St. Jude Medical Inc. has alerted patients and doctors that some of its implanted heart devices have battery defects that may cause premature power loss. The problem comes from lithium deposits that form after the pacemaker sends electricity to the heart, eventually making the device inoperable. The implants are intended to treat irregular heart rhythms that may cause heart attacks and failures. Two deaths have been reported in relation to the issue. In response, the St. Paul, Minn. company that manufactures its cardiac devices in Sylmar said batteries should be replaced immediately after patients receive an alert from the device. The defect affects a subset of its Fortify, Quadra and Unify devices. Specific model and serial numbers are available on the company’s website. Other major manufacturers, including Medtronic Inc. and Boston Scientific Corp., have also recently reported failures and recalls in their implantable heart devices.