MINNEAPOLIS — The pressure of Derek Chauvin’s knees on George Floyd’s neck and back made it virtually impossible for the handcuffed man to breathe as he was pinned face down on a street and would have killed any healthy person, an expert on the respiratory system testified Thursday.

Martin Tobin, a Chicago-area pulmonologist and critical care doctor who specializes in the science of breathing, testified that the pressure of Chauvin “jamming” his knees into Floyd’s body cut off oxygen and led to brain damage within minutes, sparking an arrhythmia that caused his heart to stop. He characterized Chauvin’s knee as being on Floyd’s neck “the vast majority of the time.”

“One second, he’s alive, and one second, he’s no longer,” Tobin said as he narrated a clip of a bystander’s video zoomed in to show Floyd’s face pressed into the asphalt, while the former police officer’s knee pressed unrelentingly on his neck as Floyd slowly stopped moving. “That’s the moment the life goes out of his body.”

In graphic, gripping detail, Tobin, an expert witness for the prosecution in Chauvin’s ongoing murder trial, described how Floyd’s left lung had been rendered useless, almost as if it had been removed surgically, by the weight and restraint of Chauvin, who also was using a pain compliance technique on Floyd’s left hand, squeezing his fingers and lifting his wrist toward a handcuff — further restricting his breathing.

Tobin was the first of several prosecution experts Thursday to repeatedly refute the defense’s attempts to claim that Floyd’s health and small amounts of fentanyl and methamphetamine in his system caused his death.

Tobin, who is not charging the prosecution for his services, called the jury’s attention to a snippet of police body-camera video showing the knuckles of Floyd’s right hand stiffly angling toward the back tire of a squad car parked next to where officers were restraining him on the ground. Tobin described it as an “extraordinarily significant” moment showing how Floyd was using his entire body to grab for breath.

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“This tells you that he has used up his resources and he’s now literally trying to breathe with his fingers and knuckles,” Tobin testified, explaining the act of breathing as a “pump handle” in which the human body cranks its way to oxygen. “This is his only way to try and get air.”

The doctor said Chauvin appeared to be using most of body weight to restrain Floyd. He estimated that with the body armor and other gear Chauvin was wearing, the officer had placed about 92 pounds of weight on Floyd’s neck, which narrowed his airway. “It’s like breathing through a drinking straw, but it’s worse than that,” Tobin testified.

Several times throughout his testimony, Tobin, a critical care doctor for nearly 46 years and who has studied breathing for most of his career, turned to the jury and urged them to feel their own necks and throats as he spoke in great detail about how the human body takes in breath. Most jurors did so — even though Hennepin County District Court Judge Peter A. Cahill interrupted and told them they did not have to, in response to a defense objection.

The veteran lung doctor repeatedly testified that the officer’s restraint and the way it limited Floyd’s oxygen intake could have killed “anybody.”

“A healthy person subjected to what Mr. Floyd was subjected to would have died as a result,” Tobin told the jury.

Tobin was one of several medical experts to testify for the prosecution as the trial shifts to what caused the 46-year-old Black man’s death as he was restrained by officers during an investigation of a counterfeit $20 bill that was allegedly passed at a local market on Memorial Day.

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William Smock, a police surgeon with Louisville Police Department, later concurred with Tobin’s findings, attributing Floyd’s death to “positional asphyxia.”

“Which is a fancy way of saying he had no oxygen left in his body,” Smock told the jury.

Prosecutors have argued the pressure from Chauvin’s knee was a “substantial causal factor” in Floyd’s death. Last June, Hennepin County Medical Examiner Andrew Baker, who is expected to testify Friday, formally declared Floyd’s death a homicide, listing “cardiopulmonary arrest complicating law enforcement subdual, restraint, and neck compression” as the cause of death.

Eric Nelson, Chauvin’s defense attorney, has argued that his client was following his training and department policies on use of force and that Floyd died of existing health issues and drugs in his system, not from the pressure of Chauvin’s knee.

But prosecutors have sought to expand on Baker’s findings. They have argued that Floyd died of asphyxia when Chauvin used “excessive and unreasonable force” on Floyd, “grinding and crushing him … until the very life was squeezed out him,” as prosecutor Jerry Blackwell put it during opening statements.

In a notable move, prosecutors did not open the medical aspect of their case Thursday with Baker’s testimony — but rather with Tobin, who in giving his opinion that Floyd died of low oxygen, emphasized that it wasn’t just the pressure from Chauvin’s knee on his Floyd’s neck, but also the pressure on his back, that led to his death.

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In recent days Nelson has repeatedly sought to raise doubts about the placement of Chauvin’s knee. He has presented footage and stills from video that he has suggested show that Chauvin’s knee was resting more on Floyd’s shoulder blades, an observation that several witnesses have disagreed with.

During Thursday’s proceedings, the ninth day of testimony in the case, the prosecution called several experts aimed at weakening the defense claims about Floyd’s death, including the impact of drugs.

The jury heard from Daniel Isenschmid, a forensic toxicologist at NMS Labs, which handled bloodwork for Floyd’s autopsy and found the presence of fentanyl and methamphetamine — findings that have been seized upon by Chauvin’s defense.

Isenschmid testified that the level of methamphetamine in Floyd’s blood was “very low” — about 19 nanograms per milliliter — roughly the equivalent found in a single dose of a prescription drug.

But Isenschmid testified that while fentanyl was found in Floyd’s blood, he also found levels of norfentanyl, a substance produced when the body breaks down fentanyl. Floyd’s blood sample had 11 nanograms of fentanyl per milliliter and 5.6 nanograms of norfentanyl per milliliter, showing that “some of the fentanyl was metabolized,” Isenschmid said.

Isenschmid said this was significant because norfentanyl is rarely found in people who have died of a fentanyl overdose because “the body doesn’t have time to break it down.”

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People can develop a tolerance to opioids, Isenschmid testified, meaning it takes more of the drug to produce an effect. Courteney Ross, Floyd’s girlfriend, testified last week that the couple had struggled with an opioid addiction.

But Nelson sought to raise doubts about Floyd’s tolerance, suggesting he could have taken a street drug — though he didn’t address the issue of norfentanyl.

“Regardless of whether you have a tolerance or a non-tolerance, any single incident could cause an adverse reaction?” he asked.

“Well, sure, if you suddenly had a pill that was 10 times the amount of fentanyl than another one,” Isenschmid replied.

During his cross-examination of Tobin, Nelson also asked whether fentanyl in street drugs could affect people differently than legally prescribed fentanyl. Tobin agreed that it would increase the heart rate but said it would not affect respiratory rate.

Tobin said fentanyl typically cuts a person’s respiration rate by about 40 percent. But he repeatedly told the jury that Floyd’s breathing was “right around normal” until the point that he stopped breathing.

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“Did you see any depression in Mr. Floyd’s ability to breathe whatsoever before he went unconscious?” prosecutor Blackwell asked.

“No, absolutely not,” Tobin replied.

Smock, an expert on positional asphyxia who was the final witness of the day, also insisted that Floyd was not suffering a drug overdose at 38th and Chicago, the intersection where he died.

“He’s breathing. He’s talking. He’s not snoring. He is saying, ‘Please, please get off of me. I want to breathe. I can’t breathe,'” Smock testified, citing footage of the scene. “That is not a fentanyl overdose. That is somebody begging to breathe.”

He also rejected another defense argument — that Floyd could have been suffering from excited delirium, a controversial term cited by some medical examiners and police to describe the sudden in-custody death of people who may be under the influence of drugs or in an agitated state.

An officer at the scene — Thomas Lane — raised the issue at the scene, and Chauvin’s defense has suggested he continued to restrain Floyd because of concern the unconscious man would awaken and fight the officers.

Smock acknowledged excited delirium is a controversial theory in the medical community that not everyone believes. He said, however, that he does. But he said, based on his review of the videos of the scene, Floyd was not exhibiting any of traditional symptoms, including excessive sweating, rapid breathing and “superhuman strength.”

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Like Tobin, Smock pointed to Floyd’s right hand straining toward the squad car tire as proof he was fighting for breath. After a video of Floyd being restrained by officers was again played for the jury, Smock described how he observed Floyd have a seizure caused by the lack of oxygen to his brain. “You see his leg shake. But you also see and you can hear the handcuffs shake,” Smock said, raising his hands and shaking them toward the jury.

Smock testified that the officers should have performed CPR on Floyd “as soon as he was unconscious.”

“He should have been rolled over. We have documentation on the video that the officer says, ‘I can’t find a pulse,’ ” Smock said.

The doctor paused, seeming to struggle for words and shaking his head as he looked toward the jury.

“Why?” Smock said. The officers should have intervened earlier, he told the jury, but “clearly when they can’t find a pulse, CPR should have been started.”