A Tennessee man cried out in pain as he was executed on Tuesday for the 1988 shooting deaths of his girlfriend and her two young daughters.

Byron Black, 69, was killed by lethal injection for the deaths of Angela Clay, 29, and her daughters, Latoya, nine, and Lakeisha, six, despite uncertainty over whether his implanted defibrillator would shock his heart when the chemicals took effect.

He spent the morning participating in a church service inside his cell, and had his final meal at 4.45 am, comprising pizza with mushrooms and sausage, followed by doughnuts and butter pecan ice cream.

As the pentobarbital flowed through his veins on Tuesday morning, he appeared to be in visible distress as he was strapped to a gurney and covered in a sheet with IV lines running to his body at the Riverbend Maximum Security Prison in Nashville, according to the Tennessean.

Just two minutes after the curtain to the execution room swung open, Black could be heard sighing heavily and breathing heavily.