Impulses of S-A Block Area

Micro-electrode studies in 13 excised rabbit preparations were done to determine the site of S-A block. Previously the fastest S-A conduction occurred in the area connecting the sinus node and the right branch of the crista terminalis. When a small cut was made in this area, 5 of the 13 hearts showed the first degree S-A block. Two hearts exhibited the second degree S-A block of the Wenckebach phenomenon and 3 showed the third degree block after cutting the same area. All blocks were transient and, within 1 hour after cutting, they were of the first degree. When the caval area was subsequently cut in these 10 hearts, all of them developed the complete S-A block. Three other hearts showed the complete S-A block by cutting only the connectable area of the sinus node with the right branch of the crista terminalis. The first cutting of the caval area surrounding the sinus node excepting the above mentioned connection did not change the S-A conduction time. Conclusions from these experiments reveal that the dominant sino-atrial conduction occurs through the connectable area of the sinus node with the right branch of the crista terminalis. Functional impairment to this region is essential and necessary to the development of any degree of S-A block.