In this image from video, Dr. David Fowler testifies as Hennepin County Judge Peter Cahill presides, Wednesday, April 14, 2021, in the trial of former Minneapolis police Officer Derek Chauvin at the Hennepin County Courthouse in Minneapolis, Minn. Chauvin is charged in the May 25, 2020 death of George Floyd. (Court TV via AP, Pool)

MINNEAPOLIS (AP) — George Floyd died of a sudden heart rhythm problem due to his heart disease while being restrained by police, a retired forensic pathologist testified for the defense Wednesday at former Officer Derek Chauvin’s murder trial, contradicting experts who said Floyd succumbed to a lack of oxygen from the way he was pinned down.

Dr. David Fowler, a former chief medical examiner for the state of Maryland and now a member of a consulting firm, said the fentanyl and methamphetamine in Floyd’s system, and possible carbon monoxide poisoning from auto exhaust, were contributing factors.

“All of those combined to cause Mr. Floyd’s death,” he said on the second day of the defense case.

He also testified that he would classify the manner of death “undetermined,” rather than homicide as the county’s chief medical examiner ruled. Fowler said the death had too many conflicting factors, some of which could be ruled homicide and some that could be considered accidental.

Chauvin attorney Eric Nelson is trying to prove that the 19-year Minneapolis police veteran did what he was trained to do and that Floyd died because of his illegal drug use and underlying health problems.

Prosecutors say Floyd died because Chauvin’s knee was pressed against Floyd’s neck or neck area for 9 1/2 minutes as the 46-year-old Black man lay pinned to the pavement on his stomach last May, his hands cuffed behind his back.

Fowler listed a multitude of factors: Floyd’s narrowed arteries, his enlarged heart, his high blood pressure, his drug use, the stress of his restraint, the vehicle exhaust, and a tumor or growth in his lower abdomen that can sometimes play a role in high blood pressure by releasing “fight-or-flight” hormones.

Fowler said all of those factors could have acted together to cause Floyd’s heart to work harder and suddenly stop.

Previous witnesses have noted that a sudden heart rhythm problem does not necessarily produce visible signs on autopsy but can be inferred from circumstances such as a victim suddenly clutching one’s chest and collapsing.

Nelson questioned Fowler extensively about carbon monoxide, which displaces oxygen in the bloodstream of people who breathe it in. Fowler said it could have contributed to oxygen depletion in Floyd, noting that he was facing the tailpipe end of a vehicle. But there is no way to know for sure because, he acknowledged, Floyd’s blood was never tested for carbon monoxide.

Nelson similarly tried to introduce another possible explanation on Tuesday when he raised questions about excited delirium, or what a witness described as a potentially lethal condition that can include agitation, incoherent speech and extraordinary strength.

Several top Minneapolis police officials, including the police chief, have testified that Chauvin used excessive force and violated his training. And a number of medical experts called by prosecutors have said Floyd died from a lack of oxygen because the way he was restrained restricted his breathing.

Fowler said the prone position alone does not affect a person’s ability to breathe — testimony that contradicts other witnesses who said the position Floyd was in was inherently dangerous.

He also testified that Chauvin’s knee was not applied with enough pressure to cause any bruises or scrapes on Floyd’s neck or back. He further said that Chauvin’s knee on Floyd was “nowhere close to his airway” and that Floyd’s speaking and groaning showed that his airway was still open.

And he said that Floyd did not complain of visual changes or other symptoms consistent with hypoxia, or insufficient oxygen to the brain, and that he was coherent until shortly before he suddenly stopped moving.

“The bottom line is, moving air in and out, and speaking and making noise is very good evidence that the airway was not closed,” Fowler said.

Fowler handled a case similar to Floyd’s in Maryland in 2018, when a 19-year-old Black man, Anton Black, died after three officers and a civilian pinned him for more than five minutes as they handcuffed him and shackled his legs.

The family brought a federal lawsuit that included Fowler, whose autopsy concluded that the stress of the struggle probably contributed to Black’s death but found no evidence that restraint directly caused it. It also found no evidence of asphyxia, or a lack of oxygen.

Chauvin, a 45-year-old white man, is on trial on charges of murder and manslaughter in Floyd’s death after his arrest on suspicion of passing a counterfeit $20 at a neighborhood market. The video of him as he gasped that he couldn’t breathe touched off worldwide protests, violence and a furious examination of racism and policing in the U.S.

The defense hasn’t said whether Chauvin will take the stand.

Testifying could open him up to devastating cross-examination, with prosecutors replaying the video and forcing Chauvin, one freeze-frame moment at a time, to explain why he kept pressing down on Floyd.

But taking the stand could also give the jury the opportunity to see and hear any remorse or sympathy Chauvin might feel. He would be able to take off the COVID-19 mask that he has to wear while seated at the defense table.

The only time Chauvin has been heard publicly defending himself was when the jury listened to body-camera footage from the scene. 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.”

Earlier Wednesday, Judge Peter Cahill turned down a defense request to acquit Chauvin, rejecting claims that prosecutors failed to prove Chauvin’s actions killed Floyd. Requests for an acquittal are routinely made midway through a trial and are usually denied.

The defense began its case on Tuesday with Nelson going straight to the question at the center of the trial — whether Chauvin’s actions were reasonable.

Police officers are allowed certain latitude to use force. Legal experts say a key issue for the jury will be whether the officer’s actions were reasonable in those specific circumstances.

A use-of-force expert for the defense, Barry Brodd, a former Santa Rosa, California, police officer, testified Tuesday that Chauvin was justified in keeping Floyd pinned to the pavement, saying Floyd kept on struggling.