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Concentration-dependent biphasic proarrhythmic and antiarrhythmic effects of quinidine in causing torsade de pointes (TdP) in the absence (left) and presence (right) of 1 nmol/L ATX-II in female rabbit isolated hearts paced at 1 Hz. Numbers in parentheses indicate the number of hearts with TdP and the total number of hearts studied. Therapeutic concentrations range of quinidine is given in the box at the top of the figure.
