LAUSR

The following editorial refers to the article published by Sillanmäki et al. titled ‘‘Relationships between electrical and mechanical dyssynchrony in patients with left bundle branch block and healthy controls’’ in the XX edition of the Journal of Nuclear Cardiology. Mechanical contraction of the heart muscle is initiated by electrical activation of myocytes. Left ventricular mechanical dyssynchrony (LVMD) refers to a difference in the timing of mechanical contraction or relaxation between different segments of the left ventricle (LV). It is important to acknowledge that mechanical dyssynchrony does not equal electrical dyssynchrony which is defined by an asynchronous electrical activation of the LV leading to a prolonged QRS duration ([ 120 ms) on the ECG, and is usually the result of a left bundle branch block (LBBB). Although electrical and mechanical dyssynchrony often coincide, electrical and mechanical dyssynchrony are commonly not present at the same time in a given patient. Cardiac resynchronization therapy (CRT) was developed with the goal to treat mechanical dyssynchrony with the hope it will improve cardiac performance and improve outcomes. Right at the getgo the simple assumption was made that electrical dyssynchrony and mechanical dyssynchrony track well with each other, and since it is so much easier to measure and quantify electrical dyssynchrony (presence of absence of a LBBB and QRS width), the electrical dyssynchrony on ECG became the designated target of CRT. Currently, presence of electrical dyssynchrony is among the inclusion criteria for CRT in heart failure (HF) patients. Other criteria include left ventricular ejection fraction (LVEF) B 35% and New York Heart Association class II-IV despite optimal medical therapy. Under those criteria, biventricular pacing has been shown to reduce all-cause mortality. Nonetheless, among patients who meet the criteria, around 30% to 40% fail to show an improvement in clinical symptoms and cardiac function, suggesting the need for a more efficient selection of CRT candidates to improve patient outcomes and decrease costs. Today we know that diseased hearts with electrical dyssynchrony do not equally respond to CRT. The two key determinants of treatment response that emerged are QRS width and presence of LBBB. Meta-analysis of three randomized trials suggests that a wide QRS duration[ 150 ms, regardless of QRS morphology, is most important for CRT response. Further, the majority of clinical trials that studied CRT have found that it is mostly efficacious in patients with LBBB. More recent work now suggests that LBBB in itself may represent a previously unrecognized cause of LV dysfunction and may impair left ventricular recovery. In the present analysis the authors set out to investigate the LBBB and its relationship with mechanical dyssynchrony using myocardial perfusion imaging (MPI) and vector electrocardiography (VECG, another surrogate of mechanical myocyte activation, derived form an ECG analysis). In a retrospective analysis the authors selected 45 patients with LBBB and 24 controls out of a cohort of 994 patients. The authors found a large degree of electromechanical dissociation (* 40% of cases). Electromechanical dissociation refers to the fact that patients with LBBB (electrical Reprint requests: Salvador Borges-Neto, MD, Duke Department of Radiology and Division of Nuclear Medicine, 2301 Erwin Road, Durham, NC 27710; [email protected]; [email protected] J Nucl Cardiol 2019;26:1240–2. 1071-3581/$34.00 Copyright 2018 American Society of Nuclear Cardiology.