Intercaval block in normal canine hearts - Role of the terminal crest

Background-The intriguing monotony in the occurrence of intercaval conducti on block during typical atrial flutter suggests an anatomic or electrophysi ological predisposition for conduction abnormalities. Methods and Results-To determine the location of and potential electrophysi ological basis for conduction block in the terminal crest region, a high-de nsity patch electrode (10x10 bipoles) was placed on the terminal crest and on the adjacent pectinate muscle region in 10 healthy foxhounds. With a mul tiplexer mapping system, local activation patterns were reconstructed durin g constant pacing (S1S1=200 ms) and introduction of up to 2 extrastimuli (S -2, S-3). Furthermore, effective refractory periods were determined across the patch. If evident through online analysis, the epicardial location of c onduction block was marked for postmortem verification of its endocardial p rojection. Marked directional differences in activation were found in the t erminal crest region, with fast conduction parallel to and slow conduction perpendicular to the intercaval axis (1.1+/-0.4 versus 0.5+/-0.2 m/s, P<0.0 1). In the pectinate muscle region, however, conduction velocities were sim ilar in both directions (0.5+/-0.3 versus 0.6+/-0.2 m/s, P=NS). Refractory patterns were relatively homogeneous in both regions, with local refractory gradients not >30 ms. During S-3 stimulation, conduction block parallel to the terminal crest was inducible in 40% of the dogs compared with 0% in th e pectinate muscle region. Conclusions-Even in normal hearts, inducible intercaval block is a relative ly common finding. Anisotropic conduction properties would not explain cond uction block parallel to the intercaval axis in the terminal crest region, and obviously, refractory gradients do not seem to play a role either. Thus , the change in fiber direction associated with the terminal crest/pectinat e muscle junction might form the anatomic/electrophysiological basis for in tercaval conduction block.