A pulmonologist told jurors that Derek Chauvin pressed 86.9 pounds onto the neck of George Floyd, who tried to push himself off the pavement with his fingertips.

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MINNEAPOLIS — It was a video everyone in the courtroom has been shown repeatedly, of George Floyd facedown on the street with Derek Chauvin’s knee on his neck. But this time, it was slowed down so the jury could see the briefest widening of Mr. Floyd’s eyes — what the expert witness on the stand on Thursday said was his last conscious moment.

“One second he’s alive, and one second he’s no longer,” said the witness, Dr. Martin Tobin, adding, “That’s the moment the life goes out of his body.”

Dr. Tobin, a pulmonologist who specializes in the mechanics of breathing, presented the prosecution’s first extended testimony on a central question in the murder trial of Mr. Chauvin: how George Floyd died. “You’re seeing here fatal injury to the brain from a lack of oxygen,” Dr. Tobin said.

Dr. Tobin said that Mr. Chauvin and other police officers had restricted Mr. Floyd’s breathing by flattening his rib cage against the pavement and pushing his cuffed hands into his torso, and by the placement of Mr. Chauvin’s knees on his neck and back.

The doctor pinpointed the moment he said Mr. Floyd had shown signs of a brain injury, four minutes before Mr. Chauvin lifted his knee from his body.

After two days of sometimes tedious law enforcement testimony on procedures and policy, jurors appeared to be riveted by Dr. Tobin’s ability to break down complex physiological concepts, at times scribbling notes in unison.

Dr. Martin Tobin, a pulmonologist who specializes in the mechanics of breathing, testified Thursday on a central question in Derek Chauvin’s trial.
Still image, via Court TV

Leaning into the microphone, tie slightly askew, Dr. Tobin used his hands and elbows to demonstrate how people breathe. He gave anatomy lessons by asking jurors to palpate their own necks, and showed an artist’s rendering of how three officers, including Mr. Chauvin, had been positioned on Mr. Floyd.

He delivered his opinions without a shred of ambivalence, noting that his conclusions were based on “very precise” scientific knowledge like the level of oxygen when someone loses consciousness.

Dr. Tobin said he had watched portions of the video evidence hundreds of times. He had calculated what he said was the exact amount of weight Mr. Chauvin had placed on Mr. Floyd’s neck (86.9 pounds), clocked Mr. Floyd’s respiratory rate and marked the instant he took his final breath: 8:25:15 p.m.

He reassured jurors that many of the medical terms they have heard during the trial — hypoxia, asphyxia, anoxia — all mean essentially the same thing, “a drastically low level of oxygen.”

His testimony may help prosecutors overcome the fact that the official autopsy report did not use the word “asphyxia,” and seemed to make irrelevant the exact position of Mr. Chauvin’s knees, which has come up several times.

“I don’t think I’ve seen an expert witness as effective as this,” said Mary Moriarty, the former chief public defender of Hennepin County, who has been following the televised trial. “He appears to be the world’s foremost expert on this, and he explained everything in English, in layman’s terms.”

The jury has heard repeatedly that police officers are taught that restraining people facedown is dangerous. Dr. Tobin walked the jury through exactly why, explaining first that simply being in the prone position reduces lung capacity.

On top of that, a knee on the neck compressed Mr. Floyd’s airway, he said, and the weight on his back alone made it three times harder than normal to breathe.

Dr. Tobin discounted the oft-repeated adage that someone who can talk can breathe, calling it “a very dangerous mantra to have out there.”

It is technically true, he said, but “it gives you an enormous false sense of security.”

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

Using video stills, Dr. Tobin showed the efforts that Mr. Floyd had made to free his torso enough to breathe, trying to use his shoulder, his fingertips and even his face, smashed flat against the pavement, as leverage against the weight of Mr. Chauvin.

He pointed out the toe of Mr. Chauvin’s boot lifting off the pavement, and the telltale kick of Mr. Floyd’s legs that, he said, indicated that he had suffered a brain injury 5 minutes and 3 seconds after Mr. Chauvin first placed his knee on Mr. Floyd’s neck.

Still image, via Court TV

The prosecution used Dr. Tobin to pre-emptively poke holes in the defense’s argument that Mr. Floyd’s death was caused by his use of fentanyl, underlying heart disease and other physical ailments.

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

Dr. Tobin was born in rural Ireland, went to medical school in Dublin and spoke with a soft Irish lilt. He is a physician in pulmonary and critical care medicine at Edward Hines Jr. V.A. Hospital and Loyola University Medical Center in the Chicago area and has been practicing for more than 40 years, but this was his first time testifying in a criminal case.

He said that he had testified numerous times in medical malpractice cases, and that he had waived his usual fee of $500 an hour for the Chauvin trial.

Experts say that working for free could cut two ways, either impressing the jury or suggesting that the witness was biased in favor of one side. Mr. Chauvin’s lawyer, Eric J. Nelson, tried to highlight the latter possibility. “You agreed to waive your hourly rate for this time?” he asked. “You felt it was an important case, right?”

Dr. Tobin disputed a defense assertion that an elevated level of carbon dioxide found in Mr. Floyd’s blood was the result of fentanyl use, attributing it instead to the length of time he was not breathing before he was given artificial breaths in an ambulance.

He said that if fentanyl had been having a significant effect, Mr. Floyd’s respiratory rate would have been slower than normal, and that if Mr. Floyd’s heart disease had been severe, it would have been more rapid. Instead, the rate was normal, he said.

Mr. Nelson pushed back, continuing to press his argument that Mr. Floyd’s death might have been an overdose.

He asked if Mr. Floyd’s breathing may have been inhibited if he had taken fentanyl in the moments before police officers brought him to the ground. Dr. Tobin agreed that it could have been but said that Mr. Floyd had never gone into a coma, as he would have done if he were overdosing.

The prosecution called to the stand two more witnesses on Thursday who undermined the claim that Mr. Floyd died of an overdose. Daniel Isenschmid, a forensic toxicologist at N.M.S. Labs in Pennsylvania, where Mr. Floyd’s blood was tested, said the level of fentanyl in his system was far from being obviously fatal. He said it was common for intoxicated drivers who had used fentanyl and survived to have an even higher level of the drug than was found in Mr. Floyd’s blood.

Dr. William Smock, the police surgeon for the Louisville Metro Police Department and a professor of emergency medicine at the University of Louisville School of Medicine, said Mr. Floyd would have had a much more depressed disposition if he were experiencing a fentanyl overdose. “He’s breathing. He’s talking. He’s not snoring,” Dr. Smock said.

“He is saying, ‘Please, please get off of me. I want to breathe. I can’t breathe.’ That is not a fentanyl overdose. That is somebody begging to breathe.”

Reporting was contributed by Nicholas Bogel-Burroughs and Tim Arango from Minneapolis, John Eligon from Kansas City, Mo., and Sheri Fink and Haley Willis from New York.