A respiratory expert testified on Thursday that George Floyd had died from lack of oxygen and police officer Derek Chauvin's knee was on his neck "more than 90 per cent of the time" that he was handcuffed, face down in the street.

Dr Martin Tobin, a pulmonologist, told the jury at Chauvin's murder and manslaughter trial that he had watched videos of Floyd's May 25, 2020 arrest "hundreds of times."

"Mr Floyd died from a low level of oxygen," Tobin told the nine-woman, five-man jury hearing the high-profile case in a heavily guarded Minneapolis courtroom.

"This caused damage to his brain," he said, and arrhythmia - an irregular heartbeat - which "caused his heart to stop."

The 45-year-old Chauvin was seen in a video taken by a bystander kneeling on the neck of a handcuffed Floyd for more than nine minutes as the 46-year-old African-American man complained repeatedly that he "can't breathe."

The video of Floyd's arrest touched off protests against racial injustice and police brutality in the United States and around the world.

Tobin said Floyd's breathing weakened because he was face down on the street, handcuffed and with Chauvin and other officers on his neck and back.

Eric Nelson, Chauvin's defence attorney, has suggested at several points during the trial that Chauvin's body weight was actually on Floyd's shoulder or back at times and not on his neck.

Tobin disagreed.

"Officer Chauvin's knee is virtually on the neck for the vast majority of the time," he said, "more than 90 per cent of the time."

Pre-existing conditions not a factor

The Irish-born Tobin is testifying as an expert witness for the prosecution.

The Chicago-based doctor said he has testified previously at medical malpractice trials but this is his first criminal trial and he is not being paid.

Chauvin, who has pleaded not guilty, faces up to 40 years in prison if convicted of the most serious charge - second-degree murder.

Chauvin, a 19-year-old veteran of the Minneapolis Police Department, was fired from the force after Floyd's death.

Prosecutors are seeking to prove that Floyd's death was due to asphyxiation, while Chauvin's defence claims it was due to illegal drugs in his system and underlying health conditions.

Tobin dismissed defence claims that any pre-existing medical conditions may have contributed to Floyd's death.

"A healthy person subjected to what Mr Floyd was subjected to would have died as a result of what he was subjected to," he said.

Forensic scientist Breahna Giles testified on Wednesday that pills containing methamphetamine and fentanyl were found in Floyd's car and in the police car, with some pills having saliva that matched Floyd's DNA.

Several high-ranking Minneapolis police officers have testified that Chauvin's use of force against Floyd was excessive.

Minneapolis police chief Medaria Arradondo said Monday that Chauvin violated the department's training policies and its "values."

Floyd was being arrested on suspicion of passing a counterfeit $20 bill in a nearby store.

A paramedic told the jury last week that Floyd was already dead when an ambulance arrived and that Chauvin was still kneeling on his neck.

Police officers are rarely convicted in the United States when charges are brought and a conviction on any of the counts against Chauvin will require the jury to return a unanimous verdict.

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