Class IA antiarrhythmics affect risk of torsades de pointes how?

• A. Increase risk of torsades de pointes and prolong action potential duration
• B. Decrease risk of torsades de pointes
• C. No effect on TdP
• D. Only class III prolongs AP duration

Answer: (A)

Class IA antiarrhythmics prolong repolarization by blocking potassium channels in addition to their sodium channel effects. This lengthens the action potential duration and the QT interval. Prolonged QT provides a substrate for early afterdepolarizations, which can trigger torsades de pointes, so these drugs increase the risk of TdP. It’s not correct to say they decrease risk or have no effect, and it’s not true that only class III prolongs the AP duration—class IA do as well, though through a different mechanism.