<prompt> The antiarrhythmic drug which decreases both rate of depolarization phase 0 as well as rate of repolarization of myocardial fibres isVerapamilQuinidinePropranololLignocaine  <response>: 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 