Skip to main content
Open this photo in gallery:

Chicago-based breathing expert Dr. Martin Tobin testifies during the trial of former Minneapolis police officer Derek Chauvin, in Minneapolis, Minn., on April 8, 2021.POOL/Reuters

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 defence theory that Mr. 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 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 illustrate a point, Dr. Tobin told the jury that Mr. Floyd’s breathing was severely constricted while Mr. Chauvin and two other Minneapolis 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.

Dr. Tobin, analyzing images of the three officers restraining Mr. Floyd for what prosecutors say was almost 9½ minutes, testified that Mr. Chauvin’s knee was “virtually on the neck” for more than 90 per cent of the time.

He cited several other factors that he said also made it difficult for Mr. 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.

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

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

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

Mr. Chauvin, 45, is charged with murder and manslaughter in Mr. Floyd’s May 25, 2020, death. Mr. Floyd was arrested outside a neighbourhood market after being accused of trying to pass a counterfeit US$20 bill.

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

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

During cross-examination, Mr. Chauvin’s lawyer Eric Nelson pressed Dr. Tobin on that common misconception, pointing to earlier testimony that Minneapolis officers are trained that if people can speak, they can breathe.

Mr. Nelson has argued that Mr. Chauvin did what he was trained to do and that Mr. 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 Dr. Tobin said he analyzed Mr. Floyd’s respiration as seen on body-camera video and explained that while fentanyl typically cuts the rate of respiration 40 per cent, Mr. Floyd’s breathing was “right around normal” just before he lost consciousness.

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

Another prosecution witness, Dr. Bill Smock, an expert on deaths from asphyxia, backed up Dr. Tobin’s assessment. Dr. Smock said Mr. Floyd did not have symptoms of a fentanyl overdose such as constricted pupils and decreased breathing. He said Mr. Floyd’s actions were the opposite, because he was pleading for air.

“That is not a fentanyl overdose. That is somebody begging to breathe,” said Dr. Smock, the police surgeon for the Louisville, Ky., department. He said Mr. Floyd died of “positional asphyxia,” a lack of oxygen because of the position of the body.

Also Thursday, a forensic toxicologist said that he tested blood drawn from Mr. 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 were also in Mr. Floyd’s system.

Dr. Isenschmid put the level of fentanyl in Mr. Floyd’s blood at 11 nanograms per millilitre. For perspective, he said that testing of more than 2,000 people arrested for driving under the influence of fentanyl revealed an average concentration of 9.59, and dozens of them had levels higher than Mr. Floyd’s – and lived.

On cross-examination, Mr. Nelson suggested there was no way to know how much fentanyl Mr. Floyd had ingested, and Dr. Isenschmid agreed. The defence attorney also said it’s impossible to know the concentration of fentanyl in illegal street drugs.

“Every single pill you take, it becomes a unique experience for the person, right?” Dr. Isenschmid agreed.

In his own turn on the stand, Dr. Tobin used simple language, with terms such as “pump handle” and “bucket handle” to describe the act of breathing for the jury. He explained that when the airway narrows, breathing becomes far 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 Mr. Chauvin was in a near-vertical position, with his toes off the ground, half of Mr. Chauvin’s body weight with his gear included – or 91.5 pounds – was directly on Mr. Floyd’s neck.

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

Mr. Chauvin’s lawyer has repeatedly shown the jury still images from the video that he said depicted Mr. Chauvin’s knee on Mr. 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.

Our Morning Update and Evening Update newsletters are written by Globe editors, giving you a concise summary of the day’s most important headlines. Sign up today.

Follow related authors and topics

Authors and topics you follow will be added to your personal news feed in Following.

Interact with The Globe