FIGURE 5.

Continued pacemaker function in the failed ablation goat (recovered sinus node, rSN) may be due to small areas of surviving “peripheral” sinus node (SAN) cells that are able to generate diastolic depolarization and thus pacemaker function. (A) Right atrial preparation taken from a goat in the rSN group. The ablated area is shown by the white circle, and the superior and inferior vena cava (SVC, IVC) are annotated for orientation. In vivo epicardial mapping had confirmed that the leading pacemaker was in this region indicating recovery of SAN function. (B) Spontaneous action potentials showing diastolic depolarization was recorded by means of sharp microelectrodes in the region indicated by the white circle in panel (A), the apparently ablated SAN. (C) The corresponding histological section (Masson’s trichrome) from this region confirms a thin endocardial strip of surviving nodal cells (shown encircled by yellow dotted line) surrounded by a region of fibrosis (blue) and dark nuclei presumably of necrotic cells. In this example, the application of endocardial radiofrequency energy was not able to generate a full-thickness ablation lesion, meaning a small region of endocardial cells were spared. (D) Histology section (Masson’s trichrome) at the level of the ablated SAN from a successfully ablated animal (subsidiary atrial pacemaker, SAP group). The ablated region is circled by the red dashed line and incorporates a full-thickness radiofrequency ablation lesion extending from the epicardium to the endocardium and encompasses the whole area where the SAN would be expected to be found (yellow dashed line). The section shows dense fibrosis (blue staining) with disrupted myocardium (purple staining) and no identifiable clusters of nodal cells.