Cincinnati-based Christ Hospital faces allegations that could cost it up to $400 million and expose its nationally known heart center as the beneficiary of an illegal kickback scheme. The complaint from a retired cardiologist charges the hospital and the Ohio Heart & Vascular Center with conspiring to steer patients and revenue to each other. The suit comes as the hospital is already spending tens of millions of dollars on new computer systems and other startup costs as it withdraws from the Health Alliance of Greater Cincinnati.