Genesis of high frequency notching of QRS complexes in an in vivo cardiac model.

High frequency notching of the QRS complex is associated with transmural infarction, cardiomyopathies, and ventricular hypertrophy from any cause. The mechanism producing notching is unknown; but the presence of a discrete anatomic lesion is not an essential feature. The hypothesis that notching was produced by activation across, rather than along, myocardial fibers was investigated by stimulation at 12 points around a clock electrode attached to the epicardium while mapping isoschronous lines in the area activated. All fibers at the subendocardial layer beneath the clock electrode were ligated by a pursestring suture. Propagation direction, as measured by isoschronous maps, produced more notched QRS compleses when the path was across, rather than paralled with, the myocardial fibers. Using grouped data and a 5 times 6 table, notches versus the angle formed between fiber direction and orientation of the direction of travel were shown to be related (P less than 0.001). The hypothesis that cross-fiber activation enhances notching was confirmed. retrograde activation did not increase notching nor did ligation of subendocardial fibers.