Which histologic findings are characteristic of first-degree atrioventricular (AV) block?

Updated: Jan 06, 2020
  • Author: Jamshid Alaeddini, MD, FACC, FHRS; Chief Editor: Jose M Dizon, MD  more...
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Under light microscopy, an atrioventricular node (AVN) is seen to be composed of a thick mesh of tiny pale cells, which anastomose with one another via short pluridirectional cytoplasmic projections. Under electron microscopy, 4 types of cells are observed in the AVN: transitional cells, P cells, common myocardial cells, and Purkinje cells.

Three functional regions have been described in the AVN on the basis of their differing conductive properties: atrionodal (AV), nodal (N), and nodal-His (NH). Cells in the N region have slower conduction times than the other regions and have no automaticity properties. Cells of the AN and NH regions have faster conduction times and display spontaneous diastolic repolarization activity.

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