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Tachyarrhythmias can arise from three different mechanisms. First, increased automaticity from more rapid phase 4 depolarization can cause arrhythmias. Second, in certain conditions, spontaneous depolarizations during phase 3 (early afterdepolarizations; EAD) or phase 4 (late afterdepolarizations; DAD) can repetitively reach threshold and cause tachycardia. This appears to be the mechanism of the polymorphic ventricular tachycardia (torsades de pointes) observed in some patients taking procainamide or quinidine and the arrhythmias associated with digoxin toxicity. Third, the most common mechanism for tachyarrhythmia is reentry. In reentry, two parallel pathways with different conduction properties exist (perhaps at the border zone of a myocardial infarction or a region of myocardial ischemia). The electrical impulse normally travels down the fast pathway and the slow Source: Cardiovascular Disorders: Heart Disease, Pathophysiology of Disease: An Introduction to Clinical Medicine, 7e pathway (shaded region), but at the point where the two pathways converge the impulse traveling down the slow pathway is blocked since the tissue is Citation: Hammer GD, McPhee Pathophysiology of Disease: Anwhen Introduction to Clinical Available at: occur in the fast refractory from the recent depolarization viaSJ. the fast pathway (a). However, a premature beatMedicine, reaches 7e; the 2013 circuit, block can http://mhmedical.com/ Accessed: May 13, 2017 pathway, and the impulse will travel down the slow pathway (shaded region) (b). After traveling through the slow pathway the impulse can then enter the © 2017 McGraw-Hill Education. Alldelay rightshas reserved fast pathway Copyright in retrograde fashion (which because of the recovered excitability), and then reenter the slow pathway to start a continuous loop of activation, or reentrant circuit (c).