A panel of outside experts to the Food and Drug Administration voted 9-2 to recommend approval of the mechanical heart-assist device, called the HeartWare ventricular assist device (HVAD). HeartWare shares were halted pending the panel outcome.

The FDA usually follows panel advice, although it is not required to and a final decision is expected later.

Panelists said the device helped save lives, although they expressed doubts about the company's conduct of clinical trials and about missing safety data.

"Overall, this is a novel device for a desperate population, and I hope it will be available soon," said panel member Dr. David Slotwiner, of the Long Island Jewish Medical Center.

Heart failure is a chronic condition with no cure, in which the heart is too weak to pump enough blood to meet the body's needs. In the most severe cases, people may need a heart transplant.

HeartWare's device helps people with failing hearts pump blood while awaiting a transplant. The device is already approved in Europe.

It is estimated that some 50,000 people around the world are candidates for heart transplant, but only some 5,000 patients per year get them.

But reviewers from the FDA said earlier this week they were concerned about device-related clotting rates and stroke. FDA reviewers and outside experts to the agency also said clinical trial results may have missing data, meaning they could over-estimate how well the device works.

"In terms of safety, these data have more holes in them than Swiss cheese," said panel member Dr. Gregory Dehmer, of Texas A&M University.

Panel members called for HeartWare to conduct further safety trials if the FDA approves its device.

If approved, the device will compete with Thoratec Corp's HeartMate II, which has been sold in the United States since 2008. It is currently the only blood pump available to people with heart failure awaiting a transplant.

Thoratec shares fell 1.5 percent in after-market trading.

Many investors have been betting HeartWare's device will eventually eclipse the HeartMate II since it is smaller and takes less time to implant. However, concerns about pump-associated thrombosis, or the formation of blood clots, as well as stroke issues, could hurt market share.

HeartWare said it changed the instructions for doctors using the device to reduce the risk of clots and there was no evidence strokes in clinical trials were linked to its device.

"Each device is going to have its relative strengths and our patients deserve these options," said Dr. Keith Aaronson, a paid consultant for HeartWare and professor of internal medicine at the University of Michigan. "And as a cardiologist treating patients with advanced heart failure, I feel strongly we need to have competition in this field."

(Editing by Gary Hill, Bernard Orr and Andre Grenon)

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