the probable basis of most arrhythmias, requiring at least three criteria in the heart: a loop circuit, unidirectional block, and slowed conduction. Impulses enter the loop circuit and divide in both directions (blocked in one direction only), negotiate the loop circuit to the area of block where the slowed conduction has allowed the impulse to arrive at a time when the tissue proximal to the unidirectional block has recovered and will permit its passage in the opposite direction.