A hospital in Lewiston is one of 71 medical centers nationwide, and the only one in Maine, to test a procedure that it said holds promise of helping millions of Americans with blocked heart valves get treatment without requiring surgery. The procedure also holds potential to decrease hospital stays and costs. Central Maine Heart and Vascular Institute, which is part of the Central Maine HealthCare system, tested a medical device called a trans-catheter aortic valve replacement, or TAVR. That device resembles a mesh circle that is bound tightly when inserted into a patient’s groin artery and threaded into the heart, where it expands and replaces a clogged aortic valve.