CHICAGO — St. Louis-based Ascension Health Alliance has signed a memorandum of understanding to acquire Oklahoma-based Marian Health System, the parent of three regional not-for-profit health care organizations in the mid and central states.

Ascension, a Roman Catholic system that is the largest not-for-profit health care provider in the nation, said a final agreement could be reached late this year or early in 2013, following a due diligence period.

Terms of the tentative agreement were not made public. The systems cautioned: "At this point, no assumptions should be made regarding changes to the existing credit structures of each of the three health systems of Marian Health System."

Ascension operates more than 70 acute-care hospitals and nine specialty hospitals in 21 states and the District of Columbia. Its facilities generate $15.5 billion in annual operating revenue and it has more than $4 billion of outstanding debt, rated AA-plus by Fitch Ratings and Standard & Poor's.

Marian, also Catholic, was founded in 1989. It is the parent of three regional systems with 36 affiliated hospitals and 150 clinics. Its groups include Via Christi Health in Wichita, Kan., which currently has an affiliation with Ascension; Ministry Health Care, based in Milwaukee; and St. John Health System in Tulsa.

The nation has seen a wave of health care consolidation as hospitals grapple with changes under national health care reform and capital needs while seeking to remain competitive.

The announcement shows Ascension's thirst for expansion was not abated by other recent acquisitions. "This step is part of an ongoing effort to strengthen Catholic health care in the U.S. The systems are all committed to serving the community and advocating for the common good," Ascension president Anthony R. Tersigni said in a statement.

Ascension and California-based Daughters of Charity Health System earlier this year announced the signing of a memorandum of understanding expected to lead to Daughters and its six hospitals joining Ascension. Late last year, Ascension acquired Alexian Brothers Health System, which is based in suburban Chicago.

Separately, last year Ascension formed Ascension Health Care Network, a joint venture with the private-equity firm Oak Hill Capital Partners, to provide alternate funding for the acquisition of Catholic hospitals and other facilities.

Ministry operates 14 hospitals in Wisconsin, one in eastern Minnesota, and 46 clinics that generate $1.2 billion in operating revenue annually. Ministry is rated A-plus by Standard & Poor's. It has $590 million of debt and benefits from a 52% market share in its coverage areas and solid balance sheet and operating margins. Its challenges have included the need for increased capital spending and volume declines.

Via Christi Health operates 11 hospitals, several outpatient clinics, one rehabilitation center, 12 senior-living facilities and a managed care organization that generate $1 billion in annual revenues. Fitch assigns an A-plus to the system's $466 million of debt. The system benefits from a leading market share in the competitive Wichita market.

St. John operates eight hospitals and three urgent care facilities in Oklahoma with 50% ownership in a health maintenance organization and collective annual operating revenues of nearly $1 billion. Standard & Poor's rates the system's $327 million of debt A with a positive outlook. The system benefits from improved operations, market share gains and volume growth.

The debt of Marian's systems is secured by the individual obligated group of each system, according to rating reports.

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