All of the following statements are true about quinidine except:: (A) It blocks myocardial Na+ channels primarily in the open state, (B) It has no effect on myocardial K+ channels, (C) It produces frequency dependent blockade of myocardial Na+ channels, (D) It delays recovery of myocardial Na+ channels
Answer is B. (Ref: KDT 6/e p511, 512) Quinidine is a class I antiarrhythmic agent. It blocks Na+ channels in the open state and also delays the recovery of these channels. Blockade by these antiarrhythmic agents is frequency dependent. Class Ia antiarrhythmics, in addition blocks myocardial K+ channels also.