Normal variations and pathologic changes in structure of the cardiac conduction system and their functional significance.

The rarity with which the cardiac conduction system is carefully examined in cases of sudden unexpected cardiac death is deplorable. Whatever the reasons for this failure, it is clearly one of the major lost opportunities for improving our understanding of a major national heart problem. To illustrate some of the morphologic changes found in such cases, two categories are discussed: normal variations (which may themselves promote electrical instability) and changes due to disease. During normal postnatal morphogenesis of the atrioventricular (AV) node and His bundle, there are a variety of derangements that may cause malfunction of those critical structures. In addition to large vessel coronary disease and platelet disorders, narrowing of small coronary arteries may also lead to ischemic damage in the conduction system. Certain intracardiac tumors and systemic diseases of an infiltrative or inflammatory nature may also involve elements of the conduction system, leading to arrhythmias or conduction disturbances with their clinical counterparts of syncope and sudden death. How any of these and many related anatomic changes may participate in the pathogenesis of electrical instability of the heart deserves much more careful study, but an essential requirement will be the wider practice of making careful clinicopathologic correlations.