This sentence has been confusing me for some time now:

"Perhaps the most important difference between quinidine and procainamide is the less prominent antimuscurinic action of procainamide. Therefore, the directly depressant actions of procainamide on sinoatrial and atrioventricular nodes are not as effectively counterbalanced by drug-induced vagal block as in the case of quinidine."

So, how exactly does drug-induced vagal block counterbalance the effects of quinidine?

If quinidine has antimuscurinic action, that means it inhibits the parasympathetic control to the heart. If it inhibits the parasympathetic influence, and parasympathetic control is dominant out of the two systems, that makes the sympathetic effects more pronounced, i.e. increased. If sympathetic effects on the heart are 'increased,' that means that quinidine's action of depressing the SA/AV nodes are being opposed, or 'counterbalanced,' since sympathetic stimulation will obviously be excitatory.

So, quinidine would induce better counterbalancing of its depressant effects through its antimuscurinic action which leads to sympathetic dominance, than would procainamide.

That's my best guess.

I've been up too long to figure this out, but I really needed to write it down somewhere to get to the bottom of it.