Muga Results

Here's hoping someone can help with my question. I was scheduled to start taxol/herceptin this week. I finished 4 cycles of A/C and had MUGA which shows my rate go from 58% to 46%. I will not be starting herceptin, but will start the taxol. My onc wants to recheck MUGA in 4 weeks and told me not to be worried. Ha ha ha. Did the A/C do permant heart damage? My major concern with being er-pr- and Her2+++ is that I may not get the herceptin and have a recurrance and die from this nasty disease.
Anyone have any thoughts about this and is the original 58% show a healthy or possible weak heart?
Thanks so much.
Rocky