title: Mechanism of prolonged electromechanical delay in late activated myocardium during left bundle branch block.
abstract: During left bundle branch block (LBBB), electromechanical delay (EMD), defined as time from regional electrical activation (REA) to onset shortening, is prolonged in the late-activated left ventricular lateral wall compared with the septum. This leads to greater mechanical relative to electrical dyssynchrony. The aim of this study was to determine the mechanism of the prolonged EMD. We investigated this phenomenon in an experimental LBBB dog model (n = 7), in patients (n = 9) with biventricular pacing devices, in an in vitro papillary muscle study (n = 6), and a mathematical simulation model. Pressures, myocardial deformation, and REA were assessed. In the dogs, there was a greater mechanical than electrical delay (82 ± 12 vs. 54 ± 8 ms, P = 0.002) due to prolonged EMD in the lateral wall vs. septum (39 ± 8 vs.11 ± 9 ms, P = 0.002). The prolonged EMD in later activated myocardium could not be explained by increased excitation-contraction coupling time or increased pressure at the time of REA but was strongly related to dP/dt at the time of REA (r = 0.88). Results in humans were consistent with experimental findings. The papillary muscle study and mathematical model showed that EMD was prolonged at higher dP/dt because it took longer for the segment to generate active force at a rate superior to the load rise, which is a requirement for shortening. We conclude that, during LBBB, prolonged EMD in late-activated myocardium is caused by a higher dP/dt at the time of activation, resulting in aggravated mechanical relative to electrical dyssynchrony. These findings suggest that LV contractility may modify mechanical dyssynchrony.
authors: Institute for Surgical Research, Oslo University Hospital, Norway.
authors: Kristoffer
authors: K
authors: Russell
authors: Otto A
authors: OA
authors: Smiseth
authors: Ola
authors: O
authors: Gjesdal
authors: Eirik
authors: E
authors: Qvigstad
authors: Per Andreas
authors: PA
authors: Norseng
authors: Ivar
authors: I
authors: Sjaastad
authors: Anders
authors: A
authors: Opdahl
authors: Helge
authors: H
authors: Skulstad
authors: Thor
authors: T
authors: Edvardsen
authors: Espen W
authors: EW
authors: Remme
mesh_terms: D000368:Aged; D000818:Animals; D002037:Bundle-Branch Block / Q000175:diagnosis / Q000503:physiopathology* / Q000628:therapy; D058406:Cardiac Resynchronization Therapy; D003198:Computer Simulation; D004195:Disease Models, Animal; D004285:Dogs; D004562:Electrocardiography; D004576:Electromyography; D022062:Electrophysiologic Techniques, Cardiac; D005260:Female; D006329:Heart Conduction System / Q000503:physiopathology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D008955:Models, Cardiovascular; D009200:Myocardial Contraction; D010210:Papillary Muscles / Q000503:physiopathology*; D000091402:Prohibitins; D011817:Rabbits; D013997:Time Factors; D016277:Ventricular Function, Left; D017725:Ventricular Pressure