Reentrant excitation around a fixed obstacle in uniform anisotropic ventricular myocardium

The purpose of this study was to investigate the role of tissue anisotropy and dispersion of refractoriness on initiation of reentrant ventricular tachycardia (VT).

A ring of perfused uniform anisotropic ventricular epicardium in Langendorff-perfused rabbit hearts was created by an endocardial freezing technique. High-resolution mapping (248 channels) was used to analyze epicardial activation of the left ventricle. One to three premature beats were induced at a total of 272 points in 17 experiments (16 different points around the ring in each experiment). Reentrant VT could be initiated at 43 of the 272 points tested. The cycle length of VT was stable and ranged from 128 to 198 msec (mean, 161 +/- 19 msec). Correlation between conduction velocity and the angle between the circulating activation wave and epicardial fiber orientation showed that in segments of the ring where conduction was perpendicular to the fiber axis, mean conduction velocity was 25 +/- 5 cm/sec compared with 60 +/- 7 cm/sec when conduction was parallel to the fiber orientation. Analysis of the site of unidirectional conduction block showed that in 41 of 43 cases, block occurred while the impulse was propagating parallel to the fiber orientation. Measurement of the refractory periods at either side of the line of unidirectional block showed that only in 12 of 43 cases did block occur while the impulse was propagating into an area with a longer (more than 10 msec) refractory period.

In uniform anisotropic ventricular myocardium, reentrant VT is initiated because lowering the stimulating efficacy of the depolarization wave by premature beats leads to preferential conduction block parallel to the fiber orientation.