Before I was on BP meds my heart at times was racing (tachycardia) and the doctor told me that high BP can cause that. Is this the only doctor you have ever seen? A diastolic number of 90 for 20-25 yrs is not good since the new guidelines are anything over 120/80 is pre-hypertensive, over 140/90 is high. Since your bottom number never went over 90 maybe that is why he/she is letting you ride on your buttom number for the last 25 yrs.
I have never heard that connection, but I'm not sure it isn't possible. I'm wondering why no medication for a consistently high diastolic pressure. What does your doctor say? Maybe he/she has a great reason for not prescribing.
Do you have SVT or VT? By SVT, do you mean supraventricular tachycardia or sustained ventricular tachycardia?