Current concepts relating coronary flow, myocardial perfusion and metabolism in left bundle branch block and cardiac resynchronisation therapy.

Cardiac resynchronisation therapy (CRT) improves mortality and symptoms in heart failure patients with electromechanically dyssynchronous ventricles. There is a 50% non-response rate and reproducible biomarkers to predict non-response have not been forthcoming. Therefore, there has been increasing interest in the pathophysiological effects of dyssynchrony particularly focusing on coronary flow, myocardial perfusion and metabolism. Studies suggest that dyssynchronous electrical activation effects coronary flow throughout the coronary vasculature from the epicardial arteries to the microvascular bed and that these changes can be corrected by CRT. The effect of both electrical and mechanical dyssynchrony on myocardial perfusion is unclear with some studies suggesting there is a reduction in septal perfusion whilst others propose that there is an increase in lateral perfusion. Better understanding of these effects offers the possibility for better prediction of non-response. CRT appears to improve homogeneity in myocardial perfusion where heterogeneity is described in the initial substrate. Novel approaches to the identification of non-responders via metabolic phenotyping both invasively and non-invasively have been encouraging. There remains a need for further research to clarify the interaction of coronary flow with perfusion and metabolism in patients who undergo CRT.