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MINNEAPOLIS A medical expert in the physiologyof breathing testified in the murder trial of Derek Chauvin that the way George Floyd was restrained handcuffed behind his back, face-down on the ground, with a knee on his neck prevented him from breathing properly.

Dr. Martin Tobin, called as an expert witness by the prosecution, said the cause of Floyd's death washypoxia, or a low level of oxygen that led to asphyxia, or suffocation. The overall effect of the restraint was almost"asif a surgeon had gone in and removed the lung," he said, referring to Floyd's left lung.

"A healthy person subjected to what Mr. Floyd was subjected to would have died as a result of what he was subjected to," Tobin said.

Chauvinis charged with second-degree murder, third-degree murder and second-degree manslaughter in Floyd's death. The defense argues Floyd died as a result of the drugs in his system and underlying medical issues, but prosecutors say Floyd was killed by Chauvin's knee on his neck for more than nine minutes.

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Dr. Bill Smock, called by the prosecution as an expert witness on drug tolerance who reviewed "thousands of pages of documents" in Floyd's case, took the witness stand Thursday afternoon.

Smock spent more than 20 years at a Level 1 trauma center in Louisville and teaches emergency medicine to paramedics, medical students and others in Louisville. He's edited four textbooks and has worked as an assistant medical examiner. He also is the police surgeon for the Louisville Police Department. Smock said he specializes in forensic analysis on people who have suffered major injuries but have not died.

Questioned by prosecutor Jerry Blackwell, Smock said he concluded that Floyd had died of positional asphyxia, "which is a fancy way of saying he had no oxygen in his body."

Smock said he also researched other potential causes of death, including excited delirium, a state where someone's respiration and breathing rate is up, and they're "out of control." Smock said excited delirium is a controversial diagnosis because "there isn't 100% agreement on what excited delirium is." However, Smock added, "in my opinion, it is real."

In this image taken from video witness Dr. Bill Smock, a Louisville physician in forensic medicine testifies as Hennepin County Judge PeterCahill presides Thursday, April 8, 2021, in the trial of former Minneapolis police Officer Derek Chauvin at the Hennepin County Courthouse in Minneapolis, Minn.(Photo: AP)

Responding to questions from Blackwell, Smock said Floyd showed none of the symptoms of excited delirium, including failing to respond to instructions from law enforcement officers, excessive sweating and far higher than normal strength.

Smock also said he ruled out a Fentanyl overdose as a potential cause of death. Symptoms of an overdose of the synthetic drug can be slower respiration rates, "or they're not respiring at all." Such an overdose would normally cause eye pupils to constrict said Smock. Instead, Floyd's respiration rate was normal and his pupil's were dilated, Smock testified.

The level of methamphetamine found in Floyd's system was "an extremely low level," Smock said, not suggestive of an overdose from that drug.

Smock also addressed the lack of bruising on Floyds body. He added that "you can be fatally strangled, die of asphyxia, and have absolutely no bruising." Bruising depends on where, how much and how long pressure has been applied, he said.

Trying to underscore Smocks medical opinion on the cause of death, Blackwell played part of a police body camera video and audio of the struggle with Floyd. "Listen to Mr. Floyd's voice, he's speaking with full volume," Smock said, adding, "You will hear his voice get weaker and weaker."

As the court played video of Floyd crying out "Mama,"his niece, Tiffany Hall, put her hand on her head and avoidedwatching the video. Shebreathedheavily and appearedto cry, dabbing her eyes with tissues as Floyd could be heard saying "I cant move."

At one point in the video, Smock highlighted Floyd pushing against the tire of the police patrol car in an attempt to breathe. "He's trying to get his right chest off the pavement so he can breathe," Smock said. That echoed earlier testimony from Tobin, the expert in the physiology of breathing.

Blackwell also focused Smock on the police officers duty to provide medical care to Floyd. Smock said the officers should have started CPR "way before" paramedics arrived and began to administer it a point several Minneapolis police officers have made in recent days. "As soon as Mr. Floyd was unconscious, he should have been rolled over," Smock added.

During cross-examination, lead defense attorney Eric Nelson got Smock to acknowledge that he's not certified as a pathologist. Asked by Nelson, Smock acknowledged there's no evidence from Floyd's autopsy that his airway had been obstructed. "The evidence is not from the autopsy, it is on the videotape, sir," Smock replied.

Smock said the autopsy showed Floyd had evidence of heart disease. Nelson asked whether Floyd's struggle with police officers could be likened to a cardiac stress test, when someone is placed on a treadmill with electrodes that record heart response as the treadmill's speed and difficulty level increases. Smock disagreed.

Asked by Blackwell, Smock said: "There was absolutely no evidence at autopsy, anything to suggest that Mr. Floyd had a heart attack."

Daniel Isenschmid, a forensic toxicologist who analyzed George Floyd's hospital blood and urine collected from theHennepin County Medical Examiner's autopsy, took the witness stand Thursay afternoon. Isenschmid works at NMS Laboratory in Horsham, Pennsylvania and previously worked at various medical examiner's offices.

He told jurors hefound fentanyl and methamphetamine in Floyd's blood. The amount of meth was consistent with a prescribed dose a"very low" amount, he said.

On fentanyl and opioids or opiates, Isenschmidsaidthe impact on a person taking the drug can vary widely depending on tolerance.The state triedto show that Floyd not only had a high tolerance for drugs, but that the amount in his system was average to minimal.

In this image taken from video, witness Daniel Isenschmid, a forensic toxicologist, testifies as Hennepin County Judge PeterCahill presides, Thursday, April 8, 2021, in the trial of former Minneapolis police Officer Derek Chauvin at the Hennepin County Courthouse in Minneapolis, Minn.(Photo: AP)

Isenschmidsaid he also found evidence of caffeine, smoking and THC in Floyd's body. Morphine was not found in the blood, but rather in the urine sample, which could indicate that the drug was taken prior to the day Floyd died, he said. Isenschmid said he also foundnarcan in Floyd's system, which can be indicative of someone undergoing treatment.

On cross-examination, lead defense attorney Eric Nelson tried to show that it was possible that Floyd ingested more fentanyl during the arrest a part of his case that drugs played a role in Floyd's death but the drug had not broken down by the time he died. Nelson also tried to bring home the point that drugs can vary from pill to pill, hit to hit, and could adversely impact a person's reaction. Isenschmid agreed.

When prosecutor Erin Eldridge resumed questioning, Isenschmid agreed the levels of meth in Floyd's system "were lower than 94% of the driving under the influence population" so low that it likely wouldn't cause an effect on Floyd.

Dr. Martin Tobin,a physician who has been working in respiratory physiology for 40 years, testified Thursday that Floyd died from a "low level of oxygen," which caused damage to his brain and an abnormal heartbeat. Tobin was called as an expert witness by prosecutors and examined records and video in the Floyd case, but he did not conduct an examination of Floyd's body.

Tobin said he watched videos of Floyd's arrests "hundreds of times"and found Chauvin's left knee was on Floyd's neck for the majority of the time. The combination of Floyd being handcuffed behind his back, the officers' manipulation of the cuffs, and the pavement beneath Floyd combined to interfere with Floyd's ability to breathe, Tobin testified.

"It's like the left side is in a vise. It's totally pushed in, squeezed in from the street at the bottom, and then from the way the handcuffs are manipulated," he said. "That totally interferes with central features of how we breathe.

In this image from video, Dr. Martin Tobin testifies as Hennepin County Judge PeterCahill presides Thursday, April 8, 2021, in the trial of former Minneapolis police Officer Derek Chauvin at the Hennepin County Courthouse in Minneapolis, Minn.(Photo: AP)

Tobin said images from the videos show Floyd trying to use his right fingers and knuckles to push the right side of his lungs up to get air into them. "This tells you he has used up his resources and he's literally trying to breathe with his fingers and knuckles," Tobin said.

Tobin looked at the jurors as he testified, and everyjuror took notes. Read more on his testimony here.

The prosecution has said Chauvin is not only culpable in Floyd's death but that he also failed to carry out his duty to provide basic care when Floyd was in medical distress and then became unresponsive.

Several Minneapolis police department officials testified Chauvin violated department policy by failing to move Floyd on his side to ease his breathing once he had been restrained face-down on the ground.

"When someone is in our custody, we have an obligation to provide for their care," Minneapolis Police Chief Medaria Arradondo told jurors. That's true even if an officer is applying defense tactics, the chief said. "They're still in our custody," he said. "They have rights."

The defense has argued Chauvin and the other officers were unable to care for Floyd because they were distracted and threatened by a crowd of vocal, upset bystanders. "As the crowd grew in size, seemingly so too did their anger," lead defense attorney, Eric Nelson, told jurors. Read more.

Jurors have heard from 30 witnesses so far, all have been called by the prosecution in the George Floyd murder trial. USA TODAY

Sgt. Jody Stiger, a Los Angeles Police Department officer who has conducted about 2,500 use-of-force reviews in his career,told jurors Wednesday that Chauvin used "deadly" force on George Floyd and kept his knee on Floyd's neck for more than nine minutes.

In this image from video, witness Jody Stiger, a Los Angeles Police Department sergeant testifies as Hennepin County Judge Peter Cahill presides Wednesday, April 7, 2021.(Photo: AP)

Stiger said the initial force used on Floyd was appropriate because Floyd was resisting arrest as officers tried to get him into their patrol car. However, after officers forced Floyd to the ground, "they should have de-escalated the situation," Stiger said. Instead, the officers continued to intensify the situation, he said.

Stiger said the number of officers on the scene outweighed any threat posed by Floyd, who was not actively resisting while he was in the prone position. He said "no force should have been used after he was in that position." But the pressure continually exerted by Chauvin "raised the possibility of death," he said. More here.

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Derek Chauvin trial, day 9: Doctor testifying for prosecution says 'what Mr. Floyd was subjected to' would kill healthy person - USA TODAY