Background: The roles of complex muscle sleeve geometry and fiber orientation in the pulmonary veins (PVs) in wave-front propagation are poorly understood.
Methods and results: We mapped the left superior PV (LSPV, n=7) and left inferior PV (LIPV, n=4) of dogs with 420 bipolar electrodes (1-mm resolution) and performed detailed histological examination. In the anterior LSPV-left atrial (LA) junction, myocardial muscle fibers were oriented perpendicular to PV blood flow. A wedge filled with connective tissues led to a complete muscle separation or an abrupt increase in muscle thickness between the PV and LA (0.42+/-0.12 versus 2.0+/-0.31 mm, P<0.01). Distal LSPV pacing resulted in conduction block at the anterior PV-LA junction, with double potentials. In contrast, the posterior LSPV-LA junction showed gradual muscle thickening and a fiber orientation parallel to the blood flow. The maximum PV muscle thickness in the anterior PV-LA junction is thinner than that in the posterior junction (0.83+/-0.15 versus 1.3+/-0.38 mm, P<0.01). Distal LIPV pacing showed multiple PV-LA breakthroughs, with segmental conduction block in the anterior PV-LA junction. The conduction block corresponded to segmental PV-LA muscle disconnection. Complex fiber orientations in the PV muscle sleeves away from the PV-LA junction were responsible for intra-PV conduction delay or block during rapid PV pacing.
Conclusions: We conclude that segmental muscle disconnection and differential muscle narrowing at PV-LA junctions and complex fiber orientations within the PV provide robust anatomical bases for conduction disturbance at the PV-LA junction and complex intra-PV conduction patterns.