Arrhythmias from Ischemic and Border-Zone Tissue | AI Learning Guide

Ischemic tissue conducts slowly because its cells are partially depolarized. If a wave of electricity enters that slow zone, the tissue ahead may have already recovered by the time the wave arrives, so the wave can turn around and re-excite tissue it just passed. That is reentry, and it is the dominant mechanism. The border zone is the worst offender because it has the steepest gradient between slow and fast conduction. A second mechanism is automaticity: border-zone Purkinje fibers, exposed to high potassium and catecholamines, start firing on their own. A third is simple block, which forces the wavefront into alternate paths and creates the electrical chaos that becomes fibrillation. The clinical takeaway is that the first hours after occlusion are the most dangerous for lethal arrhythmia.

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Three arrhythmia mechanisms in ischemia

Reentry: slowed conduction through partially depolarized tissue allows a wavefront to circle back on recovered tissue.
Abnormal automaticity: border-zone Purkinje fibers depolarize spontaneously under ischemic conditions.
Conduction block: failure of conduction forces alternate pathways and creates the heterogeneity that supports fibrillation.

Why the border zone is the arrhythmogenic hotspot

The border zone contains a steep spatial gradient between ischemic and normal tissue. Conduction velocity, action potential duration, and refractory period all change abruptly across this boundary. That heterogeneity is the raw material for reentry and block, and it explains why arrhythmias often originate at the infarct margin rather than its center.