A total of 745,000 pacemakers have been confirmed as having cyber-security issues that could let them be hacked.
The Food and Drug Administration revealed that 465,000 pacemakers in the US were affected, in an advisory note about a fix to the problem.
The pacemaker's manufacturer, Abbott, told the BBC there were a further 280,000 devices elsewhere.
The flaws could theoretically be used to cause the devices to pace too quickly or run down their batteries.
However, Abbott said it was not aware of any cases of this happening, adding that it would require a "highly complex set of circumstances".
The Department of Homeland Security has said that an attacker would need "high skill" to exploit the vulnerabilities.
The affected pacemakers are branded as having been made by St Jude Medical, which was acquired by Abbott earlier this year.
Patients are being advised to ask their doctors about an available firmware update at their next scheduled appointment.
The pacemakers can receive the revised code by being placed close to a radio wave-emitting wand in a process that lasts about three minutes.
Pacemakers manufactured after 28 August will come with the new firmware pre-installed.
"As with any firmware update, there is a very low risk of an update malfunction," the FDA said.
The regulator noted a very small number of St Jude devices had lost all functionality after a firmware update in the past.
Abbott said some patients might opt to continue with the old firmware as a consequence.
"In some cases, doctors and patients will decide that the risks that could be associated with performing the new pacemaker firmware update for some patients may outweigh the benefits," it said in a note to pacemaker users.
"If you do not receive the update, your pacemaker will continue to function as intended, and you can receive the update at any future time."
The benefit of allowing the pacemakers to send and receive data wirelessly is that patients can pair them with a transmitter at home that monitors the devices as they sleep and can potentially alert them to medical problems.
A hedge fund, Muddy Waters Research, first warned the media in August 2016 that the cardiac equipment had security flaws and claimed they could be exploited by "low-level hackers".
The investment company also revealed it had bet St Jude's shares would drop after it had been told of the issues by security company MedSec.
"[St Jude's] apparent lack of device security is egregious, and in our view, likely a product of years of neglect," Muddy Waters said at the time.
St Jude responded by saying it stood behind the security and safety of its equipment and sued its accuser for defamation.
However, shortly after Abbott bought St Jude in January, the FDA confirmed there were vulnerabilities in the company's wireless home monitor system, which were subsequently addressed.
Then, in April, the watchdog said Abbott had failed to properly investigate wider cyber-security concerns.
Even so, the medical company's legal action against Muddy Waters continues.