The Age of Reason for Gated SPECT MPI to Deal With Cardiac Dyssynchrony

Cardiac synchrony is defined as coordinated distribution of electrical activation through a prescribed path across the heart, leading to a harmonic contraction of contractile elements. The onset of contraction is dependent on two main factors including the pace of electrical waves through the conductive system and the response of contractile segments to the electrical stimulation as mechanical activation. When the propagation of mechanical activation is reasonably uniform, the blood pump is expected to work efficiently and in a synchronous way. Intraventricular and interventricular synchrony is also expressed as spreading the activation in a uniform way across one or the two ventricles, respectively (1). Dyssyn-chrony, on the other hand, occurs when one of these two essential elements of electrical or mechanical activation fails to work properly (2). The well-known indicator of electrical dyssynchrony, a wide QRS complex, has long been the main criterion for assessment and treatment of cardiac dyssynchrony by different device-based procedures including implantable cardioverter-defibrillator (ICD) and more recently cardiac resynchronization therapy (CRT) (3, 4). Over a decade ago, CRT was established as a tool to not only improve clinical symptoms but also reverse ventric-ular remodeling, increase left ventricular (LV) ejection fraction (EF), decrease the severity of mitral regurgita-tion, reduce hospitalizations and more importantly improve survival of patients with heart failure (5-8). CRT was initially recommended for patients with wide QRS (≥ 120 ms) and depressed LVEF (≤ 35%) with heart failure symptoms in the New York Heart Association (NYHA) class III or ambulatory class IV as conventional patient selection criteria (4, 5). Recent studies, however, extended the usefulness of CRT in some specific patients with milder degrees of heart failure (class II or even I) (9, 10). Nevertheless, response to therapy has not been satisfactory as it was supposed to be. About one third of patients who undergo CRT with standard criteria do not benefit clinical and/or LV functional improvement (11-13). On the contrary, CRT could be beneficial in another subset of patients who have mechanical dyssynchrony without a concomitant wide QRS complex. Therefore, electrical dyssynchrony, conventionally determined by a wide QRS, does not necessarily reflect mechanical dyssynchrony (14). Given the fact that direct assessment of mechanical dyssynchrony would be significantly a better marker of CRT success than the comprehensive electrocardiogram (15), different imaging modalities including echocardiog-raphy (16, 17), computed tomography (18), magnetic resonance imaging (19) and different types of radionuclide scintigraphy have been proposed. Among all, echocar-diography …