Even after George Floyd took his final breath and went motionless under Derek Chauvin's knee, the since-fired Minneapolis police officer kept the pressure on for another three minutes and 27 seconds, a Chicago-based lung and critical care expert testified Thursday.

Dr. Martin Tobin, a Chicago physician who has written textbooks and specialized in respiratory and critical care medicine for decades, showed through graphics and video how Floyd pressed the fingers of his cuffed right hand to the ground and his knuckles against a police squad-car tire as he fought in vain to open up his side to pull air into his right lung.

"Mr. Floyd died from a low level of oxygen," Tobin said. "This caused damage to his brain that we see, and it also caused a [pulseless electrical activity] arrhythmia that caused his heart to stop."

Floyd's cause of death remains at the heart of the case. Prosecutors used two out-of-state experts Thursday to bolster their argument that Floyd died because he couldn't breathe while handcuffed on his stomach, pinned under the body weight of Chauvin, who was aided by former officers J. Alexander Kueng and Thomas Lane.

In his cross-examinations, defense attorney Eric Nelson again worked to show that Floyd could have died from a heart attack or overdose caused by illicit drug use and underlying health problems, including arterial blockages. When prosecutor Jerry Blackwell asked Tobin if Floyd's pre-existing health conditions had anything to do with his death, the doctor replied, "none whatsoever."

Prosecutors are expected to call Hennepin County Medical Examiner Andrew Baker on Friday. He performed the autopsy on Floyd, ruling the cause of death to be "cardiopulmonary arrest complicating law enforcement subdual, restraint, and neck compression." He also listed hardening and thickening of the artery walls, heart disease and drug use as "other significant conditions." Fentanyl and methamphetamine were also found in Floyd's system.

Chauvin is on trial for second-degree murder, third-degree murder and manslaughter. The three other fired officers — Kueng, Lane and Tou Thao — are scheduled for trial in August on charges of aiding and abetting him. Thao held off the bystanders while the others restrained Floyd.

Tobin showed jurors how Chauvin and Kueng manipulated Floyd's handcuffs high on his back while pinning him. "It's like the left side is in a vise, it's being pushed in from the street at the bottom and the way the handcuffs are manipulated … totally interferes with central features of how we breathe," he said.

Tobin said Floyd lifted his right shoulder to create chest space to draw breath. "Basically, on the left side of his chest … it's as if a surgeon almost went in and removed the lung … and left him totally reliant on his right side," the doctor said.

In one photo, Tobin pointed out how the knuckles on Floyd's right hand braced the ground. "To most people, this doesn't look terribly significant, but to a physiologist this is extraordinarily significant, because this shows he has used up all his resources and showed he is trying to breathe with his fingers and knuckles."

With his left lung useless, the doctor said Floyd was trying to pull air into his right side. Another photo showed Floyd's knuckles on the tire as he tried to open his chest. "This is his only way to get air into the right lung," Tobin said.

The knee on Floyd's neck also restricted his airway to a space narrower than a small straw, Tobin said.

Tobin turned his attention to the amount of weight being pressed onto Floyd's neck. He noted that Chauvin kept his upper body erect, his knee on Floyd's neck and his toe lifted, increasing the weight pressed on Floyd's neck from about 86 pounds to more than 91.

Tobin described the moment he believes Floyd suffered brain damage as evidenced by how Floyd extended his leg in a kick about 5 minutes into the restraint. The video showed Floyd's leg rising and Lane's hand pushing it back down.

Tobin, whose speech carried the lilt of his native Ireland, often spoke directly to the jury and demonstrated on himself how to feel the anatomy of the neck. Many jurors joined him, feeling their own necks.

But his testimony involved some of the most painful video viewing of the trial now in its second week of testimony. Tobin said Floyd lapsed into unconsciousness shortly before 8:25 p.m. on May 25 when his oxygen levels plummeted. Floyd stopped breathing 23 seconds later.

As a slowed-down bystander video was shown of Floyd's face, Tobin described Floyd's death.

"You can see slight flickering and then it disappears, so one second he's alive and one second he's no longer," Tobin said. "That's the moment the life goes out of his body."

After Floyd's breathing ceased, the doctor said that Chauvin's "knee remained on the neck for another 3 minutes and 27 seconds."

Tobin noted that one video showed Kueng checking Floyd for a pulse and saying he didn't find one. From that moment, the doctor said he determined that Chauvin's knee remained on Floyd for another 2 minutes and 44 seconds.

Tobin, who is based at Hines VA hospital in Chicago and Loyola University, added, "The cause of the low level of oxygen was shallow breathing; small 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."

Under cross examination, Nelson's questioning prompted Tobin to acknowledge that he based some of his scientific conclusions on a number of assumptions, given that no one was at the scene to take various measurements, including Chauvin's net weight with his gear and Floyd's breathing patterns.

Tobin countered that his calculations "are based on direct evidence, they are based on research [and] very few assumptions."

Nelson asked Tobin about the 70 to 90% blockage found in Floyd's coronary arteries and whether that may have been a contributing factor in his death.

Tobin said if it was a contributor, Floyd would have complained of chest pain and had a rapid respiratory rate, but he did not. Nelson also pointed out that Floyd had no bruising on the neck, despite Chauvin's actions.

Blackwell also seized on Nelson getting Tobin to agree that fentanyl can be deadly. Tobin told Blackwell that a fentanyl user who dies first lapses into a coma. Floyd did not.

Unlike many experts called to testify, Tobin said he is not being paid. He said he has testified in dozens of malpractice cases, but this is his first time as a witness in a criminal proceeding. He said the prosecution approached him about testifying, and he agreed because, "I thought I might have some knowledge to explain how Mr. Floyd died."

Another later witness supported much of what Tobin said. Dr. Bill Smock, the police surgeon of the Louisville Police Department, said he was being paid $300 an hour and had spent about 10 hours reviewing videos from Floyd's arrest and other case records.

Smock also concluded that Floyd died from a lack of oxygen and not a fentanyl overdose. Smock said that people who overdose on fentanyl become sleepy, snore, have constricted pupils and their breathing rate falls. With Floyd, Smock said "that is not a fentanyl overdose. That is someone begging to breathe."

Smock said Floyd also showed no signs at the scene of a diagnosis called excited delirium, in which someone afflicted grows extremely hot, moves about rapidly and exhibits superhuman strength.

During the afternoon, jurors heard from a forensic toxicologist at NMS Labs, which tested Flood's blood for illicit drug use and found fentanyl and methamphetamine.

David Isenschmid said that while fentanyl was found in Floyd's blood, so was norfentanyl, which is metabolized fentanyl. Overdose victims who die rarely have norfentanyl in their blood, he said.

He testified that Floyd's ratio of fentanyl to norfentanyl was 1.96. This is compared with the average ratio of 9.05 in postmortem cases and 3.2 in driving under the influence cases of people who survive. Floyd's level of methamphetamine was even lower compared to a sample of driving under the influence meth cases.

"Does this show Mr. Floyd was below the average and even below the median in DUI cases?" prosecutor Erin Eldridge asked. "Yes," he said.

Rochelle Olson • 612-673-1747

Paul Walsh • 612-673-4482