Amber Depp feared Johnny Depp would choke to death on his own vomit after a binge, a court heard.
The actress, 34, was giving her testimony in Depp's libel trial against NGN when she was played audio of an argument in which she admits she hit him.
But Heard said she had hit him to wake him up as she worried he would vomit in his sleep and choke to death, a common way for drug users to die.
Depp, 57, is suing NGN for an article that appeared in The Sun in 2018 which branded him a "wife beater" - a claim he strenuously denies.
He has accused Heard of being the abusive one in their marriage, which she has denied.
Speaking about hitting him, Heard explained: "He was falling asleep constantly or passing out after a binge. He vomited nearly nightly.
"As anyone knows who has loved an addict it's a common way for them to die.
"I would hear him choking, coughing and vomiting. I would turn his head to the side so he didn't choke.
"I was worried about Johnny's life."
Heard said Depp then got angry.
She said: "This is the context of this argument. He had gone into the bathroom. He was raging. He fell against the door on the floor.
"I heard some glass bottles break. He passes out. He falls. I tried to open the door.
"As soon as he reaches his arms out and pushed the body in it ran over him. I pushed him."
Lawyers for NGN are defending their case based on Heard's 14 allegations of abuse, which include claims of bust-ups between 2013 and 2016.