The antiarrhythmic drug which decreases both rate of depolarization (phase 0) as well as rate of repolarization of myocardial fibres is

QUINIDINE Mechanism of action:- Quinidine blocks myocardial Na+ channels in the open state--reduces automaticity and maximal rate of 0 phase depolarization in a frequency dependent manner. Prolongation of APD is due to K+ channel block, while lengthening of ERP is caused by its moderate effect on recovery of Na+ and K+ channels. At high concentrations it also inhibits L type Ca2+ channels. ECG:- Quinidine increases P-R and Q-T intervals and tends to broaden QRS complex. Changes in the shape of T wave may be seen reflecting effect on repolarization. Ref:- kd tripathi; pg num:-529