutaneous implantable cardioverter-defibrillator (S-ICD) is a promising option for patients with hypertrophic cardiomyopathy (HCM). Patients with HCM can present markedly abnormal electrocardiograms (ECGs), and there are no data on what percentage of patients with HCM fail the prerequisite S-ICD vector screening. OBJECTIVE: The purpose of this study was to determine the failure rate of the prerequisite vector screening using 1 or 2 acceptable vectors stratified for risk profile for sudden cardiac death and predictors of failure. METHODS: ECG recordings from consecutive patients with HCM simulating the S-ICD sensing vectors were analyzed with the S-ICD screening tool. Eligibility was defined by 1 or 2 appropriate vectors. Medical history, ultrasound characteristics, and 12-lead ECG characteristics were analyzed and the individual arrhythmic risk at 5 year was determined to study potential predictors of failure. RESULTS: One hundred sixty-five (118 men; mean age 51 ± 16 years) patients were analyzed. Twenty-two patients (13%) had a high risk of sudden cardiac death, 33 (20%) had intermediate to high risk, and 110 (67%) had low risk. Twenty-six patients (16%) had no suitable vector, including 8 of 22 high-risk patients (36%). The primary cause of failure was high T-wave voltages in 25% of the vectors analyzed. T-wave inversions in >2 leads on the surface 12-lead ECG (odds ratio 15.6; 95% confidence interval 4.9-50.3; P < .001) and prior myectomy (odds ratio 8.4; 95% confidence interval 2.1-33.1; P = .002) were significantly associated with screening failure in a multivariable model. CONCLUSION: Currently available preimplant screening algorithms recommended by the manufacturer are associated with a significant failure rate in patients with HCM, particularly in the high-risk subgroup.

Use of an electrocardiographic screening tool to determine candidacy for a subcutaneous implantable cardioverter-defibrillator.

BACKGROUND: An electrocardiographic (ECG) screening test has been developed to identify patients being considered for a totally subcutaneous implantable cardioverter-defibrillator (S-ICD) at risk for T-wave oversensing. OBJECTIVE: The purpose of this study was to determine the proportion of potential S-ICD recipients who fail the ECG screening test and to identify predictors of failure. METHODS: Patients who already have an ICD but are not receiving antibradycardia pacing are representative of patients who might be considered for an S-ICD. One hundred such outpatients were enrolled in the study. Surface rhythm strips were recorded along the sensing vectors of the S-ICD system and the screening template applied. Clinical and standard ECG characteristics of patients who failed the test were compared to those who passed. RESULTS: Patients had the following characteristics: 72% male, age 57 ± 16 years, body mass index 29 ± 6 kg/m(2), left ventricular ejection fraction 43% ± 17%, QRS duration 109 ± 23 ms, QTc interval 447 ± 39 ms, 44% had coronary disease, and 55% had heart failure. Among the 100 patients, 8% failed the screening test. There were no differences in patient clinical characteristics and most standard ECG measurements. However, patients with T-wave inversions in standard ECG leads I, II, and aVF had a 45% chance of failing. CONCLUSION: Eight percent of potential S-ICD patients were not eligible for the S-ICD after failing the screening test designed to identify patients susceptible to T-wave oversensing. Patients with T-wave inversions in leads I, II, and aVF on a standard ECG were 23 times more likely to fail. More work is needed in S-ICD sensing algorithms to increase patient eligibility for the S-ICD.