For the study, Starling's team collected data on 837 patients who received the device from 2004 through mid-2013.
They found 72 clots occurred in 66 patients. Beginning in March 2011, the number of patients experiencing clots three months after the device was implanted jumped from 2.2 percent to 8.4 percent by January of 2013.
Before March 2011, the average time clots appeared after implanting the device was 18.6 months. After that, clotting started appearing at 2.7 months after implantation, they noted.
Among patients who developed clots, 11 needed heart transplants, with one of these dying in the month after the transplant. Another 21 patients received new pumps.
Of those who did not receive a new pump or new heart, 48 percent died.
The device's maker defended its safety profile.
"The HeartMate II is the most widely used and studied device of its kind; it has been implanted in more than 16,000 patients worldwide and has demonstrated excellent clinical outcomes," according to a statement by Thoratec Corp.
"In fact, since the initial clinical trial, survival has improved, quality of life has remained consistently high, and complication rates in general have declined," the statement said.
There are a number of factors that could contribute to clotting, the company said, including underlying patient condition, ongoing patient and device management practices, pump implantation technique, and device-related factors.
"We have performed extensive analysis on HeartMate II and have not identified any change that would cause the increase observed in the INTERMACS registry," Thoratec said.
For more information on ventricular assist devices, visit the U.S. National Heart, Lung, and Blood Institute.
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