The effects of procainamide on conduction in anisotropic canine ventricular myocardium

Although conduction velocity in cardiac tissue is dependent on fiber orientation, the influence of commonly used antiarrhythmic agents on conduction longitudinal and transverse to such fibers is unknown. We evaluated the effects of procainamide on conduction velocity and intracellular potentials in vitro during conduction longitudinal and transverse to fiber orientation in epicardial strips obtained from areas of uniform fiber orientation from 15 adult mongrel dogs. Ventricular epicardial strips demonstrated marked anisotropy. At a pacing cycle length of 1000 msec, mean conduction velocity longitudinal to fiber orientation averaged 0.602 +/- 0.051 m/sec and mean conduction velocity transverse to fiber orientation was 0.186 +/- 0.024 m/sec, resulting in a ratio of longitudinal to transverse conduction velocities of (theta L/T) 3.27 +/- 0.38. After the addition of procainamide, conduction velocity decreased to 0.532 +/- 0.062 m/sec longitudinal to fiber orientation and to 0.174 +/- 0.023 m/sec transverse to fiber orientation resulting in a decrease of theta L/T to 3.09 +/- 0.37 (p less than .05 vs control). Before the addition of procainamide, when pacing at progressively shorter cycle lengths, conduction velocity longitudinal to fiber orientation was relatively unchanged, whereas conduction velocity transverse to fiber orientation decreased resulting in an increase in theta L/T. After the addition of procainamide, conduction velocity at shorter pacing cycle lengths decreased both longitudinal and transverse to fiber orientation demonstrating the well-known use-dependent effect of procainamide. However, in contrast to control conditions, conduction velocity longitudinal to fiber orientation was slowed by a greater extent than the conduction transverse to fiber orientation, resulting in an even greater decrease in theta L/T. To investigate the effect of differences in drug binding during propagation in different directions, we examined conduction velocity during alternations in pacing direction and compared it with velocity during steady-state pacing. At a pacing cycle length of 1000 msec, no difference was observed between the initial conduction velocity after changing pacing directions and the steady-state conduction velocity. At pacing cycle lengths shorter than 1000 msec, when changing from transverse to longitudinal conduction, there was an initial drop in normalized conduction velocity that was present on the first beat of longitudinal conduction; however, with continued pacing in a longitudinal direction there was a further decrease in conduction velocity.(ABSTRACT TRUNCATED AT 400 WORDS)