Exactly. you have guessed it very right. let me add a bit more to it. actually, both Quinidine and Procainamide are Class 1A anti-dysrrythmic drugs. their main cardiac effect is depressant via blockage of Na channels and all that you know. besides that, both drugs have mild intrinsic anti-muscarinic effects(mainly increased SA nodal rate and increased heart rate),this definitely counter-acts their primary depressant effect,so the drug that has lesser anti-muscarinic effect will be more efficacious in manifesting its primary anti-depressant effect i.e Procainamide. okay. thats it!