WASHINGTON—U.S. health regulators warned surgeons Thursday to stop implanting a heart pump made by Medtronic due to power failures recently tied to cases of stroke and more than a dozen deaths. Medtronic said earlier in the day it would stop selling its Heartware assist device, which is intended to boost blood circulation in patients with heart failure. Recent company data showed the device can unexpectedly stop working and sometimes fail to restart, the Food and Drug Administration (FDA) said. The company said it hasn’t been able to pinpoint the root cause in every case. Surgeons can use implants from other manufacturers, the FDA said in a memo to physicians. About 4,000 people worldwide currently have the Medtronic device, roughly half of them in the United States. For those patients, the agency said U.S. doctors should contact the company for the latest guidance on managing the device, which includes managing blood …