Apr. 8 highlights for the murder trial of Derek Chauvin Day 9

Dr. William Smock, the police surgeon and director of the Clinical Forensic Medicine Program for the Louisville Metro Police Department, testified that George Floyd died of positional asphyxia, or lack of oxygen. 

"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. "When the body is deprived of oxygen, in this case from pressure on his chest and back, he gradually succumbed to lower and lower levels of oxygen until it was gone and he died."

Smock testified that he came to that conclusion after ruling out other causes of death, including fentanyl overdose.

Smock, a former medical examiner, is the second witness to testify Thursday that Floyd died from low or no oxygen. 

Earlier, Dr. Martin Tobin, a world-renowned expert on breathing, said Floyd died from a low level of oxygen.

"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 (pulseless electrical activity) arrhythmia because his heart stopped," Tobin said.

CORRECTION (April 8, 2021, 6:53 p.m. ET): A previous version of this post misidentified an expert witness who testified Thursday about George Floyd's cause of death. It was police surgeon Dr. William Smock, not forensic toxicologist Daniel Isenschmid.

Prosecutors called a Kentucky police surgeon and emergency medicine physician with specialized training in forensic medicine to the stand Thursday afternoon.

Dr. William Smock, who is the police surgeon at the Louisville Metro Police Department and a professor of emergency medicine at the University of Louisville, also trains police on topics such as strangulation and asphyxia and forensic evaluation of gunshot wounds.

He said he is an expert in asphyxial death and strangulation.

A legal analyst said Thursday that Derek Chauvin’s defense had a “real uphill climb” in cross-examining prosecution witness Dr. Martin Tobin, a pulmonologist and renowned expert on breathing.

“These are the scariest kinds of expert witnesses because they've forgotten more about the topic than the attorney will ever know,” NBC News legal analyst Danny Cevallos said. “And this is the kind of witness that can really beat you up, unless you have some really focused cross examination.”

Tobin repeatedly cast doubt on the role of fentanyl in George Floyd’s death, a key argument of the defense. He also testified Chauvin’s knee on Floyd’s neck and back was “extremely important” when considering what caused the lack of breathing that led to his death.

Cevallos said the problem for the defense is that the prosecution does not need to prove Chauvin’s knee on Floyd’s neck was “the sole cause of death,” but instead that it was a “substantial factor.”

“They went a long way towards that today,” he said, while adding the defense has yet to call their witnesses. 

Cevallos added the prosecution’s use of demonstrative exhibits and images along with Tobin’s testimony were the kind of tactics that “can make a point very sticky in a jurors mind in a way that just words might not.”

Derek Chauvin's attorney, Eric Nelson, attempted to obfuscate the testimony of Dr. Martin Tobin, a pulmonologist and breathing expert, seemingly to confuse jurors. 

Earlier Thursday, Tobin testified that George Floyd died of a lack of oxygen from being pinned on his stomach with his hands cuffed behind him and his face against the pavement. Nelson tried to challenge that.

The defense has argued that Floyd's drug use and underlying health problems were what killed him. Nelson asked Tobin if his scientific conclusions were based on assumptions, given that no one was at the scene to take various measurements such as Derek Chauvin's weight.

Tobin, who appeared unflappable while he was being questioned by Nelson, responded that his calculations "are based on direct evidence," research and "very few assumptions."

Mary Moriarty, the former chief public defender of Hennepin County, where Chauvin is being tried on charges of murder and manslaughter, said Tobin "was very effective in teaching the jury about complicated concepts in plain English."

"I am not sure I've ever seen an expert this effective," she tweeted. 

The state called a forensic toxicologist to the stand Thursday afternoon.

Dr. Daniel Isenschmid is a forensic pathologist for NMS Laboratory in Pennsylvania. The lab did testing of blood work in the case of George Floyd’s death.

An autopsy found fentanyl and methamphetamine in his system.

Isenschmid testified the type of fentanyl found in Floyd's system is not usually found in fatal overdose victims, suggesting that Floyd did not overdose. He said that the fentanyl found in Floyd’s system was lower than levels found in 94% of DUI cases.

The defense has claimed drug use, underlying health conditions and the adrenaline flowing through his body led to Floyd's death.

Near the end of Dr. Martin Tobin’s testimony, Derek Chauvin's attorney again attempted to link the fentanyl found in Floyd's system to this death, a central part of the defense's case. The renowned expert pulmonologist made clear he did not believe there was a connection.

“Generally speaking, fentanyl can also cause death as a result of low oxygen to the brain, right?” defense attorney Erik Nelson asked.

“But it would have to be through respiratory depression,” Tobin responded.

The defense repeated the question, looking for a yes or no response. Tobin would not concede that the science was that simple.

Prosecutor Jerry Blackwell followed up by asking Tobin if people go into a coma before they die if the cause of death is fentanyl. Tobin replied that they do.

“Was Mr. Floyd ever in a coma?” Blackwell then asked.

Tobin responded with a simple: “No.”

Mary Moriarty, the former chief public defender of Hennepin County, where Derek Chauvin is being tried on charges of second- and third-degree murder and second-degree manslaughter, weighs in on how difficult it can be to cross examine an expert:

Chauvin's attorney, Eric Nelson, is cross-examining Dr. Martin Tobin, a lung and critical care specialist at the Edward Hines, Jr. VA Hospital and Loyola University’s medical school in Chicago.

Tobin, a pulmonologist and world renowned expert on breathing, has corrected Nelson a number of times. Nelson asked Tobin a series of open-ended questions, including what he believes caused George Floyd's death. Tobin testified earlier Thursday that Floyd died "from a low level of oxygen."

Dr. Martin Tobin, a renowned expert on breathing, testified Thursday that George Floyd’s respiratory rate was normal before he lost consciousness and fentanyl “was not having an effect” on his respiratory centers based on his rate of breathing.

Tobin said while Floyd was restrained on the ground he was still experiencing a normal respiratory rate of 22. He said if fentanyl were causing a depression of his respiratory centers, one would expect a 40 percent reduction in his respiratory rate.

He said if Floyd’s respiratory rate was 22, then one would expect a respiratory rate of 10 if fentanyl was having an effect.

Tobin agreed with prosecutor Jerry Blackwell that he did not see a depressed rate of breathing in Floyd and that his respiratory rate was normal just before he lost consciousness. 

The defense has argued that fentanyl, which was found in Floyd's system at the time of his death, and other drugs can lead to insufficient oxygen in the body as part of the defense argument that Chauvin was not the cause of Floyd's death.

Dr. Martin Tobin, a renowned pulmonary expert, testified Thursday that the idea of “if you can speak, you can breathe” is both “very dangerous” and “highly misleading.”

Tobin said even though such a statement was technically true in the sense that when a person speaks, they are exhaling, it also gives “an enormous false sense of security.”

“Certainly at the moment that you are speaking, you are breathing, but it doesn't tell you that you're going to be breathing, five seconds later,” he said.

Tobin added that at the time Floyd was restrained on the ground and telling officers he could not breathe he still had oxygen in his brain.

“And again, it's a perfect example of how it gives you a huge false sense of security because very shortly after that we're going to see that he has a major loss of oxygen in the way that he moves his leg,” he added.

Tobin said this showed how dangerous the concept of “if you can speak, you can breathe,” actually was.

“Yes, that is true on the surface, but highly misleading. A very, very dangerous mantra to have out there,” he said.

Dr. Martin Tobin testified that he was aware that George Floyd had pre-existing conditions as mentioned in his autopsy and medical records. 

"Do you have an opinion to a reasonable degree of medical certainty as to whether a person who had none of those pre-existing conditions, a healthy person, would have died under the same circumstances as Mr. Floyd?" prosecutor Jerry Blackwell asked him.

"Yes. A healthy person subjected to what Mr. Floyd was subjected to would have died," Tobin said. 

The prosecution has said Floyd died from Chauvin's kneeling on his neck. The Hennepin County Medical Examiner's Office classified Floyd's death as a homicide that occurred while he was being restrained by police.

Floyd's cause of death was listed as a "cardiopulmonary arrest complicating law enforcement subdual, restraint, and neck compression." Under "other significant conditions" it said Floyd suffered from hypertensive heart disease, and listed fentanyl intoxication and recent methamphetamine use. 

A renowned pulmonary expert testified that he calculated Derek Chauvin’s knee was on George Floyd’s neck for more than 3 minutes after “there was not an ounce of oxygen left” in his body.

Dr. Martin Tobin said at 8:25 p.m. on the day he died Floyd reached the point where he did not have “an ounce of oxygen left in his entire body” as Chauvin continued to have his knee pressed against his neck while he was handcuffed on the ground.

“The knee remained on the neck for another three minutes and two seconds after we reached the point where there was not one ounce of oxygen left in the body,” he added.

Dr. Martin Tobin, a world-renowned expert on breathing, testified as jurors were shown a photo in which Derek Chauvin's toe is lifted slightly off the pavement as he kneels on George Floyd's neck.

Tobin says that by his calculation, at one point, 91.5 pounds — half of Chauvin's body weight and half the weight of his gear — was on Floyd's neck.

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

A pulmonary expert testified Thursday morning about why the knee on George Floyd’s neck and back was “extremely important” when considering what caused the lack of breathing that led to his death.

Dr. Martin Tobin loosened his tie, unbuttoned the top button on his shirt and instructed the jurors to feel their necks as he testified about why a knee on the neck was so important.

He said that a knee on Floyd’s neck was “extremely important because it's going to occlude the air getting in through the passageway.”

He walked the jurors through a demonstration of their own neck anatomy, pointing them to the  hypopharynx, which he called “vulnerable.”

“The hypopharynx is very important for understanding this case for a number of reasons. Because it's so vulnerable, because it has no cartilage around it, it's going to be an area that is compressed,” he said. “It's extremely small to breathe through and it becomes very important for being able to continue to breathe through."

Tobin added that Chauvin's knee was on that area of Floyd's neck at points, with enough pressure to cut off his breathing.

Dr. Martin Tobin, a pulmonologist, testified Thursday that he believes there were four forces that caused George Floyd's shallow breathing and led to his death. 

"The main forces that are going to lead to the shallow breath are going to be that he's turned prone on the street, that he has the handcuffs in place combined with the street and then that he has a knee on his neck and then that he has a knee on his back and on his side," Hines said. "All of these four forces are ultimately going to result in the low tidal volume, which gives you the shallow breaths that we saw here." 

The prosecution has argued that George Floyd died from Derek Chauvin kneeling on his neck for more than 9 minutes.

The first witness called Thursday in the murder trial of Derek Chauvin was a physician in pulmonary and critical care medicine who said George Floyd died from a "low level of oxygen."

Dr. Martin Tobin is a physician with the Edward Hines Jr. VA Hospital and Loyola University Medical School in Chicago. He has been working in respiratory physiology for about 46 years and has served as an expert witness in court before.

Tobin said he was asked to review medical records and watch videos related to the case. He said he believed Floyd died of a "low level of oxygen" that caused damage to his brain and also led his heart to stop beating.

The state is expected to call multiple medical experts over the next two days.

MINNEAPOLIS — Officer Derek Chauvin had his knee on George Floyd’s neck — and was bearing down with most of his weight — the entire 9 1/2 minutes the Black man lay facedown with his hands cuffed behind his back, a use-of-force expert testified Wednesday at Chauvin’s murder trial.

Jody Stiger, a Los Angeles Police Department sergeant serving as a prosecution witness, said that based on his review of video evidence, Chauvin knelt on Floyd's neck or neck area from the time officers put Floyd on the ground until paramedics arrived.

“That particular force did not change during the entire restraint period?” prosecutor Steve Schleicher asked as he showed the jury a composite image of five photos taken from the various videos of the arrest.

“Correct,” Stiger replied.

During his lengthy testimony Monday, Minneapolis' police chief minced no words in condemning the actions of Derek Chauvin, the former officer who is charged with murder in the death of George Floyd.

Still, Arradondo's testimony was rare. That he was joined by a string of other law enforcement officers was remarkable and a sign that the so-called blue wall of silence is crumbling.

Paul Butler, a law professor at Georgetown University Law Center, said the blue wall means "that sometimes police officers close rank and — right or wrong — they're blue."

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