For 400,000 people across the country with failed kidneys, dialysis care is a matter of life and death. It is also a lucrative business, in part because Medicare pays for such treatment regardless of age. But as for-profit clinics and chains have grown to control about 85 percent of the dialysis market over the last decade, researchers have documented starkly higher mortality rates in centers owned by for-profits compared with nonprofits. Now the New York State Public Health and Health Planning Council is set to vote Thursday on a deal to turn over dialysis at four of New York City's public hospitals to a for-profit franchise called Big Apple Dialysis despite government data showing the company's centers did not perform as well as the hospitals themselves.