Expert at Derek Chauvin trial testifies that George Floyd suffocated because officers restrained him facedown and handcuffed

MINNEAPOLIS – A physician with 46 years of experience in the physiology of breathing offered critical testimony in Derek Chauvin's murder trial Thursday, describing why George Floyd couldn't breathe while Chauvin and other officers held him facedown and handcuffed on the street last Memorial Day.

Dr. Martin Tobin took jurors through the repercussions of the officers' use of force that, he testified, slowly suffocated him. After Floyd was pulled from a police cruiser, he was placed on his stomach on hard asphalt. His hands were handcuffed behind his back, and officers cranked his arms up against his body, putting pressure against his chest. And Chauvin's left knee was on Floyd's neck while his right was on Floyd's back and side, Tobin said, compressing Floyd's lungs even more.

"It's like the left side (of Floyd's body) is in a vise. It's totally pushed in, squeezed in from the street at the bottom," Tobin said. "And then, from the way the handcuffs are manipulated, that totally interferes with central features of how we breathe."

Tobin told jurors that "a healthy person subjected to what Mr. Floyd was subjected to would have died as a result," potentially undercutting the defense's argument that Floyd died from a combination of his struggle with officers, health conditions and drugs.

Tobin has been an ​expert witness in many court cases, most involving allegations of medical malpractice. But this was his first criminal case, which he said is one reason he didn't seek payment when the prosecution contacted him.

His testimony came on the ninth day of the trial after prosecutors had presented video evidence captured by officers' body-worn cameras and bystanders, as well as the testimony of eyewitnesses, Minneapolis police officials, paramedics, an emergency room doctor, and an expert in the use of force.

The testimony of medical experts like Tobin is expected to be key in the trial of Chauvin, who is charged with second- and third-degree murder and second-degree manslaughter.

Floyd was 'trying to breathe with his fingers and knuckles'

Tobin said the actions by Chauvin and two other officers, J. Alexander Kueng and Thomas Lane, compressed Floyd's airway and forced him to take shallow breaths. That prevented his lungs from getting rid of carbon dioxide and replenishing oxygen.

Over time, Tobin said, Floyd's oxygen level dropped to the point that it led to a seizure. He said Floyd's cause of death was lack of oxygen, which also caused brain damage and an abnormal heartbeat.

Tobin said he watched videos of Floyd's arrests "hundreds of times" and calculated that Chauvin's left knee was on Floyd's neck for the majority of the 9 minutes and 29 seconds he was restrained on the ground.

The effect of the officers' actions and Floyd's position was almost "as if a surgeon had gone in and removed the lung," Tobin said, referring to Floyd's left lung.

Tobin said images from the videos show Floyd trying to use his right fingers and knuckles to raise the right side of his chest to get air into his lungs. "This tells you he has used up his resources and he's literally trying to breathe with his fingers and knuckles," Tobin said.

Some of the jurors sat up and took notes when Tobin said a healthy person would have died if they were restrained like Floyd was. Many jurors jotted notes when he described the moment Floyd stopped breathing and how long he said Chauvin's knee remained on Floyd's neck afterward: 3 minutes and 2 seconds.

90 pounds of pressure on Floyd's neck, doctor testifies

Floyd's position restricted a part of the airway called the hypopharynx, which is vulnerable because it is so narrow, Tobin said.

As Chauvin pressed his knee into Floyd's hypopharynx area, Tobin said, it became harder for Floyd to breathe than it would have been if he been inhaling through a drinking straw.

At one point, Tobin asked jurors to feel their own necks to locate the hypopharynx, spurring a brief interruption as the lawyers conferred with Hennepin County Judge Peter Cahill. The judge told jurors they didn't have to follow his instructions, but nearly all did anyway.

Tobin testified about an image from a video showing the toe of Chauvin's left boot lifted off the ground as he kneeled on Floyd. At that point, Tobin testified, 91.5 pounds were pressed directly onto Floyd's neck. Even when Chauvin's toes were on the ground, he said Floyd had 86.9 pounds on his neck.

Tobin said Floyd's lung capacity dropped by nearly a quarter when he was placed in the prone position. Once Chauvin pressed his knee onto Floyd's neck, his lung capacity dropped again, for a total reduction of 43%, Tobin calculated.

"Now the work Mr. Floyd has to perform is huge," Tobin said. "With each breath, he has to fight against the street, fight with the small volumes he has, and try to lift up the officer's knee with each breath, and has to also lift up the effect of the officer pumping up his arm – the handcuffed arm."

Defense attorney Eric Nelson tried to discredit Tobin's calculations by questioning the assumptions about Floyd's health and Chauvin's weight. Tobin said he made "very few assumptions."

Nelson noted that an autopsy showed that Floyd's hypopharynx was not damaged, nor was his neck bruised. Tobin said he "wouldn’t expect there to be anything found” because oxygen deprivation doesn't leave a fingerprint.

Doctor addresses claim that Floyd could breathe because he was speaking

Floyd cried out, "I can't breathe" more than 20 times. Officers can be heard saying on body-camera video that if he could talk, he could breathe.

Tobin said that is a "dangerous" assumption because it doesn't mean someone still will be breathing in the next moment. Being able to speak meant Floyd's brain was functioning, Tobin said, and it meant he did respirate the moment before.

"It gives you a huge, false sense of security because very shortly after that, we're going to see that he has a major loss of oxygen in that he moves his leg," Tobin said. "So it tells you how dangerous is that concept of ... 'If you can speak, you can breathe.' Yes. That is true. On the surface, but highly misleading, very, very dangerous."

A normal trachea has a diameter between the size of a quarter and a dime, Tobin said. When the trachea is restricted to 15% of that, he said, "you are still able to speak."

Drug use, medical issues not a factor, Tobin says

Tobin said he reviewed Floyd's medical records, knew about his past illnesses, and saw that a toxicology report found fentanyl and methamphetamine in his system. He said Floyd's medical conditions and drug ingestion were not relevant to his death.

Floyd's average rate of respiration would have been faster if he were suffering from the symptoms of heart disease, Tobin said. He calculated that Floyd's breathing rate was the same as a healthy person, even though fentanyl typically represses breathing.

The significant increase in Floyd's carbon dioxide levels as measured by paramedics and hospital emergency room doctors is "completely explained" by the lack of oxygen that Floyd suffered from the restraint, Tobin said.

'That's the moment the life goes out of his body'

Tobin calculated that Floyd was able to speak during the first 4 minutes and 51 seconds that he was on the ground under the officers. That meant oxygen was still reaching his brain.

Tobin testified that the video then showed Floyd struggling to create an area for oxygen to enter his lungs, lifting his right shoulder up and down to expand the lung space, and rocking his hips and spine.

At 5 minutes and 3 seconds, Floyd kicked out one leg, Tobin testified. That signaled he had suffered a hypoxic seizure – a seizure from the lack of oxygen – leading to a fatal brain injury.

After that, Tobin said, the placement of Chauvin's knee was no longer relevant because he had already sustained a massive brain injury.

Tobin testified that Floyd lost consciousness at 8:24 p.m. As an intensive care doctor, Tobin said he could tell when a patient lost consciousness "by how you flick your eyes, or how you constrict the muscles in your face."

"You can see from his eyes, he's conscious. Then, he isn't," Tobin said. "That's the moment the life goes out of his body."

At the moment Floyd lost consciousness, Tobin said, his blood oxygen level would have been 36 mm Hg, well below the normal level of 89 mm Hg for a man his age. Less than a minute later, Floyd would have had no oxygen left in his blood, he said.

"The knee remained on the neck for another 3 minutes and 2 seconds," Tobin said, "after we reached the point where there’s not an ounce of oxygen left in the body." For the last 2 minutes and 44 seconds, Tobin said, the officers held the position even though they knew Floyd had no pulse.

This article originally appeared on USA TODAY: Dr. Martin Tobin speaks on George Floyd breathing: Derek Chauvin trial