LAUSR

Left ventricular mechanical dyssynchrony (LVMD) characterizes the difference in timing of mechanical contraction between different segments of the left ventricle. Notably, LVMD is not equal to electrical dyssynchrony, which is commonly measured by the QRS duration. While both are commonly present at the same time, they can also present independently of each other. Dyssynchronous mechanical systolic contraction is of relevance due to its relationship with adverse outcomes and hypothetical role in patient selection for and procedural guidance of cardiac resynchronization therapy (CRT). Asynchronous mechanical contraction results in an impaired left ventricular contractile efficiency. Gated Single Photon Emission Computed Tomography (GSPECT) Myocardial Perfusion Imaging (MPI) is used to diagnose and determine prognosis in epicardial coronary artery disease (CAD). Over the last decade, advances in image processing have allowed an assessment of LVMD via post-processing and creation of series of three-dimensional images of the left ventricle that correspond to sequential time points in a cardiac cycle. The so-called phase analysis determines the variability in the timing of contraction of different LV segments. Notably, recent advances now also allow the analysis of diastolic LVMD, which identifies asynchronous LV relaxation resulting in impaired LV filling. Importantly, systolic and diastolic LVMD have different underlying mechanisms and determinants and have independent predictive utility. Measuring LVMD has a prognostic value in a number of patient cohorts. Patients with heart failure and a reduced ejection fraction without history of myocardial infarction, who had significant LVMD had a higher risks of cardiac events irrespective of QRS width and ejection fraction. In a separate cohort of patients with coronary artery disease, GSPECT-measured LVMD had a relationship with all-cause mortality and cardiovascular mortality independent of electrical dyssynchrony. Likewise, a number of studies confirmed that LVMD measured by GSPECT MPI in patients with ischemic and non-ischemic cardiomyopathies was an independent predictor of clinical outcomes. An advantage of using GSPECT MPI over other techniques such as echocardiography to measure LVMD is the low inter-observer variability, the ability for postprocessing of images, the ability to identify scar, and quantify ischemia. Taken together, this could provide clinicians some of the key tools to guide optimal CRT lead placement as both contractile reserve and delayed contractility are central factors for appropriate therapeutic response. CRT is most commonly used in the setting of a left bundle branch block (LBBB) in patients with systolic heart failure. LBBB induces electrical and mechanical ventricular dyssynchrony, with detrimental effects in an already failing LV. The intent of CRT is to improve coordination of global LV contraction 15 and ideally it is doing so by targeting/stimulating areas of the LV that have the greatest delay in contraction. In clinical practice, the greatest delay of contraction is usually determined without imaging guidance and with support of electrical mapping of some form. Among patients who meet Class I indications for CRT, 30% to 40% of patients fail to show an improvement in clinical symptoms and cardiac function. This suggest continued efforts to find more efficient ways to select candidates for CRT and Reprint requests: Marat Fudim, MD, Department of Medicine, Duke University Medical Center, 2301 Erwin Road, Durham, NC, 27710; [email protected] J Nucl Cardiol 2020;27:431–3. 1071-3581/$34.00 Copyright 2019 American Society of Nuclear Cardiology.