How to sample the cardiac conduction system.

In order to examine the cardiac conduction system, the heart is sampled in two blocks for paraffin embedding. The cardiac conduction system is removed in two blocks for paraffin embedding. It is important to "save the pacemaker" by avoiding the customary cut of the right heart margin by driving lengthwise the intercaval bridge. Indeed, by this technique one happens to slash diagonally the sino-atrial node, together with the Crista Terminalis. Block 1 consists of a portion of the right atrial wall including the lateral half of the funnel of the superior vena cava, sulcus and crista terminalis. This first block includes the sino-atrial node, its atrial approaches, the crista terminalis (with the upper 2/3 of the posterior internodal tract, the proximal part of the middle and anterior tracts), and the sino-atrial node’s ganglionated plexus (Fig. 1). Block 2 consists of the lower portion of the atrial septum, the trigonum fibrosus, and anterior contour of the coronary sinus and the upper 2/3 of the ventricular septum. This second block contains the atrio-ventricular node, His bundle, bifurcation, and bundle branches. The two blocks are cut serially.