stress myocardial perfusion scan

on monday i am having a stress test scan on my heart, using dobutamine as i am not fit enough to treadmill as i have been diagnosed with a 50% blockage on my aorta(descending). although all of my other arteries and viens are normal. when i was a child i had a heart murmur which was said to be commen but to keep an eye on it . when i was 21 it was heard again and an echocardiogram showed nothing again when i was 30 a blip showed up on an ecg.  i am now 42 and have read that a murmur could be a slight problem with a heart valve and that if a problem exists that i shouldent have this drug induced stress test. ime probebly worrying needlessly but at the moment my confidence in my doctors is very low as other mistakes have been made. question... how safe is the stress scan?