MINNEAPOLIS (NewsNation Now) — George Floyd died of a lack of oxygen from being pinned on his stomach, facedown on the pavement with his hands cuffed behind him, a medical expert testified Thursday at former Officer Derek Chauvin’s murder trial.

Floyd’s breathing, while he was being held down by Chauvin and other officers, was too shallow to take in enough oxygen, which in turn damaged his brain and caused pulseless electrical activity (PEA) arrhythmia that made his heart stop, said Dr. Martin Tobin, a lung and critical care specialist at the Edward Hines, Jr. VA Hospital and Loyola University’s Medical Center in Chicago. Tobin said this process could also be described as asphyxia or hypoxia.

Tobin has been engaged in medical research regarding the breath and the study of lungs since 1981.

He took the stand as part of an effort by prosecutors to establish that it was Chauvin’s actions — not Floyd’s illegal drug use and underlying health conditions, as the defense contends — that killed the 46-year-old Black man last May in an arrest on suspicion Floyd used a fake $20 bill to buy cigarettes at a convenience store.

Taking direct aim at the defense theory of underlying health conditions, Tobin said, “A healthy person subjected to what Mr. Floyd was subjected to would have died as a result of what he was subjected to.”

Tobin, analyzing a graphic presentation of the three officers pinning Floyd for what prosecutors say was almost 9 1/2 minutes, said Chauvin’s knee was “virtually on the neck for the vast majority of time.” He said it was “more than 90% of the time in my calculations.”

Tobin reviewed various videos of the arrest and testified that several factors in addition to Chauvin’s knee on Floyd’s neck created a type of “vise” grip on Floyd’s breathing. Those additional factors included how the handcuffs were applied, the hard street, the knee on his back, and being in the prone position.

“The street is playing a huge part, it’s coming in from the front, and totally preventing every action happening from the front,” said Tobin about Floyd’s inability to get full breaths.

Worsening the effect, Tobin pointed out that Officer J. Kueng held Floyd’s left hand upward, and Chauvin’s right knee compressed Floyd’s side, meaning “the ability to expand his left side is enormously impaired.” Often, when the lungs are constricted, the body will try and use the shoulders to assist in breathing, said Tobin.

Tobin discussed frames from the video that he said showed Floyd trying to push his chest up from the street using his fingers and his face as leverage as he struggled for breath.

Kueng and two other officers are set to face a trial later in the year on charges of aiding and abetting Chauvin in killing Floyd.

Using anatomical diagrams and excerpts from videos of the arrest, Tobin used simple language, with terms like “pump handle” and “bucket handle” to describe the act of breathing for jurors. At one point, he invited them to “examine your own necks, all of you in the jury right now” to better understand the effect of a knee on a person’s neck. The judge later clarified that the jury did not have to participate.

Gesturing to his own neck, the physician and medical school professor gave an anatomy lesson explaining why the placement of Chauvin’s knee on the hypopharynx was crucial to blocking oxygen from entering the airway.

Tobin explained to jurors what happens as the space in the airway narrows, saying breathing then becomes “enormously more difficult,” like “breathing through a drinking straw.”

Tobin testified that if the hypopharynx – the bottom part of the throat – becomes totally obstructed, it takes just seconds to reduce the level of oxygen to where it would result “in either a seizure or a heart attack.”

At times Chauvin was nearly vertical when pressing his knee into Floyd’s neck, said Tobin. He estimated that when the officer’s toes were on the ground, the body weight compressing the neck was 86.9 lbs. and when Chauvin’s toes were off the ground, 91.5 lbs. was directly compressing the neck.

While Floyd was speaking for the first 4 minutes 51 seconds, there was not complete compression of his neck. However, Tobin said using a person’s vocalization or talking as a measure for breathing isn’t an adequate gauge as brain damage happens quickly.

“It’s a true statement, but it gives you an enormous false sense of security,” Tobin said. “Certainly at the moment you’re speaking you are breathing, but it doesn’t tell you if you’re going to be breathing five seconds later.”

The pulmonologist specified the first 5 minutes, 3 seconds as key to Floyd’s breathing and that after that time period, he suffered brain damage.

But Tobin said where Chauvin had his knee after the five-minute mark would not make much of a difference, because at that point, Floyd had already experienced brain damage.

“We can tell from the movement in his leg the level of oxygen in the brain,” said Tobin, clarifying that a “straightened out leg” is a body movement clinicians often see as a signal of brain damage.

Chauvin attorney Eric Nelson has repeatedly shown the jury still images from the video that he said showed Chauvin’s knee was on Floyd’s shoulder blade. But nearly all of those images were captured more than five minutes into the ordeal, according to video time stamps.

The Associated Press and Reuters contributed to this report. Reporting by Jonathan Allen and Brendan O’Brien of Reuters, and Amy Forliti, Steve Karnowski and Tammy Webber of AP.