St. Jude Medical lost share price for a second day on Thursday after a device made by a company that it plans to buy was the subject of safety alert from the government.

Little Canada-based St. Jude has offered to buy California heart-pump maker Thoratec Corp. for $3.4 billion, which would be its largest deal ever. Analysts said Thoratec’s marquee blood-pumping machine would make a solid addition to St. Jude’s line of implantable medical devices for all stages of heart failure.

On Wednesday, just hours after Thoratec reported 9 percent revenue growth in its most recent quarter, the Food and Drug Administration administered a dose of sobering news — the company’s best-known device carries health risks greater than were initially thought.

Thoratec’s HeartMate II pumps blood for the sickest heart-failure patients by routing it around the heart using a tube with a continuously spinning propeller inside. Sales of the device comprised nearly 90 percent of Thoratec’s revenue in the quarter. But the FDA’s “safety communication” Wednesday cited recent evidence that the pump develops more dangerous blood clots than was initially known.

A 2014 study in the Journal of Heart and Lung Transplantation found 6 percent of nearly 7,000 pumps implanted between 2006 and 2013 developed blood clots, or “pump thrombosis,” after six months. That compares with rates of 1.6 percent at one year and 3.8 percent at two years documented in clinical trials before the FDA approved the device.

“Pump thrombosis remains an uncommon but potentially catastrophic complication,” the 2014 study authors wrote.

Wednesday’s FDA message also noted a higher-than-expected rate of strokes in patients who got a competitor device, made by Massachusetts’ HeartWare.

The agency cited clinical data from a conference in April that found patients with HeartWare’s device had more than double the rate of strokes as a control group using Thoratec pumps — 29 percent vs. 12 percent at two years. Meanwhile, the agency has received reports that both devices may be associated with uncontrolled bleeding, possibly related to changes in how the patients take blood-thinning drugs when they get the device.

“We believe the benefits of these [devices] continue to outweigh the risks,” the FDA letter says. “However, the FDA also believes it is important for health care providers and patients to be aware of this important information.”

St. Jude declined to comment Thursday. Thoratec and HeartWare did not respond to requests for comment.

Medicare pays about $150,000 for implantation of a left ventricular assist device, or LVAD, as the heart pumps are formally known.

Thoratec shares dropped about 3 percent after the alert was announced midmorning Wednesday, but by the end of Thursday the stock was back up to $62.91, for a loss of less than a percentage point since the FDA letter went out.

St. Jude stock dropped a small amount Wednesday, but by the end of Thursday it had dropped to $71.94, down 3 percent since the FDA announcement.


Twitter: @_JoeCarlson