Yes, there are different forms of svt some more dangerous than others. Afib is one of the more dangerous svts but the real threat comes after the person returns to normal sinus rhythm because there is a higher risk of throwing a blood clot. Accessory pathway svts, especially ones like wpw that are a bit more dangerous than the more run of the mill svts. Accessory pathway svts are characterized by having an extra muscle fiber in your heart that allows the signal to get caught in a loop. In wpw syndrome the extra pathway connects via the atria and ventricles. I had an svt called avnrt which was an extra path in the avnode. The classic sign of them is that they can start and stop on a single beat and are very very fast in the 200s or more. These are the types of svts that one should not let sustain for long periods of time. I had an episode that lasted 8 hours and it really took a toll on me but at the time I really didn't know what was going on but in general one should head to the er if their tachycardia sustains for more than an hour. That said, because my heart was in good shape I recovered within a few days with the bigger risk of svt being long term wear and tear on the heart. That said, a rate of 140 is not incredibly high and not one too many doctors get super overly concerned about. Has your svt been identified? Is the 140 a rate at resting or moving about? Do you drink plenty of water? How is your blood pressure? Do your episodes start and stop in one beat? Have you had your thyroid checked? Have you had your sodium levels checked? Do you take medications for anything? A rate of 140 may indicate simple sinus tachycardia but the problem with that is it can be hard to pinpoint the cause which is usually from something outside the heart so you need to get a full workup to see if there are any underlying conditions that may be contributing to the tachycardia. A rate of 140 though I think may be helped with simple beta blockers. So what did the cardiologist tell you. Have you been in to be evaluated? I would say try to drink as much water as you can at this point and see if it helps you feel better but do go and get checked out so you can hopefully resolve the issue if you can find an underlying cause. Take care and feel better soon.
I had 54 years of accessory path SVT called AVRT. My heart rate would jump 240bpm and was a high as 312bpm when I was a kid. I endured long episodes at this rate. A heathy, young adult should have no problem with SVT episodes. It can become a problem as you age though and it is within the realm of possibility that it could set you up for a heart attack. As a young adult I competed at very high levels in skating and cycling while having SVT. You just never knew if today was going to be a "bad" day, and it sometimes was.