Specialized Conducting Fibers in the Interatrial Band

The interatrial band (Bachmann's bundle) was studied both in situ, in the adult dog, and in vitro, using excised canine atria. Electrograms recorded from this structure in the intact animal had a double intrinsic deflection during sinus rhythm, and responded to rapid rates of atrial stimulation with alternation in both configuration and cycle length. Potassium infusions which produced atrial arrest with a sinoventricular rhythm did not abolish electrical activity in the interatrial band. Single fibers of the interatrial band, studied in vitro with microelectrodes, had distinctive transmembrane potentials different from those of ordinary atrial muscle fibers. Action potentials were characterized particularly by a high rising velocity (maximum dv/dt), of the same magnitude as that recorded from Purkinje fibers, and a prominent plateau. These fibers were sensitive to acetylcholine and more resistant to potassium arrest than ordinary atrial fibers. Conduction velocity in the interatrial band was consistently higher than in ordinary atrial muscle. Plots of sequential activation time against linear distance showed different conduction velocities in parallel linear paths, with the highest velocity in the path on the crest of the interatrial band. Due to rapid conduction through the interatrial band, simultaneous activation of right and left atrial points was demonstrated. It is concluded: 1) The interatrial band is not a homogeneous structure, but contains two fiber types. 2) In addition to ordinary atrial muscle, specialized conducting fibers are present in the interatrial band. 3) Impulse spread in the interatrial band is not radial or uniform. Rather, it occurs through several linear paths which probably have infrequent cross-connections.