Pacing in heart failure patients with narrow QRS: is there more to gain than resynchronization?

Ventricular dyssynchrony is defined as uncoordinated regional myocardial contraction and relaxation and may be either interventricular or intraventricular. Most of the clinical focus has been on left ventricular (LV) intraventricular dyssynchrony, which in principle may have 3 different origins. First, dyssynchrony may have an electric origin as in left bundle-branch block, which causes nonuniform timing of electric activation. Second, there may be disturbances in excitation-contraction coupling, and this would be apparent as delay in electromechanical activation time. Presently, there are limited data on the clinical relevance of such disturbances. Third, dyssynchrony may have a primary mechanical origin and may occur in ventricles with regional impairment of function, as in myocardial ischemia when systolic shortening in different segments is out of phase. Furthermore, a primary mechanical origin may be found in ventricles with nonuniform distribution of load, and this may occur when there are regional differences in LV wall thickness or differences in local radius of curvature, including septal curvature. In these cases, segmental differences in systolic wall stress may lead to differences in timing of peak contraction.