Relationship between endocardial activation of SA valve and right atrium in domestic fowl.

We mapped endocardial activation in the isolated, perfused chicken heart to determine how excitation spreads from the sinoatrial (SA) valve, the avian pacemaker structure, to the atrioventricular (AV) ring. We found activity originating from one of three sites on the right valvule with the upper half of this valvule being the dominant origin. Only one heart exhibited an origin lower on the right valvule. We never observed activity originating from the left valvule. Depending on the valvular origin, two patterns of atrial activation were observed. The dominant pattern emerged from an upper to middle origin on the right valvule and involved two preferential conduction pathways. One pathway followed the right transverse arch and pectinate muscle medially to depolarize the AV ring from a posterior medial direction. Activation along the second pathway traveled caudally along the right valvule and then followed the sinus septum to approach the AV ring from an anterior lateral direction. An atypical pattern resulted from one heart exhibiting a lower origin on the right valvule. In this case, preferential atrial conduction only followed the lower pathway. This could represent an entrance block located cranially in the right valvule preventing normal activation of the right transverse arch and pectinate muscle.