An expert medical witness on Thursday, April 8, testified that one portion of footage from the night of May 25 showed the exact moment that George Floyd drew his last breath.

Chicago-area pulmonology and critical care specialist Dr. Martin Tobin, who reviewed footage and other materials pertaining to Floyd's death as an expert witness for the prosecution, said that, by his math, former Minneapolis Police officer Derek Chauvin then continued to kneel on Floyd's back and neck for another 3 minutes.

"At that point, there's not an ounce of oxygen left in his body," Tobin testified.

His testimony featured illustrations depicting Floyd's final moments.

This still image from video shows a mock-up of what Chicago-area pulmonologist Dr. Martin Tobin described as the force the ex-officer exerted on George Floyd in the moments before he died May 25, 2020. (Forum News Service)
This still image from video shows a mock-up of what Chicago-area pulmonologist Dr. Martin Tobin described as the force the ex-officer exerted on George Floyd in the moments before he died May 25, 2020. (Forum News Service)

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Tobin was the first to take the stand Thursday in what was a highly technical day of testimony in the trial of Derek Chauvin, facing murder and manslaughter charges in Floyd's 2020 death.

It was the first criminal case in which the Loyola University professor of medicine took part, having beforehand testified mostly in malpractice cases. He was not paid for his time but volunteered it at the request of state prosecutors, he said, because he had "some knowledge that would be helpful to explain how Mr. Floyd died."

Because Floyd was pinned in a prone position on solid ground, according to Tobin, he was essentially unable to expand his chest and, with his airway constricted by Chauvin's knee, could not draw in enough air to breathe sufficiently. He died of "low oxygen levels," the doctor said.

In this still image from video, Chicago-based breathing expert Dr. Martin Tobin answers questions Thursday, April 8, 2021, during the trial of former Minneapolis police officer Derek Chauvin for second-degree murder, third-degree murder and second-degree manslaughter in the death of George Floyd in Minneapolis (Pool via REUTERS)
In this still image from video, Chicago-based breathing expert Dr. Martin Tobin answers questions Thursday, April 8, 2021, during the trial of former Minneapolis police officer Derek Chauvin for second-degree murder, third-degree murder and second-degree manslaughter in the death of George Floyd in Minneapolis (Pool via REUTERS)

Witnesses for the prosecution Thursday added more support to the argument that Floyd died last May not because of drug use or his own underlying health conditions, but because Chauvin kneeled on his back and neck long enough to deprive him of air.

Medical experts called to the witness stand described in anatomical detail how the force exerted on Floyd in the moments before his death locked him in a struggle to breathe. That struggle ultimately, and fatally, affected his brain, they testified.

That was further evidenced by footage of his arrest, according to two witnesses. At one point in the video, as Floyd was being restrained, they said Floyd could be seen trying to push himself up with the knuckles of one cuffed hand as if trying to ease the pressure on him. In another, he could be seen kicking out with one leg and shaking his bound hands before becoming motionless, which according to their testimony indicated he had what is called an "anoxic seizure."

A second outside expert, Dr. Bill Smock, said in his testimony later Thursday that he reached a similar conclusion. He and Tobin were dismissive of the idea that Chauvin's defense has sought to play up: that drugs — fentanyl and methamphetamine — had more to do with Floyd's death than their client did.

But footage from that night last May showed otherwise, according to Smock, who testified that Floyd appeared more alert and responsive than an individual high on fentanyl ought to have been. The surgeon for the Louisville, Ky., city police force attributed Floyd's death to positional asphyxia, a form of suffocation caused by the position of one's body.

In this still image from video, Louisville Metro Police Department surgeon Dr. Bill Smock answers questions Thursday, April 8, 2021, during the trial of former Minneapolis police officer Derek Chauvin in Minneapolis. (Pool via REUTERS)
In this still image from video, Louisville Metro Police Department surgeon Dr. Bill Smock answers questions Thursday, April 8, 2021, during the trial of former Minneapolis police officer Derek Chauvin in Minneapolis. (Pool via REUTERS)

"He's breathing. He's talking. He's not snoring. He's saying, 'please, please get off of me, I can’t breathe.' That is not a fentanyl overdose. That's someone begging to breathe," Smock said in reference to arrest footage played in court. As an expert witness, Smock said he was paid $300 per hour for his court appearance Thursday.

According to a witness from a Pennsylvania lab that analyzed the contents of Floyd's bodily fluids for fentanyl and methamphetamine, which an autopsy had earlier found he consumed at some point before he died, the amount of both drugs observed in his blood and urine were low compared to a sampling of other analyses the lab performed in the past. The methamphetamine likely had no intoxicating effect, the witness told jurors earlier in the day.

Floyd's health, namely his heart condition, was likely not a deciding factor in his death either, according to the witnesses. Asked at one point by state prosecutor Jerry Blackwell if any of Floyd's medical conditions had "anything to do with the cause of his death," Tobin replied: "None whatsoever."

Smock also said Floyd did not appear from footage to be experiencing "excited delirium" the night he was arrested. The controversial syndrome, which several medical organizations do not recognize, is characterized by aggressive behavior and confusion.

Smock agreed that the diagnosis can be controversial but said he believes it to be legitimate, though not applicable to Floyd. He did not exhibit signs consistent with the syndrome, according to Smock, including aggression, excessive sweating and the inability to feel pain.

That Floyd's autopsy did not find signs of injury to his neck and parts of his throat, Smock said, also did not mean he did not die of asphyxiation.

"You can be strangled to death and have no bruises," he said.

Chauvin is charged with second and third-degree murder and second-degree manslaughter in Floyd's death. Testimony is to resume at 9:15 a.m. Friday, April 9.