George Floyd died of a lack of oxygen from being pinned to the pavement with a knee on his neck, a medical expert testified at former Officer Derek Chauvin’s murder trial Thursday, emphatically rejecting the defense theory that Floyd’s drug use and underlying health problems were what killed him.

“A healthy person subjected to what Mr. Floyd was subjected to would have died,” said prosecution witness Dr. Martin Tobin, a lung and critical care specialist at the Edward Hines Jr. VA Hospital and Loyola University’s medical school in Illinois.

Using easy-to-understand language to explain medical concepts and even loosening his necktie to make a point, Tobin told the jury that Floyd’s breathing was severely constricted while Chauvin and two other officers held the 46-year-old Black man down on his stomach last May with his hands cuffed behind him and his face jammed against the ground.

The lack of oxygen resulted in brain damage and caused his heart to stop, the witness said.

Tobin, analyzing a graphic presentation of the three officers restraining Floyd for what prosecutors say was almost 9 1/2 minutes, testified that Chauvin’s knee was “virtually on the neck” for more than 90% of the time.

He cited several other factors that he said also made it difficult for Floyd to breathe: officers lifting up on the suspect’s handcuffs, the hard surface of the street, his prone position, his turned head and a knee on his back.

Chauvin kept his knee on Floyd’s neck for 3 minutes, 2 seconds, after Floyd had “reached the point where there was not one ounce of oxygen left in the body,” Tobin said.

As prosecutors repeatedly played a video clip of Floyd on the ground, Tobin pinpointed what he said was a change in the man’s face that told him Floyd was dead. That moment happened around five minutes after police began holding Floyd down.

“At the beginning you can see he’s conscious, you can see slight flickering, and then it disappears,” Tobin said. He explained: “That’s the moment the life goes out of his body.”

Chauvin, 45, is charged with murder and manslaughter in Floyd’s death May 25. Floyd was arrested outside a neighborhood market after being accused of trying to pass a counterfeit $20 bill.

Bystander video of Floyd crying that he couldn’t breathe as onlookers yelled at Chauvin to get off him sparked protests and scattered violence around the U.S.

Defense attorney Eric Nelson has argued that the now-fired white officer did what he was trained to do and that Floyd’s death was caused by illegal drugs and underlying medical problems that included high blood pressure and heart disease. An autopsy found fentanyl and methamphetamine in his system.

But Tobin said he analyzed Floyd’s respiration as seen on body-camera video and explained that while fentanyl typically cuts the rate of respiration by 40 percent, Floyd’s breathing was “right around normal” just before he lost consciousness. Similarly, he said people with severe heart disease have very high respiratory rates.

Tobin also said the high blood level of carbon dioxide measured in the hospital emergency room can be explained by the fact that Floyd was not breathing for nearly 10 minutes before paramedics began artificial respiration, as opposed to his breathing being suppressed by fentanyl.

A forensic toxicologist also testified Thursday that he tested blood drawn from Floyd at the hospital, as well as urine from his autopsy, and found a “very low” amount of methamphetamine. Daniel Isenschmid said fentanyl and a byproduct of its breakdown in the body were also found, but he didn’t immediately say how much was present.

Earlier, Tobin explained that just because Floyd was talking and shown moving on video, it doesn’t mean he was breathing adequately. He said a leg movement seen in the video was involuntary. And he said a person can continue to speak until the airway narrows to 15%, after which “you are in deep trouble.”

Officers can be heard on video telling Floyd that if he can talk, he can breathe.

On cross-examination, Nelson pressed Tobin on that common misconception, pointing to earlier testimony that Minneapolis officers are trained that if people can speak, they can breathe.

Nelson also suggested to Tobin that fentanyl in street drugs could affect people differently than legally obtained fentanyl. He asked, too, about methamphetamine, noting that there are few reasons for which it is legally prescribed. Tobin agreed that it would increase heart rate but said it would not affect respiratory rate.

Tobin used simple language, with terms like “pump handle” and “bucket handle” to describe the act of breathing for the jury. He explained that when the airway narrows, breathing becomes “enormously more difficult” — like “breathing through a drinking straw.”

At one point, the doctor loosened his tie and placed his hands on his own neck and the back of his head to demonstrate how the airway works, inviting the jurors to examine their own necks. Most of them did so, though the judge later told them they didn’t have to.

The expert calculated that at times when Chauvin was in a near-vertical position, with his toes off the ground, half of Chauvin’s body weight with his gear included — or 91.5 pounds — was directly on Floyd’s neck.

He said it appeared that Floyd was getting enough oxygen to keep his brain alive for about the first five minutes because he was still speaking. Tobin said that where Chauvin had his knee after the five-minute mark was not that important, because at that point Floyd had already experienced brain damage.

Chauvin’s attorney has repeatedly shown the jury still images from the video that he said showed Chauvin’s knee was on Floyd’s shoulder blade, not his neck. But nearly all of those images were captured after the five-minute mark, according to the time stamps.

(AP)