Transmural fibre direction in the anterior wall of the feline left ventricle: theoretical considerations with regard to uniformity of contraction

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Abstract
Previous studies of non-uniform performance of a myocardial region have indicated that measurement of local contraction is vectorial. Myocardial performance in one direction mainly, but not exclusively, depends on performance of the wall fibres in this same direction irrespective of wall depth. In this study we therefore determined fibre direction across the anterior wall of the left ventricle in twelve cats. Based on the fibre configuration a simple model is developed to analyse and predict the degree of uniformity of contraction. Fibre direction shifted continuously across the wall, from almost alignment with global apex-to-base axis in the endocardium (90 degrees), to close to the equatorial direction (0 degrees) in the midwall, and finally to about -50 degrees in the epicardial layer. The model predicts less uniformity with reduction of preload, and in the event of subendocardial ischaemia, which fits well with experimental data. Analysis of uniformity based on the integrated vectorial contribution of all myocardial layers thus provides a way to quantify regional uniformity/non-uniformity as a regulatory mechanism of cardiac function.