Tenet Healthcare Corp. agreed to buy hospital operator Vanguard Health Systems Inc. for about $1.8 billion in cash to grow in new markets as the U.S. health-care overhaul promises to expand insurance coverage to more Americans starting next year.

Tenet, the third biggest publicly traded U.S. hospital chain, will pay $21 a share, the companies said in a statement today. That’s 70 percent above the June 21 closing price of $12.37 for Nashville, Tennessee-based Vanguard Health on the New York Stock Exchange. Tenet also will assume $2.5 billion of debt, according to the company statement.