Boston Scientific said Monday it would buy asthma product developer Asthmatx for $193.5 million in cash. The Natick, Mass.-based device giant would pay an additional $250 million for the Sunnyvale, Calif-based, privately held company contingent on reaching undeclared "revenue-based criteria" through 2019.

The deal is expected to close in the fourth quarter.

Asthmatx's flagship device is a catheter system that uses radiofrequency energy to reduce excess smooth muscle in the airways that can constrict breathing in chronic asthma sufferers.

The device, called the Alair Bronchial Thermoplasty System, received U.S. Food and Drug Administration clearance in April. Around 20 million Americans have asthma, according to the company, 10 to 15 percent of whom have severe persistent asthma.

Boston expects the transaction to be two cents dilutive to its earnings per share next year and in 2012, and to break even by 2013.

Leerink Swann, a health care investment bank, said the deal "looks to us like directionally very positive news."

"The deal - the first in some time - importantly highlights CEO Ray Elliot's determination (and ability) to move past the current portfolio's growth/profitability challenges and forge into new areas of MedTech - seeking big new markets that can potentially add growth to the BSX product portfolio," analyst Rick Wise wrote in an e-mail to investors.

Boston's stock held steady at $5.43 in recent trading.

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