The main eligibility criterion for primary prevention implantable cardioverter defibrillator (ICD) therapy, that is, left ventricular ejection fraction (LVEF), is based on large clinical trials using primarily 2-dimensional echocardiography (2DE). Presently, cardiac magnetic resonance imaging (MRI) is considered the gold standard for LVEF assessment. It has been demonstrated that cardiac MRI assessment results in lower LVEFs compared with 2DE. Consequently, cardiac MRI-LVEF assessment may lead to more patients eligible for ICD implantation with potential clinical consequences. The aim of this study was to evaluate the clinical impact of cardiac MRI-LVEF versus 2DE-LVEF assessment for ICD eligibility. A total of 149 patients with cardiac MRI-LVEF ≤35% referred for primary prevention ICD implantation who underwent both 2DE and cardiac MRI-LVEF assessment were retrospectively included. 2DE-LVEF was computed by Simpson’s biplane method. Cardiac MRI-LVEF was computed after outlining the endocardial contours in short-axis cine images. Appropriate device therapy (ADT) and all-cause mortality were evaluated during 2.9 ± 1.7 years of follow-up. The present study found that cardiac MRI-LVEF was significantly lower compared with 2DE-LVEF (23 ± 8% vs 30 ± 8%, respectively, p <0.001), resulting in 29 (19%) more patients eligible for ICD implantation according to the current guidelines (LVEF ≤35%). Patients with 2DE-LVEF >35% but cardiac MRI-LVEF ≤35% experienced a lower ADT rate compared with patients having 2DE-LVEF ≤35% (2.1% vs 10.4% per year, respectively, p = 0.02). Application of cardiac MRI-LVEF cutoff of 30% resulted in 119 eligible patients experiencing 9.9% per year ADT, comparable with 2DE-LVEF cut-off value of 35%. In conclusion, cardiac MRI-LVEF assessment resulted in more patients eligible for ICD implantation compared with 2DE who showed a relatively low event rate during follow-up. The event rate in patients with cardiac MRI-LVEF ≤30% was comparable with patients having a 2DE-LVEF ≤35%. This study suggests the need for re-evaluation of cardiac MRI-based LVEF cut-off values for ICD eligibility.
The benefit of implantable cardioverter defibrillator (ICD) therapy for primary prevention of sudden cardiac death in patients with an impaired left ventricular ejection fraction (LVEF) was demonstrated in several large randomized trials. In these trials, 2-dimensional echocardiography (2DE) is primarily used for LVEF assessment. Consequently, current guidelines recommend ICD therapy for primary prevention in patients on optimal medical therapy with an LVEF ≤35% who are in the New York Heart Association (NYHA) class II or III of heart failure or LVEF ≤30% in case of ischemic cardiomyopathy (CMP) and NYHA class I. As the guidelines do not specify the preferred method to assess LVEF, similar LVEF cut-off values for ICD eligibility are used for different imaging techniques. Presently, cardiac magnetic resonance imaging (MRI) is becoming increasingly common in clinical practice and is considered the gold standard for LVEF assessment because of its high reproducibility and accuracy. Previous studies comparing LVEF assessment by 2DE and cardiac MRI showed that they are not interchangeable. Especially in patients with impaired LVEF, most studies demonstrate that LVEF values as assessed using cardiac MRI are 3% to 7% lower compared with LVEF assessed by 2DE, resulting in a substantial reclassification of patients with regard to ICD eligibility. Consequently, cardiac MRI-based LVEF assessment might lead to an increase of patients eligible for ICD implantation, which may have important clinical consequences for the benefit of primary prevention ICD therapy. A cardiac MRI-specific LVEF cut-off value for eligibility of ICD therapy has been suggested, but follow-up data on the clinical consequences of cardiac MRI-based LVEF assessment for the eligibility of primary prevention ICD therapy are lacking. The aim of the present study was to determine the clinical consequences of LVEF assessment by cardiac MRI compared with 2DE with respect to device eligibility and outcome (appropriate device therapy [ADT] and death).
Methods
In this retrospective, observational cohort study, patients were included with a cardiac MRI-assessed LVEF ≤35% who received an ICD for primary prevention of sudden cardiac death from January 2005 to December 2012 in the VU University Medical Center. Inclusion criteria were (1) ischemic or dilated CMP, (2) assessment of LVEF using both 2DE and cardiac MRI within 6 months before ICD implantation, and (3) no reported clinical events between 2DE-LVEF and cardiac MRI-LVEF assessment. The local Ethics Committee of the VU University Medical Center approved the data collection and management of this study.
Patient characteristics before device implantation were collected from medical records: demographics, medical history of cardiovascular diseases, implantation indication, device type, medication, co-morbidities, NYHA class, QRS duration, and laboratory results. Primary prevention was defined as no history of sustained ventricular tachycardia (VT) or ventricular fibrillation (VF) (>48 hours after acute myocardial infarction). Ischemic CMP was defined as a history of (1) significant coronary artery disease, (2) myocardial infarction, (3) percutaneous coronary intervention, or (4) coronary artery bypass graft surgery.
Cardiac MRI studies were performed on a 1.5-T whole-body scanner (Magnetom Sonata/Avanto; Siemens, Erlangen, Germany) using a dedicated phased-array body coil. After survey scans, cine imaging was performed using a retrospectively ECG-gated, steady-state free precession sequence during breath holds in mild expiration. Typical imaging parameters included slice thickness of 5 mm, slice gap 5 mm, temporal resolution <50 ms, repetition time 3.2 ms, echo time 1.54 ms, flip angle 60°, and a typical image resolution of 1.3 × 1.6 mm. Stacks of 10 to 12 consecutive short-axis slices were acquired, fully covering the LV. Subsequently, endocardial borders of the LV were outlined manually in both end-diastolic and end-systolic phase. Papillary muscles were included in the LV volume. LV volumes and LVEF were computed using these analyses. The cardiac MRI-assessed LVEF data were extracted from clinical reports.
2DE was performed using commercially available ultrasound equipment, and images were obtained in the standard parasternal long- and short-axis and apical 4- and 2-chamber views. LV volumes and LVEF were calculated using the Simpson’s biplane method after manual tracing of the endocardial borders from the 4- and 2-chamber images. Papillary muscles were included in the LV volumes. All measurements were performed according the American Society of Echocardiography standards. The 2DE-LVEF data were extracted from clinical reports.
All patients underwent ICD implantation or ICD combined with cardiac resynchronization therapy (CRT). The devices were typically programmed according to the Primary Prevention Parameters Evaluation (PREPARE) study with detection rates, depending on the device manufacturer, of more than ∼180 beats/min (VT zone) and more than ∼250 beats/min (VF zone), with extended detection intervals and appropriate utilization of antitachycardia pacing (ATP) therapy. Clinical follow-up with device interrogation was routinely performed with regular intervals of 6 months. Event transmissions of patients connected with home monitoring were reviewed instantly when they occurred. All recorded events and ADT were reviewed by specialized cardiac technicians or by electrophysiologists. ADT was defined as an episode of ATP and/or defibrillation shock to terminate VT or VF. The primary end point was defined as the occurrence of first ADT. The secondary end point was defined as the combined occurrence of ADT or all-cause mortality as first event.
Continuous variables were expressed as mean ± SD. Histograms were used to determine if continuous data were normally distributed. Dichotomous and categorical data were expressed as frequencies and percentages. Comparisons of baseline characteristics of patients with and without ADT were performed using the chi-square or Fisher’s exact test for categorical or dichotomous data. Continuous unpaired data were compared using the Student’s t test or Mann-Whitney U test when appropriate. Levene’s test for equality of variances was used to verify the equal variances assumption of the Student’s t test. Furthermore, a 1-way analysis of variance was performed to compare unpaired data between all cardiac MRI-LVEF subgroups. The paired-sample t test was used to compare imaging parameters of patients with both cardiac MRI and 2DE measurements. Occurrence of ADT and mortality during follow-up was presented as incidence density (% per person-year) and stratified according to subgroups of cardiac MRI-LVEF and 2DE-LVEF. Furthermore, Kaplan-Meier curve survival analyses were performed comparing patients with 2DE-LVEF ≤35% versus >35% and cardiac MRI-LVEF ≤30% versus >30% and were tested for significance using the log-rank test. A p value ≤0.05 was considered statistically significant. All statistical analyses were performed using SPSS software package (version 20.0; IBM SPSS Statistics, Chicago, Illinois).
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