Inferior interatrial pathway in the dog.

In 20 anesthetized dogs, close bipolar electrograms consisting of two deflections were recorded along the ligament of Marshall in the posterior left atrium. At the inferior portion of the ligament the deflections were separated by an interval of 10-15 msec and at the level of the left superior pulmonary vein by an interval of 60 msec or more. Histological and electrophysiological studies indicated that both deflections were associated with striated cardiac muscle; the first deflection was designated as a left atrial muscle potential and the second as a left atrial tract potential. Although the left atrial muscle potential was coincident with the middle portion of the P wave, the left atrial tract potential occurred during some part of the P-R segment. Conduction to the left atrial tract potential was altered, delayed, or completely blocked during retrograde conduction in the atrium, rapid pacing of the atrium (300-500/min), or local cooling at the point where the ligament of Marshall joined the inferior left atrium near the origin of the coronary sinus. Crushing Bachmann's bundle caused delay in conduction to the left atrial muscle but not to the left atrial tract. Left cardiac sympathetic nerve stimulation induced an atrial activation pattern consistent with pacemaker activity in the inferior left atrium, with inversion of the P wave in lead II. Pacing from the edge of the ligament of Marshall in the posterior left atrium produced the same change in P-wave morphology. These data indicate that the left atrial tract represents a terminal, insulated tract which is activated through an inferior interatrial pathway connecting the inferior right and left atria along the coronary sinus. The terminal end of the tract showed no reinsertion into atrial musculature.