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Medical experts in Derek Chauvin trial testify that George Floyd died from lack of oxygen, restraint

A renowned expert on breathing and a doctor who trains police officers both testified that Floyd died from lack of oxygen, brought on by the officers' restraint.

MINNEAPOLIS — Editor's note: Some of the images depicted in the video and testimony are graphic. Have a question you'd like to hear our trial experts answer? Send it to lraguse@kare11.com or text it to 763-797-7215.

  • Renowned breathing expert testified that George Floyd died from 'low level of oxygen' caused by 'shallow breaths.'
  • Dr. Martin Tobin said that was caused by three things: prone position, handcuffs and Derek Chauvin's knee
  • Forensic toxicologist who tested Floyd's blood testified about fentanyl, meth levels
  • Medical expert: 'Mr. Floyd died from positional asphyxia, which is a fancy way of saying he died because he had no oxygen left in his body.'
  • Prosecutor indicated Hennepin County Medical Examiner Dr. Andrew Baker will testify Friday

A world-renowned expert on breathing and lungs delivered important testimony for the prosecution today, explaining to the jury for hours how he believed George Floyd's death was caused by a lack of oxygen due to his restraint by Derek Chauvin and two other officers.

Chauvin is charged with second-degree murder, second-degree manslaughter and third-degree murder in Floyd's death.

"Mr. Floyd died from a low level of oxygen," pulmonologist Dr. Martin Tobin told the jury. "The cause of the low level of oxygen was shallow breathing. Small breaths."

Dr. Tobin showed the jury in detail, using graphic recreations of the scene and mathematical equations, how the prone restraint, the handcuffs and Derek Chauvin's knee all worked together to deprive Floyd of oxygen.

Tobin answered crucial questions for the state to combat the defense's strategy. He said the pressure from Chauvin's knee would have caused oxygen deprivation whether it was on the neck, back or arm. He testified the phrase "If you can speak, you can breathe" is a "dangerous mantra." He also told the jury that a healthy person without Floyd's medical conditions "would have died as a result of what he was subjected to."

Tobin told the jury that he believes he can identify the exact moment Floyd died. Prosecutors showed the video clip as Tobin described what he saw.

"You can see his eyes, he's conscious, and then you see that he isn't," Tobin said. "That's the moment the life goes out of his body."

Two other important witnesses took the stand Thursday:

  • The forensic toxicologist who tested Floyd's blood after his death. He said there was fentanyl present and a "very low" level of methamphetamine.
  • A medical expert on asphyxial death and drug overdoses. He said after watching the videos of Floyd's restraint, "That is not a fentanyl overdose, that is somebody begging to breathe."

Chauvin's attorney Eric Nelson asked several questions to each witness about the possibility that underlying health conditions or drugs caused Floyd's death, a key part of his defense strategy.

The state will call Hennepin County Medical Examiner Dr. Andrew Baker, who performed George Floyd's autopsy, to the stand Friday.

RELATED: Pulmonologist: George Floyd died from 'low level of oxygen'

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Thursday, April 8

4:45 p.m.

Defense attorney Eric Nelson cross-examined the state's expert medical witness Dr. William Smock toward the end of the day Thursday, first asking him to confirm that he is not a pathologist.

Nelson asked Smock to confirm that an overdose death from methamphetamine and fentanyl would be different from a death from only fentanyl. The doctor said it would depend on the amounts.

Smock acknowledged that he did not see any physical evidence of Floyd's airway being obstructed in the autopsy. He also confirmed that Floyd said "I can't breathe" several times before he was in the prone position.

Nelson pointed out, as he did with a previous expert, that people with COVID-19 are sometimes placed in the prone position, which "assists them with their oxygenation."

Dr. Smock said he trains police officers in how to avoid causing positional asphyxia. He acknowledged that he has observed officers put a knee at the base of a person's neck, but "for short periods of time."

Nelson pointed out that Floyd had cardiovascular disease and that records show one of his coronary arteries was 90% blocked. Smock said someone may be in danger of a heart attack if it is 100% blocked.

Smock testified that for every second the brain goes without oxygen, millions of neurons and cells die.

Nelson asked if it's true that when a person stops using a controlled substance for a time, tolerance "dissipates." Smock said that's correct, but said he does not know how quickly it dissipates.

The defense also asked Dr. Smock if chronic methamphetamine use can cause heart disease, and he said yes.

Upon redirect from the prosecution, Smock said there is "absolutely no evidence" that George Floyd had a heart attack.

4 p.m.

The state called Dr. William Smock to the stand, an emergency medical physician with a specialty in forensic medicine. He teaches medical students, paramedics and police officers, and gives trainings all over the country.

Smock is an expert in asphyxial death, meaning "low levels of oxygen in the blood." 

He is also a police surgeon with the Louisville Police Department. Smock said that means he goes with SWAT teams when they deploy to make sure a doctor is present, advises the chief on health care, and evaluates injured officers.

Smock also goes to the scene when someone is hurt in police custody, and does "living forensic consultations." He said he also administers Narcan, which can treat an opiate overdose.

Prosecutor Jerry Blackwell asked Smock to talk to the jury about drug tolerance. He compared it to the different effects alcohol would have on a "naïve" drinker and an alcoholic.

Smock was paid $300 an hour by the state to review videos, reports and thousands of pages of documents related to George Floyd's death. Blackwell asked him to give his opinion on Floyd's cause of death.

"Mr. Floyd died from positional asphyxia, which is a fancy way of saying he died because he had no oxygen left in his body," Smock said.

Smock said he evaluated other possible causes, including excited delirium.

Smock acknowledged that it's a "controversial" diagnosis. "There isn't 100% agreement on what excited delirium is, but I can tell you at least in my opinion I believe it is real," Smock said.

He acknowledged that some major medical associations do not recognize it as real.

Credit: Hennepin County District Court
Ten signs of excited delirium shown to the jury during questioning of Dr. William Smock.

Smock talked through 10 signs of excited delirium and why he did not believe Floyd was showing any of them. For example, Smock said Floyd was dressed appropriately for the weather, was not attracted to glass, responded to police presence, and was complaining of pain.

Smock said in his evaluation, he saw zero of the 10 signs in George Floyd.

Blackwell also asked Smock to talk through the possibility of a fentanyl overdose, given the level of the drug found in Floyd's blood.

Smock said the telltale signs of a fentanyl overdose are snoring, decreasing respirations, and constricted pupils. He said fentanyl overdose does not cause "air hunger," which Floyd demonstrated by saying "I can't breathe" over and over.

Instead, Smock said, an overdose "can decrease the drive to breathe."

Smock said Floyd's actions shown on video do not indicate a fentanyl overdose.

"That is not a fentanyl overdose, that is somebody begging to breathe," he said.

"Their brain is going to be in sleep mode," Smock added of someone suffering a fentanyl overdose.

Smock said based on medical records he examined, Floyd used opiates for years.

"Mr. Floyd has been a chronic user," he said. "The more you use any drug, alcohol, in this case fentanyl, you build up that tolerance so it takes more drug to give you that high to affect your brain. And in this case we actually have a visit to the Emergency Department where he took seven or eight narcotic pills. They watched him and let him go."

Smock said in his practice, it's difficult to tell how a drug is affecting a patient just by looking at the amount ingested. He said the patient's behavior is more telling.

"You don't rely on the level to tell you how much it's impacted a particular patient, you look at that patient," he said.

Smock showed the jury a body camera image of Floyd's hand pushing against the ground and the tire. He made the same statement that an earlier expert witness made, that Floyd was pushing himself up attempting to breathe.

Smock told the jury that the sooner compressions and ventilation are started on a person, the greater the chance of success. Prosecutor Blackwell asked him when CPR should have been started on Floyd.

"Way before it was, as soon as Mr. Floyd is unconscious, he should have been rolled over," he said. "We have documentation on the video that the officer says 'I can't find a pulse.'"

Smock said CPR should have been started "way before" that, but "clearly" when they could not find a pulse.

3 p.m.

Defense attorney Eric Nelson began his cross-examination of forensic toxicologist Daniel Isenschmid of NMS Labs, who tested George Floyd's blood and identified fentanyl and methamphetamine.

Nelson asked Isenschmid about the substances that were found in Floyd's samples that were "below the threshold to report." Those included amphetamines, the toxicologist had told the prosecution earlier.

The defense had Isenschmid clarify the difference between fentanyl and norfentanyl, which is produced as the body breaks down the fentanyl.

"Regardless of whether you have a tolerance or nontolerance, any type of pill could cause a reaction," Nelson said. He pointed out that pills obtained on the street may not be manufactured "in a controlled environment." Isenschmid agreed that if someone were to take a more powerful pill suddenly, it may have an adverse effect.

Nelson pointed out that of the pool of NMS Labs blood samples the prosecution showed to the jury, there is no way to know which people died directly from fentanyl.

Isenschmid said on redirect from the prosecution that Floyd's ratio of fentanyl to norfentanyl was more similar to a driving population more likely to be "chronic" opiate users versus the postmortem group where use was more likely to be "acute."

2:30 p.m.

The second witness of the day didn't take the stand until 2:30 p.m., with the jury hearing from medical expert Dr. Martin Tobin for close to five hours.

Forensic toxicologist Daniel Isenschmid took the stand next. He works with NMS Labs, which processed the pills found in the cars on the scene of Floyd's arrest.

They also tested George Floyd's blood and urine, sent to them by Hennepin County during the autopsy.

Isenschmid said the "most notable findings" in the blood were the presence of fentanyl and methamphetamine.

The meth level of 19 nanograms per milliliter found in Floyd's blood was consistent with one prescription dose, and Isenschmid said he would consider it a "very low" dose.

Isenschmid said the fentanyl level was 11 nanograms per milliliter. Fentanyl levels can vary from person to person based on their drug tolerance, he confirmed.

"With chronic use, to get the same feeling that you would at a given concentration of fentanyl, you need to take more to get that effect," he said.

Isenschmid also testified that norfentanyl was found. That's the substance that is produced as the body breaks down fentanyl. That level was 5.6 nanograms per milliliter, Isenschmid said.

"It shows that some of the fentanyl was metabolized to norfentanyl," he said. "It basically shows that when we see very recent deaths with fentanyl, we frequently see fentanyl with no norfentanyl whatsoever," he said.

That's because with an overdose, the body doesn't have time to break it down, Isenschmid said.

There was also nicotine, caffeine, and evidence of prior marijuana use in the blood, he testified.

Morphine was found in the urine, Isenschmid said, which can stay there for several days and be indicative of prior use.

Isenschmid testified that sometimes in hospital blood samples, substance levels can appear higher than they were at the time of death.

1:30 p.m.

Derek Chauvin's defense attorney began his cross-examination of the state's first medical expert Thursday afternoon.

He started by asking pulmonologist Dr. Martin Tobin, who said he testified for free, what his usual hourly rate would be. Tobin said it was $500 an hour.

"You felt it was an important case, right?" Nelson asked. Tobin agreed.

Nelson also asked Tobin to confirm that Minneapolis police officers have nothing close to Tobin's level of medical expertise. He said that was true.

Nelson pointed out that Tobin said he watched videos of Floyd's restraint hundreds of times.

"You've had the luxury of slowing it down, moving it into slow motion, still-framing it," Nelson said.

Nelson asked Tobin to confirm that a "multitude of biological factors" contribute to the death of a person, and Tobin agreed.

The defense referenced testimony from a Minneapolis police department trainer earlier in the week, who admitted that the phrase "If a person can breathe, they can talk" has "been said."

As he did on direct examination, Tobin said the statement is problematic because the person may not be breathing in 10 seconds.

Nelson asked Tobin about other factors including the fact that Floyd had previously been diagnosed with COVID-19, controlled substances like nicotine in his system, and adrenaline.

Nelson asked Dr. Tobin hypothetically, if partially ingested pills containing fentanyl and methamphetamine were found in the back of the squad car with Floyd's DNA on them, when the peak effect of the fentanyl would hit.

"If there was any amount of it ingested, yes the peak would be five minutes," Tobin said.

Upon redirect, prosecutor Jerry Blackwell asked Dr. Tobin if he believed nicotine, meth or fentanyl could have caused Floyd's death. He said no.

9:20 a.m.

The prosecution called Dr. Martin Tobin to the stand as its first witness of the day Thursday. Tobin is a physician specializing in pulmonary and critical care medicine in Chicago. Pulmonology is the study of the lungs, Tobin explained to the jury when he took the stand.

Tobin is a renowned expert on breathing. Prosecutor Jerry Blackwell spent some time talking through his credentials for the jury.

Dr. Tobin has been an expert witness in medical malpractice suits, he said, but never in a criminal case. He said he has been in court about 50 times.

Tobin said he is not being paid by the prosecution. "I thought I might have some knowledge that would be helpful to explain how Mr. Floyd died," he said. Because he has never testified in this type of case, he said, he did not "wish to be paid."

Blackwell asked Tobin his medical opinion on Floyd's cause of death.

"Mr. Floyd died from a low level of oxygen, and this caused damage to his brain that we see and it also caused a PEA arrhythmia that caused his heart to stop," he said. "The cause of the low level of oxygen was shallow breathing. Small breaths."

Tobin said that Floyd was taking "shallow breaths that weren't able to carry the air through his lungs down to the essential areas of the lungs that get oxygen into the blood and get rid of the carbon dioxide."

Dr. Tobin said he watched videos of Floyd's restraint hundreds of times. Prosecutors showed the jury an artist's rendering of the restraint to better show the positions of the three officers holding him down.

"Mr. Chauvin's left knee is virtually on the neck for the vast majority of the time," Tobin said. "It's more than 90% of the time by my calculations."

Tobin testified that the handcuffs are "extremely important" to the explanation of why Floyd couldn't breathe effectively.

"It's because of the positioning of the handcuffs at the back, then how he's manipulated with the handcuffs," he said. "They're pushing the handcuffs into his back and pushing him high."

He said the hard asphalt street that Floyd is being pushed into is the other side of the equation.

"His left side and it's particularly the left side we see that, it's like the left side is in a vice," he said.

Tobin said these factors "totally interfere with central features of how we breathe."

"He was being squashed between the two sides and so this meant that he couldn't exert his pump handle," Tobin said, using the analogy of a pump to explain the mechanics of breathing to the jury.

"Then in addition because of the knee that was rammed into the left side of his chest," Floyd was unable to get any air into his left lung, Tobin said.

Tobin testified that the "knee that was rammed into the left side of his chest" had a similar effect whether or not it was directly on the neck.

It was as if a surgeon had "gone in and removed the lung," he said.

He pointed out in a body camera image that Floyd's left arm was pulled over, further hampering his ability to expand his chest.

Tobin also showed the jury a photo of Floyd's hand grasping at the street.

"He's used up his resources and he's now literally trying to breathe with his fingers and knuckles," Tobin said. "He's totally dependent on getting air into the right side. So he's using his fingers and his knuckles against the street to try and crank up the right side of his chest. This is his only way to try and get air to get into the right lung."

"The knee on the neck is extremely important" because it keeps air from getting into the passageway, Tobin said.

"To understand the knee on the neck, you need to examine your own neck," he told the jury. He pointed out the hypopharynx, which he said is "vulnerable because it has no cartilage around it" and is "extremely small to breathe through."

Tobin explained an anatomical photo of the hypopharynx to the jury. It's used for eating and breathing, he said. He pointed out that speech comes from a different part of the body.

The cross-section of the hypopharynx is the size of a dime, Tobin said, which "tells you how small and how vulnerable" the area is.

He said if Chauvin's knee had been directly on the hypopharynx and never varied, Floyd could have had a heart attack or seizure within "seconds." This didn't happen, he explained, because both Chauvin and Floyd varied their movement.

Tobin pointed out another photo that shows Floyd's face "rammed into the street." He said this indicates Floyd was trying to use his face to push himself up, attempting to get his chest off the street.

Tobin said he can calculate the amount of force coming from Chauvin's knee based on his body weight, including his gear weight, and removing the weight of his shin bone and his boot.

Separately, he said he can calculate the narrowing of the space Floyd had left to breathe through.

He displayed a chart that shows that the effort required to breathe doesn't go up that much when the airway is narrowed by 60%. However, when the airway is narrowed by 85%, "the effort to breathe increases seven and a half times."

Tobin testified that Chauvin's knee on Floyd's neck caused the narrowing of the hypopharynx.

The prosecutor showed the jury a photo that shows Chauvin's knee on Floyd's neck, with his toe lifted up above the ground.

Warning: Graphic photo below

Credit: Hennepin County District Court
Prosecutors showed this graphic as Dr. Martin Tobin testified about George Floyd's oxygen capacity.

"This means that all of his body weight is being directed down at Mr. Floyd's neck," Tobin said.

At that moment, Tobin said his calculations show that the weight on Floyd's neck was 86.9 pounds.

Blackwell asked Tobin about Floyd's oxygen levels during the first five minutes of the restraint.

"We know that his oxygen levels were enough to keep his brain alive, and the reason we know that is that he continued to speak over that time," Tobin said. "We know that he made various vocal sounds for four minutes and 51 seconds."

Tobin pointed out the point where Floyd's leg goes out straight.

"That is something that we see as clinicians in patients when they have suffered brain injury as a result of low oxygen," he said.

Blackwell asked Tobin to speak again to Chauvin's positioning.

"For the first five minutes, the left knee is on the neck virtually all of the time," he said. "By my calculation the right knee is on his back 57% of the time."

He said the reason he can't say more than 57% is because he doesn't have a good view of the rest of the time.

Tobin said that Floyd being placed in the "prone" position also had several impacts, including narrowing the hypopharynx.

He showed the jury a graphic that illustrates lung function, saying that he calculated Floyd's specific lung volume based on his age, sex and height.

Tobin said he calculated Floyd's end-expiratory lung volume (EELV) during regular moments at 3,840 mL. That is "basically the volume that's in your lung in between each breath," he said, and is also called "oxygen reserves."

He said Floyd's lung volume would have dropped by 24% just by being held in the prone position. Blackwell asked Tobin if it matters whether Chauvin's knee was on Floyd's neck, back or arm at different times.

"No it's really all amounting to the same," he said. "Whether it's on the back or rammed in against the side and down on the arm, all of these are just going to markedly impair your ability to move your chest."

When the weight of Chauvin's knee was added to the prone position, Tobin calculated there would be another 19% drop, leading to a 43% reduction in Floyd's EELV, his oxygen reserves, and his hypopharynx.

"The work that Mr. Floyd has to perform becomes huge," Tobin said. "With each breath he has to try and fight against the street, he has to fight with the small volume that he has, and he has to try and lift up the officer's knee with each breath."

Tobin used another graphic to show how narrow Floyd's hypopharynx would have gotten which each added element of restraint.

Credit: Hennepin County District Court
Prosecutors showed this graphic demonstrating the size of George Floyd's hypopharynx which each added element of restraint by Derek Chauvin.

He told Blackwell that he can identify the moment Floyd went unconscious "precisely" from the videos. He said after Floyd stopped breathing, the oxygen would have gone down to zero in 25 seconds.

Blackwell asked Tobin about studies that suggest putting someone in the prone position and putting weight on their back is not dangerous. Tobin called these studies "highly misleading." He said they focus on the person's oxygen levels, ignoring a drop in lung capacity. Tobin testified that the point at which oxygen levels fall is a "late event" that follows a decrease in lung capacity.

He also confirmed that none of the studies go on for nine minutes and 29 seconds, the length of time Chauvin's knee was shown on Floyd's neck by body cameras.

Blackwell asked Tobin whether a person without Floyd's pre-existing medical conditions would have died under the same restraint. 

"Yes, 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.

The defense is expected to suggest to the jury that the cause of Floyd's death was primarily medical conditions and drug use, rather than Chauvin's restraint. 

Blackwell asked Tobin about a "paraganglioma tumor"  Floyd was found to have during the autopsy. Tobin said he knows of six deaths across the world from this tumor. He said they were all sudden.

Blackwell also asked Tobin to comment on the idea that if a person can speak, they can breathe, which the officers restraining Floyd told the crowd.

"It's a true statement but gives you an enormous sense of false security," he said.

Speaking does mean you're breathing, Tobin said, "but it doesn't mean you're going to be breathing five seconds later."

"A very dangerous mantra to have out there," he added.

In describing the effects of oxygen deprivation on the brain, Tobin said, "If you stop the flow of oxygen to the brain you lose consciousness in eight seconds."

Tobin remarked on Floyd's leg movements that occurred after he stopped speaking, saying that they are involuntary reactions to the brain damage.

"You're seeing here fatal injury to the brain from a lack of oxygen," he said. "The leg jumps up like that as a result of a fatally low level of oxygen going to the brain."

During one body camera video, Tobin counted Floyd's breaths for the jury.

"If fentanyl is having an effect and is causing depression of the respiratory centers, that's going to cause a decrease of the respiratory rate," he said. 

He said Floyd's respiratory rate was normal at that point, 22, as opposed to what would be normal for someone on fentanyl: 10.

"There isn't fentanyl on board that is affecting his respiratory centers," Tobin concluded.

Tobin testified that Floyd made his last effort to breathe about three minutes before he was lifted up off the street by EMTs. 

Prosecutor Blackwell asked Tobin to show the jury the exact moment he believes Floyd died, using a segment of bystander video. Tobin pointed to a flickering of Floyd's eyes.

"You can see his eyes, he's conscious, and then you see that he isn't," Tobin said. "That's the moment the life goes out of his body."

9:15 a.m.

The judge briefly went over legal issues with the defense and prosecution before the jury came in to hear more testimony.

At that time prosecutor Jerry Blackwell indicated the state will call Dr. Andrew Baker to the stand on Friday. Baker is the Hennepin County Medical Examiner who performed the autopsy on George Floyd.

Wednesday, April 7

In the courtroom Wednesday, much of the testimony from expert witnesses focused around Floyd's possible drug use prior to his arrest by former officer Chauvin on May 25, 2020.

Defense attorney Eric Nelson introduced a new element to the trial on Wednesday, by suggesting that George Floyd said the words "I ate too many drugs" while he was being restrained by police. 

Special Agent James Reyerson agreed with the defense. But after a break in the courtroom, prosecutors called Reyerson back to the stand to watch a longer version of that video clip. Reyerson said in context, it sounds like Floyd is saying "I ain't do no drugs." 

George Floyd's words are not recorded in the official transcript of the body camera video. Prior to the comment by Floyd, the officers were discussing that they believed Floyd was high, and that they found a pipe.

The defense made a point of questioning BCA agents Reyerson and McKenzie Anderson about the pills found in the Mercedes SUV Floyd was in before he was arrested, and in the squad car officers tried to detain him in. Drugs were not retrieved from the vehicles until a second search of each, one of which was prompted by the defense.

A forensic chemist, Susan Neith from NMS Labs also took the stand, saying that the fentanyl levels found in the pills were normal, but the methamphetamine levels were much lower than what she usually sees.

In the tablet found in the squad car, she said the fentanyl was less than 1% and the methamphetamine purity was 2.9%.

"Generally we see higher levels in methamphetamine samples that I have tested," she said. "Majority of the time I see 90 to 100% methamphetamine."

RELATED: Derek Chauvin trial: Defense attorney zeroes in on George Floyd's drug use

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