Tachyarrhythmias can also be due to altered impulse formation producing reentry, which requires a dual pathway, slowed conduction and unidirectional block.

Figure A illustrates the potential dual pathway that is normally homogeneously activated. Under certain circumstances, slowed conduction and unidirectional block, a continuously excitable electrical circuit can develop. A premature impulse can conduct down one limb of the circuit but is blocked in the other limb. It can then enter the distal portion of the circuit that has not been stimulated because of the proximal unidirectional block, Figure B. As it conducts retrograde, the area of unidirectional block may allow retrograde conduction, Figure C. If so, and particularly with slowed conduction, the impulse may then encounter continuously repolarized and excitable tissue, thus causing either a single or recurring series of premature atrial or ventricular complexes, Figure D. Thus, reentry is a potential mechanism for a single premature complex or a sustained tachycardia in the atria, AV junction or ventricles. An example of a common reentry tachycardia is AV nodal reentry tachycardia.