What is a dangerous heart rate for teens(who exercise once a week[running laps] and sit down for about 8-10 hours a day) when scared, or in general?

Tachycardia, SV: During an episode of SVT, the heart's electrical system doesn't work right, causing the heart to beat very fast. The heart beats at least 100 beats a minute and may reach 300 beats a minute. After treatment or on its own, the heart usually returns to a normal rate of 60 to 100 beats a minute. SVT may start and end quickly, and you may not have symptoms. SVT becomes a problem when it happens often, lasts a long time, or causes symptoms. SVT also is called paroxysmal supraventricular tachycardia (PSVT) or paroxysmal atrial tachycardia (PAT).