If you are in afib that is how they will sometimes get you back into normal sinus rhythm. It is called cardioversion but basically they shock your heart back into normal rhythm if meds don't work. I also believe they do it as a precaution though the odds of someone succumbing to an svt attack are extremely rare but I am sure they want to be safe rather than sorry.
There a nmber of ways to convert. It's possible they were prepping you for something called "synchronized cardioversion", a process that fires a pulse at precisely the right moment to break the SVT. It's a hands-off process that relies on microprocessor based equipment. Rather than explain it, here's a very good video of the process: http://youtu.be/ReJo4aclOw8 Note that is takes several power levels before the conversion to NSR occurs.
I don't think you would have been in sustained VT without there being a lot more symptoms on your part though I can't say that for certain but I am sure they would have told you if it was VT as opposed to simple svt. As well maybe they put the pads on just in case you went into VT but that is rare as well. I would say if you want to know for sure ask the hospital for all your records and see what they documented.
Passing out for one. SVTs don't generally cause someone to pass out but VT might. I would say with a rate of 240 you are very likely looking at avnrt and an ablation is a pretty good bet at correcting the issue for good. The odds are super good with very very low complication rates if you ever want to consider it.
Tom's answer is pretty good.
Forget the alphabet soup of VT, afib... whatever. You went to the ER - they hooked you up a person with "MD" after their name looked at the chart and told you that you have SVT. If that's the case don't worry about anything else... (ie - dying)
Noooooowww that being said. Tom's first sentence is a good answer. There are a bazillion and 5 ways to convert an SVT. Vagals, drugs and elctroconvert... well the *second* time I went to the ER, I had a fresh out of Med School intern telling me about how he was going to give me five minutes and then electroconvert me.... to which I said (literally) " $%^ you, I'll convert myself." My thought was that his though was, "Cool, I've been waiting for a cardiac case where I can elcetroconvert somebody." So, if that was the FIRST thing they were offering... that's freakin' cowboy medicine as far as I'm concerned. And yes, I converted myself.
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