| The sequence of electrical activation in the heart significantly affects the pattern and effectiveness of contraction. Whereas the normal heart contracts in a coordinated manner, in heart failure (HF), abnormal electrical activation, most commonly left bundle branch block (LBBB), can substantially decrease function. Cardiac resynchronization therapy (CRT), a ventricular pacing technique, improves function in patients with HF and LBBB. The dependence of functional improvement on electrical and contractile synchrony, however, is unclear. To optimize CRT clinically, a better understanding of the electromechanical relationships at work is needed. In this study, the effects of electrical activation patterns on local and global contractile function were examined in normal, diseased, and paced canine hearts. Two important relationships were studied: (1) The relationship between epicardial electrical synchrony and the synchrony and effectiveness of left ventricular contraction under left ventricular and biventricular pacing in HF with LBBB. (2) The relationship between epicardial and endocardial electrical synchrony under conditions in which the heart is simultaneously activated by both intrinsic and paced pathways.; To this end, a novel method was developed to obtain local ventricular electrical signals, temporally and spatially registered to myocardial strain in the in vivo canine heart using an epicardial electrode array and tagged MRI.; It was found that, in HF with LBBB, mechanical but not epicardial electrical synchrony is needed to achieve maximum benefit from CRT. Additionally, it was found that the endocardial and epicardial surfaces demonstrate similar electrical responses to pacing; therefore, measures of electrical synchrony on one surface are indicative of synchrony on the other.; In conclusion, a novel method for in vivo electromechanical mapping of the heart was developed. This method can be used to assess normal and diseased function, to evaluate the efficacy of therapy, and to gain insight into mechanisms of electromechanical interaction. The method was used to highlight important and previously unmeasured electromechanical relationships in HF with LBBB under CRT. The results from these studies contribute to the understanding of the complex electromechanical relationships underlying this emerging and much needed therapy and may play a significant role in its clinical optimization. |
