Ambulatory surgery centers, facilities that offer outpatient surgical procedures, say their industry and the low-cost care it provides could be under threat from state regulators seeking to give their competition, Bay State hospitals, a competitive edge. At issue, according to Linda Rahm, president of the Massachusetts Association of Ambulatory Surgery Centers, is a proposed regulation by the Department of Public Health that would require ASCs to obtain state approval known as a "determination of need" before expanding.