ABSTRACT
Electrophysiological disturbances are a common finding in advanced heart failure.1 In addition to abnormalities in cardiac muscle contraction (mainly dependent on the severity of the underlying myocardial disease), abnormal electrical conduction delays the timing of atrial contraction and generates discoordinate contraction of the left ventricle (LV), which further impairs the hemodynamic performance of the failing heart. Both abnormal electrophysiological timing and contractile discoordination can be offset by cardiac resynchronization therapy (CRT) through the use of atrial-synchronized biventricular pacing. Although QRS duration is not a direct marker of mechanical dyssynchrony, CRT has been shown to reduce morbidity and mortality in patients with ventricular dyssynchrony selected almost exclusively on the basis of a prolonged QRS width.2-11
As initially proposed, CRT is based on the original and logical (but probably oversimplified) theory that synchronous biventricular pacing and LV free-wall pre-excitation are able to reduce the interventricular delay caused by left bundle branch block (LBBB) and to counterbalance the delay of activation of the LV free wall. However, even the general assumption that biventricular or LV pacing is effective
in removing the electrical component of the electromechanical delay is still under evaluation. Although the clinical results of CRT are promising – analysis of individual responses has revealed that almost 30% of patients do not exhibit any symptomatic or hemodynamic improvement: the so-called ‘non-responders.’ Current data indicate that the problem of nonresponse is multifactorial and not only related to the parameters of dyssynchrony (i.e., electrical vs mechanical) used for patient selection. However, and in order to improve clinical outcomes, investigators are seeking new markers of dyssynchrony that can prospectively identify the patients who are more likely to respond.12,13
This chapter aims at summarizing our understanding about the problem of non-response to CRT from an electrical perspective, to discuss the strengths and weakness of the QRS width as an index of dyssynchrony, and to go deeply into the relationship between electrical and mechanical dyssynchrony.
