First - MO people often have abnormal EKGs, especially women, because of fat/breast tissue on the chest wall.
I had two friends who had DS a while after me; both were my age (they were both in their early 50s at the time) and both were over 400 lbs.
1) My male friend had developed kidney issues, and was, as most men are, centrally obese. His preop EKG was abnormal. He had to undergo an "emergency" angiogram the day before his scheduled surgery (he had to travel to the Bay Area, and his COBRA insurance was about to expire). The angiogram showed an astonishingly clear cardiovascular system, giving him more confidence that he was not going to die on the table.
2) My female friend (who had an even higher BMI, being much shorter), also had a bad preop EKG, and the angiogram discovered a 95% blocked coronary artery (and others as well). She was rotorooted and had to delay surgery for 2 months, to taper off the anticoagulants. She did fine - but could have died on the table if they had operated without unblocking her coronary arteries.
So the further testing may be reassuring, if it turns out that your coronary system is fine, OR it may find and fix something that needs to be fixed for you to safely have your surgery.
Either way, it's a good thing.