Tachycardia induced AF

I've always suffered from a high HR, with minimal activity my pulse rushes up to 130-140. Usually around 90-100 at rest, altogh when really relaxed it could be as low as 55 (rare) However, I otherwise always have felt in great shape, at least until recently, when I experienced atrial fibrillation. I wonder if my high pulse over the last years (as long as I can remember) could have provided the substrate for my A-fib?

I'm 24 yrs old. My echo was normal, except for a slighty dilated left atrium.

[va_tony]

It's likely your dilated left atrium is contributing to your afib episodes. But what caused the atrial dilation? The usual causes are high BP or mitral valve regurgitation or even having afib in the past (afib begets afib). I'm not a doctor but I don't think a pulse rate of 90-100 is high enough to ever cause heart enlargement . Yes, it's true that a high heart rate will lead to atrial enlargement, but the HR's need be in the range >150 for long periods.  

If you google "Atrial Remodeling in Tachycardiomyopathy" there's a paper (J Cardiovasc Electrophysiol, Vol. 12, pp. 1404–1410, December 2001) that studied the effects of high HR in goats. They concluded you needed fairly high ventricular heart rates (240 bpm) over four weeks to cause significant dilation.  At the same time the goats developed cardiomyopathy. My guess is that if you don't currently have cardiomyopathy, your HR didn't cause the dilated atrium.