MECHANISMS OF ATRIAL-FIBRILLATION - RECEN T CONCEPTS

Atrial fibrillation (AF) is due to the presence of multiple reentry pa thways. Although this mechanism has been known for some time, new info rmation has recently been acquired about the factors of atrial vulnera bility and the conditions of myocardial alteration. There are two main factors of atrial vulnerability : intra-atrial conduction defects and abnormalities of the refractory periods. In addition, the concept of critical mass and the influence of the autonomic nervous system have t o be taken into consideration. The abnormalities of the refractory per iods liable to increase atrial vulnerability are their shortening, spa tial dispersion and poor adaptation to the heart rate. All these chang es may be demonstrated at cellular level. The product of the intraatri al conduction velocity and the duration of the refractory period defin es the wave length. The risk of developing reentry pathways increases as the wave length shortens. Moreover, the more the atrium fibrillates , the greater will be the decrease of the refractory periods, atrial f ibrillation giving rise to atrial fibrillation. Histological lesions o f the atrial tissue may be demonstrated, even in the absence of underl ying cardiac disease. They mainly consist of fibrosis, fatty degenerat ion and myocytic hypertrophy. In the long-term, atrial fibrillation le ads to a number of structural abnormalities of the atrial, and sometim es ventricular tissues, progressing to cardiomyopathy in some cases.