A triple-killer is set to be the first man executed with a working defibrillator in his chest when he's put to death in Tennessee today.

Byron Black's bid to delay the execution was denied on Monday, and he will now be killed by lethal injection.

There is uncertainty over whether the device will shock his heart, potentially prolonging his death and causing suffering when the deadly cocktail takes effect.

The 69-year-old is in a wheelchair and is said to have dementia, kidney failure, brain damage and congestive heart failure.

His cardioverter-defibrillator is a battery-powered device that delivers electric shocks to restore a regular heartbeat if needed.

Black's lawyers said a doctor should put a device over the implant to ensure it's switched off, and in July, a judge agreed it was n

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