To Investigate Whether Remnants of the "Ring Tissues" Constitute the Substrate of the Circuit of Atrioventricular Nodal Reentrant Tachycardia

Background The exact circuit of atrioventricular nodal reentrant tachycardia still remains unknown despite advances in high-density mapping, tissue histochemistry, and connexin genotyping. Theoretical calculations have derived a range of 0.8 to 5.2 cm for the length of the slow pathway, whereas that of the fast pathway should be within the range of 2.4 to 11.2 cm. Recent histology data also indicate that the distance of the right inferior extension of the atrioventricular node within the tricuspid vestibule, as judged histologically, and as measured from the site of penetration of the conduction axis in 31 hearts, is 8.2±2.4 mm, with a range from 5.2 to 13.6 mm. These results are compatible with theoretical calculations of the slow pathway previously performed by the investigators. On 3-dimensional reconstructed fluoroscopic images, the average distance between the compact AV node and the ablating electrode in cases of successful AVNRT ablation, however, is 17.2±1.6 to 17.8±0.9 mm. This clearly suggests that successful ablation interrupts the AVNRT circuit at a point beyond the identifiable right posterior extension, and at a consistent area on the the septal isthmus between the CS ostium and the TV. This site is beyond the histologically identifiable inferior extensions of the node.

There is additional evidence in animal studies that the right inferior extension continues within the vestibule of the tricuspid valve as "ring tissue", ie a remnant of the primary ring, while the left atrial vestibule is derived from the initial atrioventricular canal of the developing heart, which is itself known to be slow conducting but cannot be distinguished histologically from the left atrial myocardium. In the human heart, the primary ring extends through the atrioventricular node, and includes the bundle of His, which is rapidly conducting.

Hypothesis The investigators hypothesized that, in humans, the extent of ring tissues varies in such a way that some subjects are susceptible to atrioventricular nodal re-entry, whereas others are not. Thus, the entirety of the tricuspid vestibule, or even part of the mitral vestibule, are parts of the re-entry circuit, at least in some atypical forms with prolonged His-atrial intervals. The circumference of the right atrioventricular orifice has been measured at between 9 and 11 centimeters in patients aged less than 65 years, whereas that of the mitral orifice is from 7 to 9 centimeters. The possibility of the vestibules being involved in the circuit, therefore, cannot theoretically be excluded. Recently, high-density mapping of the re-entry circuit has provided valuable insights into the electrophysiologic patterns of this arrhythmia, identifying the circuit in the vicinity of the nodal area. All these mapping studies, however, were conducted in patients with typical AVNRT. In this setting, the remnants of the ring tissues might be short enough as not to allow proper discrimination of the circuit limbs. The investigators propose to apply high-resolution electroanatomic mapping in patients with both typical atypical AVNRT in which the circuit may contain longer circuit components. Mapping should be focused on both the septal area and the tricuspid, and, if feasible, the mitral ring.

Aims of the Study

This study, therefore, is aimed at :