However, analysts said the review by Food and Drug Administration staff raised few new issues and bet the device would be approved, sending HeartWare shares up 4.3 percent to $70.01 in morning trading on Nasdaq.
FDA staff also said clinical trial results for the mechanical heart-assist device, or the HeartWare ventricular assist device (HVAD), may have missing data and are difficult to compare with a registry of patients.
In clinical trials, HeartWare compared patients who got its device to a registry, or database, of patients with mechanically assisted circulatory support. It was the first blood pump trial to use a registry as a control arm.
FDA staff said the registry may have had more people who were critically ill, skewing results in favor of HeartWare's pump.
The staff also said 16 of the 250 people implanted with HeartWare's device had blood clot-related pump failure, and 15 of them needed to replace their pumps. HeartWare said people who needed to a new device should not be considered as failures.
(But) FDA continues to believe that the need for device exchange is a relevant consideration when assessing the device's safety and effectiveness and seeks panel input on this issue, the FDA staff said in documents posted online on Monday.
The FDA staff review comes ahead of a meeting of outside experts who were due to vote on the device on Wednesday. The FDA will make a final decision later.
Heart failure is a chronic and incurable condition, 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 heart 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.
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 who are waiting for a transplant.
Larry Biegelsen, analyst at Wells Fargo, said the FDA review did not contain any major surprises.
We believe the overall success of the pivotal trial and FDA's desire to have an alternative pump on the market to Thoratec's HeartMate II will result in approval of the HVAD later this year, he said in a research note.
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.
Biegelsen said the higher stroke rate with HeartWare's device, especially compared to the Thoratec device, will be the greatest challenge during the panel on Wednesday.
The FDA said 8 percent of patients who got HeartWare's blood pump during clinical trials had an ischemic stroke, compared to a rate of 5 to 6 percent in trials of other devices. In an ischemic stroke, a blood vessel that supplies blood to the brain is blocked by a clot.
HeartWare is also testing its device as a long-term treatment for patients who are not eligible for a heart transplant or may not be able to get one.
(Editing by Gerald E. McCormick, Maureen Bavdek, Dave Zimmerman)