Some Characteristics of Ventricular Echoes.

• Two features of A-V transmission previously attributed to a dual conduction system; namely, "abnormal" delay in the A-V propagation of early premature atrial responses, and alteration of the configuration of the corresponding ventricular electrograms, have been adequately explained as due to aberrant conduction within the intraventricular conduction system,and more specifically as the result of block in the right bundle branch. The most compelling evidence of intranodal dissociation has been the experimental demonstration of atrial and ventricular echoes. Reciprocal beats of the ventricle, triggered by premature ventricular contractions, have been described many times in the clinical cardiological literature (see, for example, Decherd and Ruskm, Kistin.' ° Less commonly, atrial reciprocal beats have also been described. In the study by Moe et al., echo responses were attributed to two intra-nodal pathways having different refractory periods, permitting an early premature beat to pass from atrium to ventricle (or vice versa) over one pathway and return to the chamber of origin over the other. Rosenblueth presented similar evidence and conclusions, but he failed to observe echo responses in the atria. He therefore postulated two pathways, but assumed that one of the communications between atrium and ventricle was "polarized"; i.e., it could transmit impulses only from atrium to ventricle. He further postulated that a premature ventricu-