George Floyd had heart disease and had consumed drugs but they were not the "direct causes" of his death, the doctor who conducted the autopsy said today at the trial of former police officer Derek Chauvin.

Andrew Baker, the chief medical examiner for Minnesota's Hennepin County, told the jury that Mr Floyd's cause of death was "restraint and neck compression" while being subdued by police.

Derek Chauvin is facing murder and manslaughter charges for his role in George Floyd's death on 25 May, 2020, which occurred during his arrest for allegedly passing a fake $20 bill.

The 45-year-old police officer, who is white, was seen in a video taken by a bystander kneeling on Mr Floyd's neck for more than nine minutes as the handcuffed 46-year-old black man complained repeatedly that he "can't breathe".

The video touched off protests against racial injustice and police brutality in the United States and around the world.

The cause of Mr Floyd's death is a central element in Mr Chauvin's high-profile trial being held in a heavily guarded Minneapolis courtroom.

Prosecutors are seeking to prove that his death was due to asphyxiation and have called several medical experts in recent days to bolster their case.

Mr Chauvin's defence claims Mr Floyd's death was due to his consumption of the illegal drugs fentanyl and methamphetamine and underlying health conditions.

Mr Baker, who conducted the autopsy on the deceased and signed his death certificate, was questioned about his findings by prosecutor Jerry Blackwell and Mr Chauvin's attorney, Eric Nelson.

The chief medical examiner said he had intentionally chosen not to watch any videos of George Floyd's death before conducting the autopsy so as not to "bias" the examination.

On the death certificate, Mr Baker cited "cardiopulmonary arrest complicating law enforcement subdual, restraint, and neck compression" as the cause of death.

"Mr Floyd's use of fentanyl did not cause the subdual or neck restraint," Mr Baker told the court. "His heart disease did not cause the subdual or the neck restraint.

"They are not direct causes, they are contributing causes," he said.

Mr Baker said Mr Floyd had a slightly enlarged heart and a narrowing of his coronary arteries. He noted that Mr Floyd, before being handcuffed and placed face down on the ground, had scuffled with the officers arresting him.

"Those events are going to cause stress hormones to pour out into your body, specifically things like adrenaline," he said.

"And what that adrenaline is going to do is it's going to ask your heart to beat faster.

"It's going to ask your body for more oxygen so that you can get through that altercation," he said. "And in my opinion, the law enforcement subdual, restraint and the neck compression was just more than Mr Floyd could take by virtue of those heart conditions."

Also testifying for the prosecution today was Lindsey Thomas, a veteran forensic pathologist.

She said she believed the "primary mechanism" of George Floyd's death was "asphyxia or low oxygen" caused by compression of his chest by the policemen on his neck and back. 

"This is not a sudden cardiac death," she said.

Derek Chauvin faces up to 40 years in prison if convicted of the most serious charge - second-degree murder.

A 19-year veteran of the Minneapolis Police Department, he was fired from the force after Mr Floyd's death.

Three other former police officers involved in the arrest are to be tried separately later this year.