Abnormal electrical activation is an interesting area of research, because it has considerable clinical relevance and because it leads to very pronounced regional differences in contraction patterns. There are two major sources of electrical asynchrony: ventricular pacing and conduction disturbances, like bundle branch block. Both lead to a situation where the electrical impulse in the ventricles is conducted slowly through the working myocardium rather than through the ventricles’ rapid conduction system. Due to fairly tight excitation-contraction coupling a region which receives the electrical impulse relatively early also starts to contract early. This contraction occurs, however, when ventricular pressure is still low. Consequently, fibers can initially shorten rapidly. This rapid shortening results in stretching of later activated regions. Altogether, contraction within the ventricle is quite dyscoordinate and inefficient. Patterns of activation and finer shortening have been determined in dogs using optical techniques, MRI tagging and, most recently, with 2-D strain analysis by speckle tracking in echocardiographic images. The complicated mechanical behavior of paced ventricles has also been simulated using a finite element model of ventricular electro-mechanics. This model was validated using the experimental measurements.
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