Elsevier

Heart Rhythm

Volume 13, Issue 2, February 2016, Pages 490-497
Heart Rhythm

Left ventricular sphericity independently predicts appropriate implantable cardioverter-defibrillator therapy

https://doi.org/10.1016/j.hrthm.2015.09.022Get rights and content

Background

Whether echocardiographic markers of remodeling are associated with ventricular tachyarrhythmias is unknown.

Objective

The purpose of this study was to determine whether a transthoracic echocardiographic (TTE) marker of spherical left ventricular (LV) remodeling is associated with appropriate implantable cardioverter-defibrillator (ICD) therapy in patients with primary prevention ICDs.

Methods

From TTE images, we calculated sphericity index (SI), the ratio of biplane LV end-diastolic volume to the volume of a hypothetical sphere with a diameter of the LV end-diastolic length, and examined the relation between SI and therapy for ventricular tachyarrhythmias in 278 patients with primary prevention ICDs and in 50 controls without structural heart disease or ventricular arrhythmias.

Results

SI in normal healthy adult subjects and in subjects receiving ICDs was 0.44 ± 0.02 and 0.65 ± 0.04, respectively (P <.001). Median time to first appropriate ICD therapy was significantly shorter in ICD patients with SI in the upper vs lower 50% of SI values (1.40 vs 2.38 years, P = .02 for conventional ICD patients; 1.54 vs 2.65 years, P = .02 for cardiac resynchronization therapy-defibrillator [CRT-D] patients). In multivariable Cox regression analysis, SI in the upper 50% was independently associated with appropriate ICD therapy after multivariable adjustment (hazard ratio 2.2, P = .03 for ICD cohort; hazard ratio 4.4, P = .01 for CRT-D cohort). SI was not associated with total mortality in either cohort.

Conclusion

SI is associated with appropriate ICD therapy, but not total mortality, in patients receiving primary prevention ICDs. These observations suggest spherical LV remodeling may predispose to ventricular arrhythmias. Furthermore, SI appears to add predictive accuracy for appropriate ICD therapy in patients with reduced ejection fraction.

Introduction

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.

Section snippets

Definitions, patient population, and data collection

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

Results

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

Discussion

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

Conclusion

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.

Clinical Perspectives

The question of which patients benefit most from primary prevention implantable

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    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.

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