Tumors of the cardiac conduction system (CCS) have rarely been reported. The CCS from 198 cardiac-related deaths (GroupⅠ),and 838 deaths from non-cardiovascular diseases or trauma (Group Ⅱ ), were studied. Sampling was done of the sinoatrial node (SAN)and atrio-ventricular node (AVN) along their long axis of each node as a single block and the His bundle (HB) perpendicular to its long axis in 2-4 blocks. Five-micron serial sections were made; tissue slices were taken intermittently, every 20th from the SAN, every 10th from the AVN, and every 30th from the HB and bundle branches (BB), by continuous slices three times. Tumors in the CCS were found in 12 cases (1.155 %), where 10 (0.965%) were primary tumors, and 2 (0.193%) were metastatic tumors. The primary tumors included 4 fibromata compressing the HB (0.386 %), 4 hemangiomata (0.386%), 1 AVN tumor (0.097 %), and 1 rhabdomyoma (0.097 %). In 8 of the 10 cases, the tumors were located in the AVN or HB. The metastatic tumors originated from lymphocytic leukemia and malignant lymphoma (histiocytic type) in lung, and were all found in the SAN. Of the 12 cases, 2 were from the group Ⅰ. Tumors in the CCS are the smallest tumors in different parts of the body, which can cause sudden death.

Objective To explore the delineation between normal variations and developmental anomalies (malformation) of human cardiac conduction system (CCS). Methods The CCS of 886 cases (737 cases were noncardiac death,149 cases died of sudden cardiac death) were examined histologically using the method designed by the authors that tissues containing the SAN and AVN were cut along the long axes of the nodes in 1-2 block and the HB perpendicular to its long axis 2-4 blocks.The morphology and causes of death in CCS of two groups were also studied. Results 1.There were congenital variations in human CCS with respect to the sizes,position and shape;2.There were postnatal variations in CCS accompanying the aging process;3.It was revealed that there were developmental anomalies in CCS with sudden cardiac death,which included fetal typed atrioventricular node (AVN) in adults, complete displacement of the AVN into the central fibrous body,complete separation of the HB into more than 3 bundles and the displacement of bifurcation of the HB into the root of tricuspid valve.These changes should not be grouped into normal variations,since they are related or potentially related to sudden cardiac deaths.Conclusion The following changes should be included in normal variation or CCS:displacement of the bifurcating portion of the HB into the pars membranacea of the ventricular septum;left sided deviation of downward and leftward displacement of that portion;displacement of less than 1/2 of the AVN in to the central fibrous body;displacement of the myocardium into the HB or LBB.;