Hennepin County's chief medical examiner testified in Derek Chauvin's murder trial Friday that George Floyd's underlying heart disease contributed to his death at the hands of Minneapolis police officers, and that being held down on the street outside Cup Foods was "just more than Mr. Floyd could take."

Dr. Andrew Baker's testimony came after three other medical experts had already testified in Chauvin's Hennepin County District Court trial on second-degree murder, third-degree murder and manslaughter charges. Cause of death is a pivotal point for prosecutors and the defense.

Baker's findings, based on his autopsy the day after Floyd died, determined he died from "cardiopulmonary arrest complicating law enforcement subdual, restraint, and neck compression." He did not cite a lack of oxygen, or asphyxia, a cause found by three prior prosecution experts, including Dr. Lindsey Thomas, a medical examiner who worked on Baker's staff from 2013 to 2017.

"In this case, I believe the primary mechanism of death is asphyxia, or low oxygen," said Thomas, a medical examiner of 37 years who retired from Hennepin County.

Questioned further by prosecutor Jerry Blackwell, she said, "There's no evidence to suggest he would have died that night except for the interactions with law enforcement."

The prosecution is trying to show that Floyd died from a lack of oxygen caused by police holding him prone and handcuffed on the pavement. Defense lawyer Eric Nelson is trying to sow reasonable doubt, raising the prospect that Floyd died by heart attack brought on by illicit drug use and health problems.

Baker, who has been the county's chief medical examiner since 2004, said the 46-year-old Floyd had "very severe" hypertensive heart disease, meaning his heart weighed more than it should, requiring more oxygen.

"Now in the context of an altercation with other people that involves things like physical restraint, it involves things like being held to the ground, it involves things like the pain that you would incur from having your cheek up against the asphalt and that abrasion on your shoulder, those events are gonna cause stress hormones to pour out of your body, specifically things like adrenaline, and what that adrenaline is going to do is it's going to ask your heart to beat faster," Baker testified. "It's going to ask your body for more oxygen so that you can get through that altercation, 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."

While earlier medical testimony doubted that Floyd's physical condition or his drug use played a role in the death, Baker said under questioning by defense attorney Nelson that "in my opinion, yes," they were factors.

Asked by Nelson whether Floyd died of asphyxiation, Baker said: "That's something we don't really see, pressure to the back of the neck resulting in strangulation."

Nelson also asked again about the amount of fentanyl found in Floyd's body during the autopsy.

The lawyer had Baker disclose what he told prosecutors earlier about whether Floyd overdosed, and Baker replied, "I don't recall specifically what I told the county attorney, but it almost certainly went something like this: Had Mr. Floyd been home alone in his locked residence with no evidence of trauma, and the only autopsy finding was that fentanyl level, then yes, I would certify his death as due to fentanyl toxicity."

Baker began his testimony by saying he was aware of little more than Floyd had died in police custody at the hospital, but he avoided videos of Floyd's arrest and death until after the autopsy. "I don't want to go into autopsies with a preconceived notion about what already happened. It might cause me to skip something," he said.

He then was shown photos he took from the autopsy.

The doctor said one image showed bruises and scrapes to Floyd's left eyebrow area. He said Floyd suffered those while "pinned against the asphalt."

Prosecutor Blackwell asked, "In the prone position?" Baker responded, "Yes."

Blackwell earlier questioned Thomas, Baker's former assistant.

She said that although Floyd's heart stopped, he didn't die from a heart attack. She also said Floyd died from subdual restraint and neck compression.

Thomas said she agreed with Baker's determination on the cause of death. However, Thomas' declaration runs counter to Baker's assessment in a meeting with the FBI that he could not say whether Floyd would have lived "but for" the officers' use of force, according to documents obtained in the case.

"Baker did not believe that the prone position was any more dangerous than other positions based on an article or journal he had read," said a summary of the meeting.

He also listed hardening and thickening of the artery walls, heart disease and illicit drug use as "other significant conditions." Fentanyl and methamphetamine were found in Floyd's system, Baker's autopsy results noted.

Thomas, however, testified that the physical stress of being prone must be taken into account.

"Mr. Floyd was already in a position where he was experiencing difficulty breathing and getting enough oxygen in this body," she said. "On top of that, now there's the physiological stress that's putting increased demand on his heart, lungs and muscles. … He's using his strength to get himself into a position where he can breathe."

In cross-examination by Nelson, he said to Thomas, "Let's assume you found Mr. Floyd dead in his residence, no police involvement, no drugs, the only thing you found were these facts about his heart. What would you conclude is the cause of death?"

She responded, "In those very narrow circumstances, I would probably conclude that the cause of death was his heart disease."

Thomas also revisited ground covered by previous medical experts concerning injuries on Floyd's body, saying they indicated his struggle to open his chest to breathe: scrapes on his knuckles, face and shoulders. The judge then had photos of the injuries distributed to the jurors, and Thomas explained what they depicted and why the images "are consistent that he is pushing himself so he can get in a position to breathe."

In the end, Baker stood firmly behind his initial ruling. "My opinion remains unchanged," Baker said as his testimony concluded. "That was my top line then, [and] it would remain my top line now."

Prosecution testimony continues Monday.

Staff writer Libor Jany contributed to this report.