When the cardiologist did every test possible, I hope he did a Thyroid function test also.(T3, T4,TSH), Hyperthyroidism
is a common cause for tachycardia
. So if it is not done do it immediately.
Do you get this fast heart beats only during anxiety
, activity or any illness. Then it may be stress/anxiety related. Have you tried any anti-anxiety drugs. If not it may be tried with the advice of your doctor.