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(Circulation. 2001;103:2521.)
© 2001 American Heart Association, Inc.

Basic Science Reports

Intercaval Block in Normal Canine Hearts

Role of the Terminal Crest

Ruediger Becker, MD; Alexander Bauer, MD; Stephan Metz, MD; Ralf Kinscherf, PhD; Julia C. Senges, MD; Kirsten D. Schreiner, MD; Frederik Voss, MD; Wolfgang Kuebler, MD, FACC; Wolfgang Schoels, MD

From the University of Heidelberg, Department of Cardiology, and the Department of Anatomy III (S.M., R.K.), Heidelberg, Germany.

Correspondence to Ruediger Becker, MD, University of Heidelberg/Department of Cardiology, Bergheimer Str 58, 69115 Heidelberg, Germany. E-mail ruediger_becker{at}med.uni-heidelberg.de

Background—The intriguing monotony in the occurrence of intercaval conduction block during typical atrial flutter suggests an anatomic or electrophysiological predisposition for conduction abnormalities.

Methods and Results—To determine the location of and potential electrophysiological basis for conduction block in the terminal crest region, a high-density patch electrode (10x10 bipoles) was placed on the terminal crest and on the adjacent pectinate muscle region in 10 healthy foxhounds. With a multiplexer mapping system, local activation patterns were reconstructed during constant pacing (S₁S₁=200 ms) and introduction of up to 2 extrastimuli (S₂, S₃). Furthermore, effective refractory periods were determined across the patch. If evident through online analysis, the epicardial location of conduction block was marked for postmortem verification of its endocardial projection. Marked directional differences in activation were found in the terminal 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<0.01). In the pectinate muscle region, however, conduction velocities were similar 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 S₃ stimulation, conduction block parallel to the terminal crest was inducible in 40% of the dogs compared with 0% in the pectinate muscle region.

Conclusions—Even in normal hearts, inducible intercaval block is a relatively common finding. Anisotropic conduction properties would not explain conduction 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/pectinate muscle junction might form the anatomic/electrophysiological basis for intercaval conduction block.

Key Words: atrial flutter • atrium • conduction • electrophysiology • mapping

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