Dear cmasse,
Did you know that the EKG's only show something AFTER you've already had a heart attack? Same thing for the bloodwork, etc. They are useless before. Keep going back to E.R. or to a cardiologist and ask for at LEAST a stress-test using dye and at best, an angiogram (or whatever it is they call it when they go in through your leg and up into your heart with a little camera, to find any blockage). If you are still having these symptoms. It does NOT sound all like costochondritis to me. Case in point, my husband was hospitalized overnight years ago at the age of only 37 just for observation, since his symptoms very similar to yours, showed nothing from the above-mentioned tests. Literally, at the moment when the doctor was in his hospital room saying he was going to release him and schedule an out-patient stress-test with dye, THAT's when he had his first heart attack. Fortunately, he was still hooked up to monitors, when his blood pressure dropped, and fortunately, this heart attack was mild... Doctor immediately said, "Change of plans, we're moving you to Intensive Care and scheduling an Angiogram." The procedure showed very severe blockage... In fact, he ended up getting a triple by-pass shortly thereafter. I thank God that the first "warning" heart attack happened where & when it did rather than on the way home or at home, since we lived an hour away from the hospital at the time. Oh, AFTER that, his EKG now ALWAYS shows that heart attack from the past, so they take him SERIOUSLY when he comes in for ANYTHING. Also, I noticed that when they re-ran bloodwork, now they could see results showing how much damage had been done to the heart.
Another case in point is myself, about your supposed costochondrtis. I accepted that diagnosis for YEARS, decades with this recurring sternum pain. Now, after many other progressive symptoms, I found out I have had hypercalcemia all these years. I recommend you have your blood tested for PTH, serum calcium, and ionized calcium. Mine turned out to show a benign hyperparathyroid tumor which caused my body to strip the bones of calcium, but not digest it out, so it deposited the calcium here & there, wreaking havoc in all kinds of places in my body... while my bones got weaker. NOW, after removing my parathyroid tumor, they are re-checking my sternum pain... doing bone scans to see if it's been eaten away so badly, or if there are calcium deposits irritating the cartilege, or if the thymus--another gland, located right beneath your sternum/breastbone--might be enlarged as a side-effect of the hypercalcemia/hyperparathyroid and the pain might be caused from the abnormal thymus pushing against my breastbone when I move a certain way. Please get this simple blood test and rule out these things. If I had pursued it decades ago instead of accepting a "Costochondritis" diagnosis, I wouldn't have damaged kidneys and ostioporosis and more... Best wishes!