WakeMed Health & Hospitals faces challenges as the Affordable Care Act reshapes the U.S. health care industry, and the hospital system must now address those challenges while also hunting for a new leader. President and Chief Executive Bill Atkinson agreed Wednesday to step down, citing "differences in the future direction of the organization," officials said Thursday. He had led WakeMed since 2003. "There's a lot of pressure on hospitals right now, especially nonprofits like WakeMed with razor-thin profit margins," said David Ridley, health sector management faculty director at Duke University's Fuqua School of Business.