Key Moments on Day 9 of the Derek Chauvin Trial

The trial of the former officer Derek Chauvin will continue on Thursday after a day of testimony focused on Mr. Floyd’s drug use on the day of his death. Mr. Chauvin’s defense has tried to argue that Mr. Floyd died from a possible overdose, but the prosecution blames the actions of Mr. Chauvin, who pinned Mr. Floyd with his knee for about nine and a half minutes.

Here are some key takeaways from recent testimony.

Mr. Floyd showed signs of a brain injury under Mr. Chauvin’s knee.

A veteran lung doctor testified on Thursday that George Floyd’s death was caused in part by Derek Chauvin’s knees pressing against his neck and back, making it impossible for him to breathe, and that Mr. Floyd showed signs of a brain injury about four minutes before Mr. Chauvin lifted his knee from his neck.

Dr. Martin J. Tobin, a pulmonologist and critical care doctor in Chicago, said in court that the combination of Mr. Chauvin’s pressure, the handcuffs pulling Mr. Floyd’s hands behind his back and Mr. Floyd’s body being pressed against the street had caused him to die “from a low level of oxygen.”

Dr. Tobin was adamant that Mr. Chauvin had caused Mr. Floyd’s death on May 25. He said that based on Mr. Floyd’s visible respiratory rate before he went unconscious, any fentanyl in his system was “not having an effect” on his breathing.

“A healthy person subjected to what Mr. Floyd was subjected to would have died,” Dr. Tobin said.

After analyzing videos of the arrest, Dr. Tobin said he determined that Mr. Chauvin had pressed his left knee on Mr. Floyd’s neck for more than 90 percent of the time that Mr. Floyd was on the ground, and that he had kept his right knee on Mr. Floyd’s back for the majority of the time as well. That pressure, combined with having his hands cuffed behind his back and pushed into the street facedown, had cut off oxygen and caused his heart to stop, Dr. Tobin said.

“He was being squashed between the two sides,” he said.

Mr. Floyd was so desperate for air at one point that he tried to lift himself off the ground by pushing his right knuckle against a police car’s tire, Dr. Tobin said.

“This is his only way to try to get air into the right lung,” the doctor said.

Dr. Tobin outlined four factors that he said had caused Mr. Floyd to lose oxygen and die: The left knee of Mr. Chauvin on Mr. Floyd’s neck, Mr. Chauvin’s right knee on Mr. Floyd’s back and side, Mr. Floyd being handcuffed as he was lying in the street, and Mr. Floyd being held in the prone position. These factors had combined to only allow Mr. Floyd to take small breaths, which were not enough to bring air to the parts of the lungs that allow oxygen to get into blood.

Mr. Chauvin’s lawyer pushed back against the doctor’s testimony.

Fentanyl and methamphetamine were found in Mr. Floyd’s system, and the possibility that they caused his death is a crucial argument for Mr. Chauvin’s defense as the trial enters a phase in which medical testimony is front and center.

Mr. Chauvin’s lawyer, Eric J. Nelson suggested on Thursday that Mr. Floyd could have died of a fentanyl overdose if he had taken the drug in the moments before police officers pushed him to the ground. Dr. Tobin said that, unlike someone who died of a fentanyl overdose, Mr. Floyd had never gone into a coma. Mr. Nelson also sought to portray the medical care that Mr. Floyd received from paramedics as lacking; it took them nine minutes to insert a tube down Mr. Floyd’s throat after they arrived on scene, he said, but the nearest hospital was a five-minute drive away.

Mr. Nelson also noted that even doctors sometimes have trouble understanding that a patient who can talk is having trouble breathing. Mr. Floyd told police officers that he could not breathe, and in response, one officer told him that it takes oxygen to talk, implying that he must be able to breathe, something medical experts have said is false or misleading.

Floyd was begging for air, not overdosing, a surgeon said.

Dr. Bill Smock, the surgeon for the Louisville Metropolitan Police Department, also testified on Thursday, saying he saw no evidence of an overdose. “Mr. Floyd died from positional asphyxia, which is a fancy way of saying he died because he had no oxygen left in his body,” he said.

Mr. Floyd was alert and had a keen sense of space, which both indicate that he was not overdosing, Dr. Smock said. In addition, Mr. Floyd was begging for air — Dr. Smock called it “air hunger” — and said a person overdosing on fentanyl would typically have the opposite response, more like someone who falls asleep or slips into a coma.

Asked whether Mr. Floyd was suffering from a fentanyl overdose, Dr. Smock said, “That is not a fentanyl overdose. That is somebody begging to breathe.”

A forensic toxicologist who testified on the issue of drug use provided the jury with data about fentanyl and methamphetamine, the two substances found in Mr. Floyd’s system after his death. The toxicologist, Daniel Isenschmid, who works at N.M.S. Labs in Pennsylvania, said that the drug profile found in Mr. Floyd’s system appeared to be more consistent with patients who were still alive, rather than those found in autopsies.

Dr. Isenschmid indicated that Mr. Floyd’s body had already processed a substantial portion of fentanyl, bolstering prosecutors’ argument that he had not overdosed. The same amount of fentanyl, he said, can have very different effects in a new user as opposed to someone who is addicted to the drug. Mr. Floyd’s girlfriend has said she and Mr. Floyd both struggled to stop using opioids.

“If a person becomes tolerant to a drug, they require more and more to get the desired effect,” Dr. Isenschmid said.