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St. Jude Medical Inc. is warning patients that the batteries on thousands of its implantable defibrillators can quickly and unexpectedly deplete, in rare cases leaving them vulnerable should they need a shock from the device to restart their faltering hearts.

The premature battery depletion has been seen in fewer than 1 percent of the devices, the company said in letters sent to doctors and patients on Tuesday. Two people died after their devices failed to work correctly because of the drained battery, while 47 others reported dizziness or fainting. The warning applies to devices made before May 2015, which are in use in about 350,000 people worldwide. St. Jude’s shares declined on the news.

The warning comes at a sensitive time for the St. Paul, Minnesota-based medical device manufacturer, which is in the midst of being purchased by Abbott Laboratories. In August, short-seller Carson Block and his investment firm Muddy Waters Capital LLC issued a report saying St. Jude’s heart devices are vulnerable to cybersecurity attacks, including a depletion of the battery. The issue isn’t related to the device’s wireless communications, which are essential for doctors to closely monitor their patients, according to St. Jude. The problem stems from the lithium-powered batteries that run the devices.