Properties and substrate of slow pathway exposed with a compact node targeted fast pathway ablation in rabbit atrioventricular node

Introduction: The properties and substrates of slow and fast AV nodal pathw ay remain unclear. This applies particularly to the slow pathway (SP), whic h is largely concealed by fast pathway (FP) conduction. We designed a new F P ablation approach that exposes the SP over the entire cycle length range and allows for its independent characterization and ablation, Methods and Results: Premature stimulation was performed before and after F P ablation with 5.4 +/- 1.9 lesions (300-mum diameter each; overall lesion size 1.4 +/- 0.5 mm) targeting the junction between perinodal and compact n ode tissues in seven rabbit heart preparations. The resulting SP recovery c urve and control curve had the same maximum nodal conduction time (165 +/- 22 msec vs 164 +/- 24 msec; P = NS) and effective refractory period (101 +/ - 10 msec vs 100 +/- 9 msec; P = NS), The two curves covered the same cycle length range. However, the SP curve was shifted up with respect to control one at intermediate and long cycle lengths and thus showed a longer minimu m nodal conduction time (81 +/- 15 msec vs 66 +/- 10 msec; P < 0.01) and fu nctional refractory period (180 +/- 11 msec vs 170 +/- 12 msec; P < 0.05), The SP curve was continuous and closely fitted by a single exponential func tion, Small local lesions (2 +/- 1) applied to the posterior nodal extensio n resulted in third-degree nodal block in all preparations. Conclusion: The posterior nodal extension can sustain effective atrial-His conduction at ail cycle lengths and account for both the manifest and conce aled portion of SP, Slow and FP conduction primarily arise from the posteri or extension and compact node, respectively.