Endocardial and epicardial activation pathways of the canine right atrium.

Endocardial and epicardial activation pathways of the canine right atrium. Am. JS Physiol. 220(l) : l-l 1. 197 1 .-Right atria1 epicardial and endocardial activation was explored in Langendorff-perfused canine hearts during sinus rhythm and during stimulation near the coronary sinus and elsewhere in the right atrium. Control experiments demonstrate close correlation to epicardial activation in situ; conduction velocity increased about 5-20y0 during perfusion. In situ overall conduction velocity in two directions along the crista terminalis was 0.9-l .2 m/set, parallel to the pectinate muscles of the atria1 roof 0.88 m/set, and perpendicular to these muscle bundles 0.56 m/set. In one experiment a conduction velocity along Bachmann's bundle of 3 m/set was found. The rapid conduction along the internodal tracts-with the possible exception of Bach-mann's bundle-can be accounted for by the parallel arrangement of myocardial fibers and does not require participation of specialized fibers. The posterior tract was the most stable inter-nodal connection, conduction along the anterior tract was not continuous in some experiments, and the middle tract was inconsistent. The limbus of the fossa ovalis is a barrier to propagated activation, deflecting it around the fossa ovalis. THE PRESENCE of a specialized conduction system in the atria has been discussed frequently in recent years. The existence of the anterior, middle, and posterior internodal tracts has been proven by the morphological studies of James (11). Recently, relatively fast conduction along these tracts during sinus rhythm has been demonstrated by the electrophysio-logical investigations of Holsinger et al. (9) who studied-them from a surgical point of view. Specialized fibers and fiber tracts have been demonstrated in the internodal tracts, in various preparations and in (different species, both by histological (16, 21) and by ,electrophysiological techniques including microelectrode An excellent review has been given by Merideth and Titus (16). However, it has not been demonstrated whether preferential conduction also occurs in the internodal tracts during rhythms originating outside the sinus node, nor whether the specialized fiber systems indeed contribute to fast conduction of excita-tion along these pathways. In fact, evidence has been found that relatively fast conduction occurs along thick bundles of atria1 myocardium, including the crista terminalis, without the participation of specialized fibers (10, 18). In order to determine whether specialized paths do function as such in the intact atria of the dog, we carefully analyzed excitation at the epicardial and endocardial surfaces of the right atrium and adjacent parts of the …