On measuring curvature and electrical diffusion coefficients in anisotropic myocardium: comments on "Effects of bipolar point and line simulation in anisotropic rabbit epicardium: assessment of the critical radius of curvature for longitudinal block" [comments and reply]

Winfree comments as follows: Notions of "curvature" and "propagation perpendicular to the activation front" inherited from electrophysiological theory of isotropic reaction-diffusion media do not apply directly to experimental data taken in uniformly anisotropic myocardium. They apply directly only after the concept of "curvature" is normalized and propagation is made to look perpendicular to activation fronts by rescaling distances to achieve isotropy. Without rescaling, the curvature-dependence of longitudinal propagation speed turns out counter-intuitively to measure transverse intercellular electrical coupling. In reply Knisley concludes that it is important to recognize that although rescaling to isotropy has the advantage of providing a single value for critical curvature, the value is not the same as the critical curvatures that theoretically occur on either axis in the heart. In general the critical radius of curvature will vary according to the angle of the wavefront relative to fibers. This may have important consequences, e.g., for the design of small pacing electrodes as Dr. Winfree suggests.