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MPR News is streaming live coverage of the trial. Some images or material discussed during the trial will be disturbing to many viewers. Watch the morning court proceedings 

The prosecution in the trial of former officer Derek Chauvin brought three medical experts to the stand Thursday in an effort to undermine defense arguments that George Floyd was killed by drugs or underlying health problems rather than a lack of oxygen.

The day started with testimony from a pulmonologist who concluded that George Floyd died from a low level of oxygen, due to “shallow breathing.”

Floyd was unable to get enough air into his body because he was placed in a prone position pinned to the street with his hands cuffed behind his back and Chauvin’s knees on his neck and back, Dr. Martin Tobin told jurors. He said videos show the officers pushing his handcuffed hands higher, further restricting his lungs. 

"It’s like the left side is in a vise. It’s totally being pushed in, squeezed in from each side — from the street at the bottom, and then the way the handcuffs are manipulated,” he said. “It’s not just the handcuffs, it’s how the handcuffs are being held, how they’re being pushed, where they're being pushed that totally interfere with central features of how we breathe."

Tobin gave jurors a brief anatomy lesson and explained how pressure on the neck affects the ability to breathe. He urged jurors to find the areas of their necks that he was talking about during his analysis, and some followed his instructions. “The knee on the neck is extremely important because it’s going to occlude the air getting in through the passageway,” he said.

He told the court that “a healthy person subjected to what Mr. Floyd was subjected to would have died.”

Prosecutors are relying on Tobin's testimony to disprove the defense's claim that Floyd died because of drugs in his system and underlying health conditions.

Jurors were shown a still image from the bystander video showing Chauvin putting nearly all his weight on Floyd's neck. In a subsequent graphic, Tobin explained that he estimated more than 90 pounds of force was being applied. Tobin said a close-up photo shows Floyd trying to use his face to push back and get more air into his lungs. 

In his analysis, Tobin said that Chauvin continued to apply his weight on Floyd for at least three minutes after there was zero oxygen left in his body. Watching the bystander video, Tobin identified 8:24:53 p.m. as the moment Floyd died. "That’s the moment the life goes out of his body."

The pulmonologist also addressed the question of whether drugs played a role in Floyd's death. The defense has been building an argument that Floyd died due to underlying medical conditions or drugs in his system, and not due to Chauvin's knee on his neck or the other two officers also holding him down with their body weight. 

But Tobin testified that Floyd's respiratory rate was normal until he stopped breathing. Had Floyd been impacted by fentanyl, Tobin said, his respiratory rate would have been suppressed.

Tobin also said the idea that “if you can speak you can breathe,” which Chauvin told Floyd while he was on ground, is “dangerous” and misleading. He said it’s possible that someone can speak while their brain is receiving oxygen but then be dead 10 seconds later. 

Defense attorney Eric Nelson pushed Tobin on how he conducted his analysis of Floyd’s death, including weights and measurements he used to estimate when Floyd’s oxygen ran out completely. But Tobin insisted that neither fentanyl nor any other underlying health problems, including Floyd’s heart disease, could have led to his death in this manner. 

While most expert witnesses are paid for their testimony, Tobin told the court that he’d volunteered his time after being contacted by the state of Minnesota because he hadn’t ever testified in a criminal trial before. 

Following a day of testimony where some jurors appeared to struggle to maintain their attention, reporters in the courtroom said that jurors paid rapt attention to Tobin’s lengthy testimony, with many taking notes. 

Forensic toxicologist David Isenschmid testified that the levels of fentanyl and methamphetamines found in Floyd’s blood was much lower than in cases where people were driving under the influence. He also said a metabolized version of fentanyl was found in Floyd’s blood that is rare in overdose victims.

The prosecution also called Dr. Bill Smock, a forensic physician who works for the Louisville Police Department. He told the court that he had concluded that “Mr. Floyd died from positional asphyxia, which is a fancy way to say that he died because he had no oxygen in his body.”

Smock said Floyd showed no signs of a condition called “excited delirium,” a condition that’s controversial in medical circles, but can lead to unpredictable behavior and excessive strength. 

He also said that a fentanyl overdose would have made Floyd appear to fall asleep. But Floyd was exhibiting signs of asphyxia, said Smock, including what he termed “air hunger” as he pleaded for air.

“He’s saying, 'Please, please get off me. I want to breathe. I can’t breathe,'" Smock testified. “That is not a fentanyl overdose. That is somebody begging to breathe.”

Nelson questioned Smock about Floyd’s health problems. Under cross-examination, Smock admitted that wrestling with officers could put extra stress on a heart, but said later that Floyd didn’t show signs of a heart attack. 

Chauvin, who was fired from the Minneapolis police force, faces murder and manslaughter charges in Floyd’s May 25 killing while in police custody. Bystander video showed Chauvin with his knee pressed against Floyd’s neck for more than nine minutes as the man lay handcuffed and face down on the pavement, pleading that he couldn’t breathe.

Medical examiner who ruled Floyd's death a homicide testifies Friday

Hennepin County Medical Examiner Dr. Andrew Baker is expected to take the stand Friday. 

With the trial's outcome expected to hinge on who or what killed Floyd while he was in police custody, Baker's highly anticipated testimony will be crucial for the prosecution and defense.

Baker, who conducted the autopsy on Floyd, 

described Floyd’s death as a homicide in his report last year

, saying Floyd went into cardiopulmonary arrest as then-officer Chauvin kept his knee on the neck of the prone, handcuffed man.

Baker's report also identified “hypertensive heart disease,” “fentanyl intoxication” and “recent methamphetamine use” as other “significant conditions.”

Baker will be called to the stand Friday by the prosecution. During an off-mic comment made before the jury was seated Thursday morning, prosecutor Jerry Blackwell told Judge Peter Cahill the state planned to call Baker on Friday.

BCA agent: Chauvin kept pressure on 4 minutes after Floyd stopped moving  

Much of Wednesday’s testimony dug deep into sometimes arcane detail around the maneuvers Chauvin and other officers used to subdue Floyd, the nature of officer training in tense situations and the composition of stains and pills found during the investigation into Floyd’s killing.

A BCA expert testified that several bloodstains in the squad belonged to Floyd and that a pill with Floyd’s DNA was also found there; testing revealed it contained fentanyl and methamphetamine.

Minnesota Bureau of Criminal Apprehension agent James Reyerson was asked by prosecutor Matthew Frank about when Floyd stopped moving. 

“Does it appear that Mr. Chauvin is using his legs to hold Mr. Floyd down?" Frank asked Reyerson, who replied, “Yes, it does.”

While Chauvin restrained Floyd on the ground for about 9 1/2 minutes, Reyerson testified in court that Floyd was not responsive for more than four minutes before paramedics arrived. 

Reyerson outlined the extent of the criminal investigation during his testimony, which he said involved interviews with about 200 citizen witnesses, as well as  assistance from about two dozen FBI agents.

Wednesday began with a prosecution use-of-force expert concluding that Chauvin's restraint of Floyd constituted an unwarranted use of deadly force. 

Jody Stiger noted that Floyd's subdued position — handcuffed, prone and pinned face down on the street — created the danger of positional asphyxia even without the pressure of Chauvin's knee on Floyd's neck. Adding the body weight increased the likelihood of death, he added. 

Stiger also said his analysis of video and images of Chauvin's restraint on Floyd showed Chauvin grasping the fingers of Floyd's left hand in what appeared to be a "pain compliance" maneuver. 

"Pain compliance is a technique that officers use to get a subject to comply with their commands. As they comply, they are rewarded with a reduction in their pain," Stiger explained.

What if there's no opportunity for compliance, the prosecution asked. Stiger responded: "Then at that point, it's just pain."

During his testimony, the prosecution dissected a few still frames from videos that captured the incident in an attempt to counter the defense's argument that Chauvin's knee was on Floyd's shoulder blade instead of his neck. The defense brought that up Tuesday. 

But in the photos presented, Stiger said Chauvin appears to be using his body weight and knee to push down on Floyd's neck.

Under cross-examination by defense attorney Eric Nelson, Stiger agreed that objective reasonable use of force standards can differ by police departments, although all follow state and federal law. But Nelson couldn't get Stiger to agree on every point specific to this case, including whether a handcuffed person can still pose a threat.

"And sometimes the use of force is instantaneous?" asked Nelson.

"Sometimes, but not in this case," Stiger replied.

The defense has suggested during the trial that the tension between the officers and the bystanders kept the officers from attending to Floyd's distress.

Stiger said Chauvin's experience —  he was a 19-year veteran of the force with hundreds of hours of training — should have been sufficient to prepare for this distraction.

The killing of George Floyd and trial of Derek Chauvin

George Floyd's death, protests and Derek Chauvin's trial

How the trial is unfolding

Watch recordings of previous days in court

Listen to the podcast

 The 12 jurors and two alternates picked to review the case include a chemist, a youth volunteer, a cardiac nurse and an IT professional.

Chauvin's lawyer is outnumbered, but has help:

 No fewer than four attorneys have appeared for the prosecution so far, compared to a single attorney to defend Derek Chauvin. 

Legion of Chauvin prosecutors, each with own role:

 Viewers may be struck by the array of prosecutors taking turns presenting their case. The choice of who does what is no accident.

Nancy Lebens, the newsroom’s deputy managing editor, answered audience questions about our reporting plans.

 Before he became a symbol in the fight for racial justice, friends say George Floyd was a “gentle giant” who sought a fresh start.

 Here’s how the site of George Floyd’s killing — 38th Street and Chicago Avenue in Minneapolis — is being reshaped.

Rescuing the plywood — and memorializing a movement:

 Two Black women are leading the effort to preserve the murals painted on storefront boards in the Twin Cities.

 Here’s what some activists tell MPR News about their experiences with race in Minnesota, why they march and what they hope for the future.

Could mask hamper Chauvin's image with jurors?

 The face mask that Derek Chauvin has been required to wear during his trial has hidden his reaction to testimony. That includes any sympathy or remorse that legal experts say can make a difference to jurors. 

Questioning blurs meaning of 'lawful but awful':

 The phrase typically refers to police shootings when the officer is found to have reasonably feared for their life and fired. Legal observers say Derek Chauvin's defense will have a hard time making that case. 

Trial breaks 'blue wall of silence,' but will it transform policing?

 "My hope is that it leads to results in terms of transforming the way that policing happened in Minneapolis," said civil rights attorney and activist Nekima Levy Armstrong. 

 Derek Chauvin's attorney has raised the concept of excited delirium as testimony examines whether reasonable force was used on George Floyd. 

Is defense blaming acquaintance in Floyd's death?

 The defense clearly thinks Morries Hall can help them raise doubt in the minds of jurors about how George Floyd died. 

How long did it take medics to reach Floyd?

 Last week, the defense questioned paramedics in their efforts to resuscitate George Floyd, and seemed to suggest a drawn-out response, potentially setting up an argument that if medical help had arrived sooner, Floyd could have been saved. 

Racism is making us sick. How can equity in medicine help us heal?

 Two doctors and a medical researcher talk about how racism affects their patients’ health — and how racism in medicine leads to inadequate medical education and poor care. 

The latest from the murder trial of Derek Chauvin. 

Questions about the Chauvin trial? Ask us

What questions do you have about the Chauvin trial? 

Get the latest updates and share your questions on Chauvin trial

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Dr. Martin Tobin, a pulmonologist, told the court Thursday that George Floyd died from a low level of oxygen, due to “shallow breathing.” He also testified that a healthy person experiencing the same restraint Floyd did “would have died.”

Chauvin trial: Restraint acted like a 'vise' on Floyd's breathing, expert says

Screenshot of Court TV video

Community activists hold pictures of two men. 

Chauvin trial: Breathing expert pinpoints moment when 'the life goes out of his body’

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