Published Online
on June 18, 2009

A more recent version of this article appeared on August 1, 2009

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Original Article

Three Distinct Directions of Intramural Activation Reveal Non–Uniform Side–to–Side Electrical Coupling of Ventricular Myocytes

Bryan J. Caldwell; Mark L. Trew; Gregory B. Sands; Darren A. Hooks; Ian J. LeGrice and Bruce H. Smaill¹

University of Auckland, Auckland, New Zealand

¹ E-mail: b.smaill{at}auckland.ac.nz

Background—The anisotropy of cardiac tissue is a key determinant of 3D electrical propagation and the stability of activation wavefronts in the heart. The electrical properties of ventricular myocardium are widely assumed to be axially anisotropic with activation propagating most rapidly in the myofiber direction and at uniform velocity transverse to this. We present new experimental evidence that contradicts this view.

Methods and Results—For the first time, high-density intramural electrical mapping (325 electrodes at ~ 4 x 4 x 1 mm spacing) from pig left ventricular (LV) tissue has been used to reconstruct 3D paced activation surfaces projected directly onto 3D tissue structure imaged throughout the same LV volume. These data from 5 hearts demonstrate that ventricular tissue is electrically orthotropic with three distinct propagation directions that coincide with local microstructural axes defined by the laminar arrangement of ventricular myocytes. The maximum conduction velocity (CV) of 0.67 ± 0.019 ms^-1 was aligned with the myofiber axis. However, transverse to this, the maximum CV was 0.30 ± 0.010 ms^-1 parallel to the myocyte layers and 0.17 ± 0.004 ms^-1 normal to them. These orthotropic conduction velocities give rise to preferential activation pathways across the LV free wall that are not captured by structurally detailed computer models which incorporate axially anisotropic electrical properties.

Conclusions—Our findings suggest that current views on uniform side-to-side electrical coupling in the heart need to be revised. In particular, the non-uniform laminar myocardial architecture and associated electrical orthotropy should be included in future models of initiation and maintenance of ventricular arrhythmia.

Key Words: mapping • structure • anisotropy • computer modeling • intramural pacing