Clinical Perspectives
The question of which patients benefit most from primary prevention implantable
Current guidelines for use of the implantable cardioverter-defibrillator (ICD) for primary prevention of sudden cardiac death (SCD) are based primarily on left ventricular ejection fraction (LVEF).1 However, although LVEF is an excellent marker of total mortality in patients with prior myocardial infarction and nonischemic dilated cardiomyopathy,2, 3, 4 it is neither sensitive nor specific for prediction of SCD.5, 6 If primary prevention ICDs are to be used in a cost-effective and clinically appropriate manner, the identification of variables predictive of SCD, not just total mortality, is critical.
Recent data implicate the potential role of adverse left ventricular (LV) remodeling in ventricular arrhythmogenesis. Noninvasive markers of fibrosis and myocardial stretch, such as B-type natriuretic peptide and N-terminal proBNP, have been linked with the development of ventricular tachycardia (VT)/ventricular fibrillation (VF) and have been proposed as potential risk stratification tools for SCD.7, 8, 9, 10 Mechanistically, LV remodeling might promote activation of stretch-activated depolarizing cation channels, resulting in alterations in action potential duration of cardiomyocytes11, 12 and alterations in cell-to-cell coupling.13 We reasoned that elevation of natriuretic peptides may reflect increased LV wall stress, which will be promoted if the LV assumes a more spherical shape than normal.14, 15
In this study, we applied transthoracic echocardiography (TTE) to record a variant of the previously developed sphericity index (SI).16, 17, 18, 19, 20 We calculated LV SI by dividing biplane LV end-diastolic volume (LVEDV) by the volume of a hypothetical sphere having the diameter equal to the LV end-diastolic length.19, 20 We hypothesized that patients with more spherically shaped ventricles (higher SI) would be more likely to develop spontaneous ventricular tachyarrhythmias, and we tested this hypothesis in a population of patients with abnormal LV systolic function receiving primary prevention ICDs.
From the TTE images, LV end-diastolic endocardial borders were manually traced for the apical 2- and apical 4-chamber views. The biplane LVEDV was calculated using commercial software (EchoPAC BT12, GE Medical Systems, Milwaukee, WI) using the Simpson method. LV end-diastolic length (d) was taken in the apical 4-chamber view at end-diastole and measured from the mitral valve to the apical endocardium. We calculated the volume of a hypothetical sphere having this length as its diameter using the
Mean SI in the control patients with a normal TTE was 0.44% ± 0.02 (95% confidence interval [CI] 0.42–0.47; see Online Supplemental Figure 1A) and was similar between genders (male 0.47 ± 0.02 vs female 0.49 ± 0.04, P = NS).
Of the 807 patients who underwent initial primary prevention ICD implantation, 278 (203 with conventional ICDs, 75 with CRT-Ds) had available TTE images of adequate quality acquired within 6 months of implantation and without an intervening hospitalization. Of the 203
In this retrospective study of patients undergoing primary prevention ICD implantation, we found that SI, a TTE metric of LV spherical remodeling, is associated with appropriate ICD therapy but not total mortality. SI is a noninvasive metric that is easy to calculate and highly reproducible. Patients with both conventional ICDs and CRT-D devices having higher SI were at significantly increased risk for appropriate ICD therapy in both univariate and multivariable analysis.
LV sphericity has been
SI, the biplane LV end-diastolic volume divided by the volume of a hypothetical sphere with a diameter of LV length, is a simple, geometric index that correlates with appropriate ICD therapy in patients with primary prevention ICDs. Future prospective studies are warranted to determine whether echocardiographic SI has prognostic utility for patients at risk for ventricular tachyarrhythmias. The question of which patients benefit most from primary prevention implantableClinical Perspectives
Changes in left ventricular geometry affect the likelihood of developing VT and VF. High blood levels of B‐type natriuretic peptide (BNP) and its N‐terminal fragment (NT‐proBNP) are associated with higher rates of appropriate implantable cardioverter defibrillator (ICD) placement and mortality.51,52 The only indicator that has been identified to be consistently associated with an increased risk of SCD in the setting of CHD and left ventricular (LV) dysfunction is LV ejection fraction (LVEF).48
We suspect the specific association of NT-proBNP with SCD is due to myocardial stretch and disordered ventricular geometry. In a separate retrospective study, we demonstrated that sphericity index (another metric of altered ventricular geometry) correlated with appropriate ICD therapies but not with all-cause mortality.23 These observations are paralleled by the relation of left ventricular end-diastolic volume index to SCD.24
The SI-diastole was defined as the ratio of the end-diastolic volume to the volume of a sphere with a diameter equal to the end-diastolic LV long axis (and similar for SI-systole) (Levine et al., 2016; Mannaerts et al., 2004). Accurate evaluation of SI is relevant for patients with myocardial infarction or implantable cardioverter-defibrillator (Levine et al., 2016; Mannaerts et al., 2004). Due to the sinusoidal (instead of cardiac) volume curve and elastic material of the LV, the motion of the LV phantom is different from the motion of the mean LV shape from the SSM.
Dr. Buxton has research grants funded by Medtronic Inc and Biosense-Webster (on Unrelated topics). Dr. Josephson has received consulting fees and honoraria from Medtronic Inc.