I can tell you that from my experience here on the forum, Diltiazem is seldom mentioned here as treatment for SVT. I think you hear the majority of people are on
Metoprolol, followed by Atenolol, Propanolol, then Sotalol all beta blockers. Then comes Verapamil, which is a calcium channel blocker like Diltiazem. Perhaps is has something to do with the nature of how a Class IV anti-arrhythmic affects the cardiovascular system.