Idioventricular rhythms occur when all overdrive pacing fails and the ventricles are left to pace on their own, and they do it verrrrry sloooooowly (20 to 40 beats per minute). Weird thing is, even if overdrive pacing fails and the ventricles are all on their own, they can get worked up and develop an accelerated rhythm (up to 250 beats per minute). Guess what that looks like? V-tach. Bummer if you call the cardiologist reporting V-tach when it's really an accelerated idioventricular rhythm masquerading as V-tach. Happily, the accelerated version generally stays <100 beats per minute, so it seems that you're on safe ground calling it accelerated idioventricular if it looks and smells like V-tach but isn't fast enough. It also looks more like a long run of PVCs.
These accelerated rhythms are tricky. I had some good times with an accelerated junctional rhythm recently; it took meds to slow down the looks-like-SVTs-but-we-don't-know-what's-lurking-beneath rhythm. This is a nice sleight-of-hand used by cardiac folks, by the way; if a patient has a fast rhythm that could be this, could be that, we cheat and give the patient drugs to slow it down at least long enough to see what it really is. Adenosine is probably the most common drug (warn your patient "you're going to feel kind of sick for minute"; do NOT warn your patient "your heart's gonna stop for a few seconds"---this produces an even more accelerated rate), but I've pushed beta-blockers and hung Cardizem drips for essentially the same purpose.