Derek Chauvin trial: Medical expert says George Floyd died of 'low level of oxygen'

Two expert witnesses testified in Derek Chauvin trial Thursday that, in their medical opinions, George Floyd died of a loss of oxygen to the body. 

Chauvin is charged with third-degree murder, second-degree murder and second-degree manslaughter in Floyd’s death last May. The trial is being broadcast live, gavel to gavel, on FOX 9 and streaming live at

Dr. Martin Tobin, one of the nation’s foremost experts in the science of breathing, was on the stand for most of the day. He is an expert witness for the state, but was not paid to testify. 

Tobin said he reviewed the medical records related to the case as well as videos of the deadly arrest and concluded Floyd "died from a low level of oxygen" that caused brain damage and a PEA (pulseless electrical activity) arrhythmia that caused his heart to stop.

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Dr. William Smock, a police surgeon and emergency room physician from Louisville, Kentucky, was a paid witness for the state. He testified that in his medical opinion, Floyd "died from positional asphyxia," or in simpler terms, he had "no oxygen left in his body." 

Jurors also heard testimony from Dr. Daniel Isenschmid, a forensic toxicologist at the NMS Laboratory in Pennsylvania who ran tests on Floyd’s blood and urine from the autopsy

Court is adjourned until 9:15 a.m. Friday. 

Pulmonologist says Floyd died of 'low level of oxygen'

The state called its first medical expert to the witness stand on Thursday: Dr. Martin Tobin, a Chicago area pulmonologist and critical care physician. He has been an expert witness on both sides of medical malpractice cases, but this is his first criminal case. He volunteered to offer his expertise in the George Floyd case; he was not paid by the state to testify. 

Dr. Tobin said he reviewed the medical records related to the case as well as videos of the deadly arrest and concluded Floyd "died from a low level of oxygen." 

"This caused damage to his brain and it also caused a PEA (pulseless electrical activity) arrhythmia that caused his heart to stop," he said. 

Tobin said Floyd’s low oxygen level was caused by shallow breathing. He testified that a number of forces led to that shallow breathing, including him being in a prone condition, handcuffed and with knees on his back and neck. That would have prevented air from being able to reach the "essential area in the bottom of the lungs where it gets oxygen into the blood and gets rid of the carbon dioxide," he said. 

The state played an animation of the officers’ position on Floyd during his deadly arrest, during which Tobin explained to the jury the impact on Floyd’s breathing and ability to get oxygen. 

When prosecutor Jerry Blackwell played some of the body camera video of Floyd’s deadly arrest, Tobin drew the jury’s attention to the motions of Floyd’s hands on the street. 

"To most people, this doesn’t look terribly significant, but to a physiologist, this is terribly significant because this tells you that he has used up his resources and he is now literally trying to breathe with his fingers and his knuckles."

Tobin explained Floyd was "using his fingers and his knuckles against the street to try to crank up the right side of his chest. This is his only way to try and get air to try to get into the right lung."

‘Not an ounce of oxygen left in his body’ 

Prosecutor Jerry Blackwell asked Tobin about the significance of Floyd’s final words, "I can’t breathe." Tobin told the jury it gave a false sense of security because of the belief that if someone is speaking, that means they are breathing. Tobin said he believes Floyd suffered a fatal loss of oxygen to the brain immediately after. 

Blackwell later played the video of what Tobin believes was Floyd’s final moments.

"You can see his eyes, he's conscious and then, you see that he isn't...and that's the moment the life goes out of his body," he said.

Tobin told the jury Floyd ended up with zero oxygen in his body, as captured in the video footage from the scene. He then testified by his calculations, Chauvin kept his knee on Floyd’s back and neck for another three minutes. 

Based on his review of the videos in the case, Dr. Tobin said he calculated there were nine minutes and 50 seconds between when Floyd took his last breath and when paramedics arrived and began to give him oxygen. 

He testified this was a crucial amount of time, which is additional evidence as to why he does not believe fentanyl was the cause of Floyd’s death. 

Tobin was asked about Floyd’s underlying health issues. He testified a fully "healthy person" would have died if subjected to the same conditions Floyd faced on the street outside Cup Foods. 

 Pills found in Floyd's vehicle contained meth, fentanyl

Topics ranged during testimony on Wednesday, starting first with the state's use of force expert and ending with forensic scientists who tested the pills found in the vehicles connected to the case.

Stiger testified that force should not have been used on Floyd while he was in the prone position, handcuffed and not resisting.

Forensic experts said the pills found in the vehicles tested positive for methamphetamine and fentanyl.

A moment of contention came when defense attorney Eric Nelson played a short video during Floyd's arrest and asked witnesses if they heard Floyd say, "I ate too many drugs."

Special Agent James Reyerson of Minnesota Bureau of Criminal Apprehension first agreed that he heard Floyd say that. After listening to a longer clip from the prosecution, Reyerson said he heard Floyd say, "I ain't do no drugs."

TIMELINE: George Floyd's death to Derek Chauvin's trial

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