Supernormality in Bachmann's bundle. An in vitro and in vivo study in the dog.

A supernormal phase of conduction between right and left atria was observed in the exposed hearts of anesthetized dogs. Interatrial conduction time of premature atrial responses was reduced by as much as 17% during the early phase of diastole, relative to the conduction interval of basic driven responses. The supernormal phase lasted from 60 to 140 msec and was greater at slower basic driving frequencies. A brief phase of supernormal excitability was also found in the specialized cells of Bachmann's bundle from puppy hearts in vitro. Supernormal conduction was not observed within the atrial appendages (i.e., in areas not supplied by specialized conducting bands); supernormal excitability was not demonstrable in cells of ordinary atrial myocardium. Although vagal stimulation abbreviated transatrial conduction time, atropine did not abolish the phase of supernormality. ADDITIONAL