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Isochronal maps of ventricular activation were analyzed in dogs 3-5 days after ligation of the left anterior descending coronary artery utilizing a 64-channel multiplexer. Isochronal maps of the effective refractory period were determined from 62 epicardial sites and correlated with the activation maps. The ischemia occurring in the surviving epicardial layer prolonged refractoriness in a spatially nonuniform manner. The resulting pattern of refractoriness on the epicardial surface resembled concentric rings of isorefractoriness which increased in duration from the normal zone to the center of the ischemic zone. The formation of an arc of functional unidirectional conduction block occurred along the gradient of refractoriness and the exact location of the arc depended on the S1-S2 interval. When a short S1-S2 failed to induce reentry, fewer adjacent sites with sufficiently disparate refractoriness formed a smaller arc of block. A subsequent S3 encountered further nonuniformly shortened refractoriness (normal areas had shortened refractoriness greater than ischemic areas) and the arc of block was lengthened. This required a longer time for the wavefront to circulate around the arc. When it then reached the distal side of the arc, refractoriness had expired proximal to the arc and reentry occurred. Similarly, nonuniform shortening of refractoriness explained why one reentrant beat may or may not produce successive reentrant beats. Therefore, the spatial pattern of refractoriness forms the substrate for the arc of unidirectional conduction block that is fundamental to the development of ventricular reentry in this experimental model.

Reentrant ventricular arrhythmias in the late myocardial infarction period in the dog. 13. Correlation of activation and refractory maps.