Experimental bases for diagnosis of left bifascicular and trifascicular block.

It seems to be justified to establish the diagnosis of fascicular blocks, in clinical tracings, on the basis of experimental findings. For this, we produced left bifascicular block as well as trifascicular block in the heart of 20 mongrel dogs. Because of left bifascicular block, the activation process is unequally delayed in high lateral and in posterior aspects of the free left ventricular wall. Nevertheless, it begins at normal time in middle left septal mass via intermediate strands of the left bundle. Consequently, left bifascicular block which resembles intermediate degree left truncular block, is recognizable by unequally delayed onset of intrinsicoid deflection in aVL, aVF and V6 and often by the manifestation of the first septal vector. Vectorcardiographic curves are diphasic and show initial and terminal slurrings. Trifascicular block delays the activation process in both ventricles. For this, the asynchronism between the electromotive forces of ventricles is lessened. The said conduction disturbance can be diagnosed by electrocardiographic and vectorcardiographic features suggesting intermediate degree RBBB as well as left bifascicular block.