Medtronic PLC has added to its line of congestive heart failure products with the purchase of HeartWare International Inc., a maker of ventricular assist devices.

Medtronic said Monday that it would pay about $1.1 billion in cash for HeartWare, a firm based in suburban Boston that had been on the acquisition hunt until activist shareholders recently pressed it to look for a buyer instead.

HeartWare's flagship product is designed to reduce the invasiveness of heart surgery and improve patient recovery times and outcomes.

"The team at HeartWare has established excellent relationships with its hospital customers and built a strong position and reputation in the marketplace," Mike Coyle, president of Medtronic's cardiac and vascular group, said in a statement.

Doug Godshall, HeartWare's chief executive, said in a statement that the deal gives HeartWare "a unique opportunity to enhance growth in the mechanical circulatory support market."

Congestive heart failure, a weakening of the heart that tends to progress slowly and often complicates the functions of other organs, is a leading cause of death in the U.S.

HeartWare, which had sales of $277 million last year, says it makes the world's smallest ventricular assist device, a pump that is implanted in the left ventricle of the heart. It raises the diminished flow of blood in a weakened heart, which strengthens the rest of the circulatory system and other organs.

HeartWare sells the pump as part of a system that includes tools for implanting it. Medtronic estimates that the size of the worldwide market for ventricular assist devices is around $800 million and that the market's annual growth, now in the mid-to-high single digits, will be above 10 percent starting around 2018.

In the transaction, Medtronic valued HeartWare shares at $58 each, nearly twice their closing price on Friday of $29.98. In trading Monday, HeartWare shares closed at $57.79, up 93 percent, and Medtronic closed at $82.38, down 1 percent. Broad-market stock indexes fell in a range of 1.5 percent to 2.4 percent.

"Despite the high premium and all-cash offer, the transaction looks attractive to us," Joanne Wuensch, analyst at BMO Capital Markets, wrote in a research note. She noted that the offer price was about 30 percent below the 52-week high for HeartWare shares and that Medtronic, with more than $12 billion in cash, could easily fund it.

HeartWare made several acquisitions in recent years, but an activist shareholder, Engaged Capital LLC of Newport Beach, Calif., in January forced the company to call off a proposed $860 million deal for Valtech Cardio, an Israeli maker of heart value replacement products.

Engaged Capital and other investors later pressed the company to seek buyers, influenced in part by the $3 billion that Little Canada-based St. Jude Medical Inc. paid for a HeartWare rival called Thoratec Corp. last year.

HeartWare in March agreed to appoint Chadwick Cornell, a former Medtronic executive, to its board of directors at Engaged Capital's urging. That move spurred media and investor speculation that Medtronic was the leading candidate to acquire HeartWare, sentiment that intensified after news in late April that St. Jude was being acquired by Abbott Laboratories.

Engaged Capital did not respond to a request for comment. JPMorgan Securities advised Medtronic on the deal. Perella Weinberg Partners represented HeartWare. Medtronic said it aims to close the deal by the end of October.