Mechanical alternancies in cardiomycytes of the myocardial sleeves of the superior vena cava and pulmonary veins as a potential source of ectopic activity of the atria

The myocardium of the right and left atria (RA and LA) continues into the superior vena cava (SVC) and pulmonary veins (PV) and forms myocardial sleeves, which are sources of ectopic activity causing atrial fibrillation. We compared the sarcomere dynamics of single cardiomyocytes from the myocardial sleeves of the SVC and PV and guinea pig atria. SVC cardiomyocytes were characterized by a longer time to peak sarcomere shortening and time to 50% relaxation than cardiomyocytes of other groups. In cardiomyocytes of the SVC, PV and right atrium, there was no correlation between the amplitude of sarcomere shortening and the length of the cardiomyocytes. The sarcomere shortening alternans was found in cardiomyocytes of the SVC and PV. The mechanical alternans and the lack of correlation between the amplitude of the sarcomere shortening and the cell size in the myocardial sleeves of the SVC and PV may point to a mechanical substrate that provokes arrhythmia.