An example of triggered activity developing in Purkinje tissue. The appearance of a delayed after-depolarization (DAD) after the fifth paced impulse is followed by a burst of triggered activity that maintains the rapid rate of impulse formation despite the cessation of electrical pacing. Triggered activity from DADs occurs in Purkinje fibers or ventricular muscle when the tissues are exposed to toxic concentrations of digitalis, catecholamines, or other interventions that increase intracellular calcium concentrations. Whereas DADs occur after the cell has achieved its maximum diastolic potential, the phenomenon of early afterdepolarization (EADs) occurs before complete repolarization has taken place. EADs can occur after exposure to drugs that prolong the action-potential duration and may account for the proarrhythmic action (discussed in the text) of several antiarrhythmic drugs.
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