A medical expert testified Thursday in the Derek Chauvin murder trial that George Floyd died from a lack of oxygen late last spring as the now-fired Minneapolis police officer knelt on the neck and then held that position for three minutes after Floyd drew his last breath.

Dr. Martin Tobin, a Chicago physician who has specialized in respiratory and critical care medicine for decades, said he has reviewed much of the evidence and concluded that “Floyd died from a low level of oxygen. This caused damage to his brain that we see, and it also caused a [pulseless electrical activity] arrhythmia that caused his heart to stop.”

Cause of death has provided a sharp divide between the state and the defense. The prosecution is saying Floyd died from a lack of oxygen, while defense attorney Eric Nelson has argued that Floyd died of a cardiac arrest and other factors.

Using “precise science,” Tobin said Floyd lapsed into unconsciousness shortly before 8:25 p.m. on May 25 as his oxygen levels plummeted.

Floyd stopped breathing 23 seconds later and “didn’t have an ounce of oxygen in his body” less than a minute after losing consciousness, Tobin said. He noted the moment Floyd died when shown the bystander video of his final moments.

“At the beginning you can see he’s conscious, you can see slight flickering and then it disappears, so one second he’s alive and one second he’s no longer,” Tobin said. “…That’s the moment the life goes out of his body.”

After Floyd’s breathing ceased, the doctor said that Chauvin’s “knee remained on the neck for another 3 minutes and 2 seconds.”

The doctor also challenged the defense contention that the powerful opioid fentanyl was a factor in Floyd’s death. Analyzing Floyd’s breathing rate by viewing one of the officer’s body-camera video, Tobin found the rate to be in the normal range. Fentanyl, he said, sharply reduces a person’s breathing rate.

Therefore, he said, “Basically it’s telling you fentanyl is not on board, it is not having an effect on his respiratory centers.”

Tobin, who is based at Hines VA hospital in Chicago and Loyola University, added, “The cause of the low level of oxygen was shallow breathing; small breaths … that weren’t able to carry the air through his lungs down to the essential areas of the lungs that get oxygen into the blood and get rid of the carbon dioxide.”

Tobin listed the factors leading to Floyd being unable to take in oxygen, among them, “He is turned prone on the street, that he has the handcuffs in place combined with the street, and that he has a knee on his neck, and he has a knee in his back and side.”

Unlike many experts called to testify in cases, Tobin said he is not being paid. He said he has testified in dozens of malpractice cases, but this is his first time as a witness in a criminal proceeding. He said the prosecution came to him about testifying, and he agreed because “I thought I might have some knowledge to explain how Mr. Floyd died.”

Using a composite video, Tobin showed jurors how Floyd was positioned with the officers on top of him and how it contributed to his inability to take sufficient breaths. He said Chauvin and officer J. Alexander Kueng manipulated Floyd’s handcuffs by “pushing them into his back and pushing them high,” which further hindered his ability to breathe.

“It’s like the left side is in a vise, it’s being pushed in from the street at the bottom and the way the handcuffs are manipulated … totally interferes with central features of how we breathe,” he said.

Tobin said Floyd also used his left shoulder in attempt to create chest space to draw a breath, but “the shoulder is a very ineffective way of breathing.”

“Basically, on the left side of his chest,” Tobin continued, “it’s as if a surgeon almost went in and removed the lung … and left him totally reliant on his right side,” the doctor said.

With a photo shown in court of Floyd’s knuckles bracing in order to get air, Tobin said, “To most people, this doesn’t look terribly significant, but to a physiologist this is extraordinarily significant, because this shows the has used up all his resources and showed he is trying to breathe with his fingers and knuckles.”

With his left lung rendered useless, the doctor said that Floyd “is totally dependent on getting air into the right side. He’s using his fingers and knuckles against [a squad tire and the street] to try and crank up his chest. This is his only way to get air into the right lung.”

The knee on Floyd’s neck also restricted “air getting into the passageway,” according to the doctor, who said it left him trying to breathe through a space narrower than a small straw. He called that “enormously difficult, and we know that from physics.”

“We know what happens physiologically, when you have this level of narrowing this is going to happen to everybody.”

Tobin then turned his attention to the amount of weight being pressed onto Floyd’s neck. He noted that Chauvin kept his upper body erect while having his left knee on Floyd’s neck while at times lifting his left toe.

“We’re talking half of his body weight and half of his gear [weight], and all of that is coming down,” the doctor said. A graphic accompanying this part of his testimony read that lifting the toe increased the weight pressed on Floyd’s neck from about 86 pounds to more than 91.

Tobin also focused on a specific passage of time of Chauvin having his knee on Floyd’s neck. He said that various video from the scene indicated Floyd suffered brain injury about 5 minutes into his restraint on the street. This is when Floyd “kicked out his leg in an extension form” and that response is when “we see he suffered brain injury from a low level of oxygen.”

Along with the left knee on the neck hindering Floyd’s ability to breathe, Tobin said Chauvin’s right knee on the back made things more difficult while “the street is playing a huge part … and totally preventing every action on the front.”

With both knees on the body, “you are now seeing a 43% reduction” in Floyd’s lung capacity.

Also Thursday, the prosecution confirmed that it intends to soon call as a witness Dr. Andrew Baker, the Hennepin County medical examiner who performed the only autopsy on Floyd soon after he died.

Baker ruled Floyd’s cause of death as cardiac arrest, namely “cardiopulmonary arrest complicating law enforcement subdual, restraint, and neck compression.” He also listed hardening and thickening of the artery walls, heart disease and drug use as “other significant conditions.” Fentanyl and methamphetamine were also found in Floyd’s system.

The prosecution, however, said in its opening statement at the start of last week that it would prove Floyd died of asphyxia, or lack of oxygen, while Chauvin knelt on his neck for more than nine minutes.

Defense attorney Nelson has argued that Floyd died of a cardiac arrest resulting from illicit drug use and ongoing health problems, including heart disease and high blood pressure.

Chauvin is on trial for second-degree murder, third-degree murder and manslaughter. The three other fired officers — J. Alexander Kueng, Thomas Lane and Tou Thao — are to go on trial in August on charges of aiding and abetting him.

Meanwhile, Thursday is the deadline that District Judge Peter Cahill set for Nelson to disclose to the court a line of questioning he hopes to pursue should he get the chance against a most reluctant potential witness.

Morries Hall was with Floyd earlier in the day and during the arrest at 38th and Chicago by Chauvin and other officers. Testimony last week from Floyd’s girlfriend implicated Hall as a provider of illicit drugs to Floyd, including in the month of his death.

Hall, 42, has been subpoenaed as a witness but has told the court that he intends to invoke his Fifth Amendment right against self-incrimination out of concern of revealing enough in testimony to prompt third-degree murder charges or other felony counts against himself.

Once Cahill sees Nelson’s questions, the judge is expected to decide whether Hall will be called to the witness stand at some point and how sweeping the defense can quiz him without prompting Hall to invoke his constitutional right based on the nature of the question.

Star Tribune staff writers Rochelle Olson and Chao Xiong contributed to this report.

Paul Walsh • 612-673-4482

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