Chauvin is on trial on charges of second-degree unintentional murder, third-degree murder and second-degree manslaughter in connection with the May 25, 2020, death of Floyd, a 46-year-old Black man.
The prosecution indicated in court that the witness they will call to rebut Fowler's testimony is Dr. Martin Tobin, a pulmonologist that testified at trial that Floyd died of "low oxygen levels".
The question of whether Mr Chauvin would testify was the subject of weeks of speculation, with legal analysts split over whether his testimony would help or hinder his case. After an ambulance had taken Floyd away, Chauvin told a bystander: "We gotta control this guy 'cause he's a sizable guy. and it looks like he's probably on something".
Dr Fowler said that the fentanyl and methamphetamine in Floyd's system, and possible carbon monoxide poisoning from a vehicle exhaust, were contributing factors.
A 19-year veteran of the Minneapolis Police Department, Chauvin was sacked from the force after Floyd's death.
Fowler, the former chief medical examiner of the state of Maryland, also introduced a new element in the defense case.
Mr Floyd was restrained by a total of three officers.
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The risks were high: Testifying could have opened him up to devastating cross-examination, with prosecutors replaying the video of the arrest and forcing Chauvin to explain, one frame at a time, why he kept pressing down on Floyd.
They also called a forensic pathologist, former Maryland chief medical examiner Dr David Fowler, who said Floyd, whose death was ruled a homicide at the hands of the police, really died of heart disease, and that the exhaust fumes of the adjacent police auto may have also poisoned him. He said Floyd's death had too many conflicting factors, some of which could be ruled homicide and some that could be considered accidental.
She said she tried to wake him a "couple of times" and he eventually woke up when police arrived in response to a report that Floyd had used a counterfeit $20 bill and pulled him out of his auto.
Chauvin's defence claims Floyd's death was due to underlying health conditions and consumption of fentanyl and methamphetamine. Floyd's death spurred global protests against police brutality.
He said Floyd was held faceddown on the ground by Chauvin and other officers next to the exhaust pipe of a running police vehicle.
Prosecutors say Mr Floyd died because the officer's knee was pressed against Mr Floyd's neck or close to it for 9 1/2 minutes as he lay on the pavement on his stomach, his hands cuffed behind him and his face jammed against the ground.
Mr Chauvin kneeled on Mr Floyd's neck even after the suspect drew his last breath.