For the first time during his trial on murder and manslaughter charges in the death of George Floyd, Derek Chauvin spoke for himself Thursday, April 15.

Taking off the face mask he wore as a coronavirus pandemic precaution, Chauvin invoked his constitutional protections against self-incrimination.

"I will invoke my Fifth Amendment privilege today," Chauvin, in a dark gray suit and dark, monochromatic blue shirt and tie, said into a microphone.

Chauvin appeared calm on video streamed live from inside the Hennepin County Courthouse, and spoke only briefly to answer procedural, yes-or-no questions from his attorney, Eric Nelson, and Judge Peter Cahill, before the jury was called back into the courtroom.

The question of whether Chauvin would testify in his own defense had until Thursday remained an open one.

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With that, the defense rested its case, and prosecutors brought back before the jury Dr. Martin Tobin, a Chicago-area pulmonologist and expert in critical care medicine. Tobin had the distinction of being the last witness to testify before jurors in the case.

Tobin, who had testified a week earlier, on April 8, was put on the witness stand for a second time largely to counter the testimony of an expert witness called by the defense a day earlier. That expert, retired Maryland medical examiner Dr. David Fowler, postulated Wednesday, April 14, that Floyd may have been exposed to enough carbon monoxide in his final moments for it to be considered as a factor in his death.

In this still image from video, Chicago-based breathing expert Dr. Martin Tobin answers questions Thursday, April 15, 2021, during the trial of former Minneapolis police officer Derek Chauvin for second-degree murder, third-degree murder and second-degree manslaughter in the death of George Floyd in Minneapolis (Pool via REUTERS)
In this still image from video, Chicago-based breathing expert Dr. Martin Tobin answers questions Thursday, April 15, 2021, during the trial of former Minneapolis police officer Derek Chauvin for second-degree murder, third-degree murder and second-degree manslaughter in the death of George Floyd in Minneapolis (Pool via REUTERS)
In this still image from video, former chief medical examiner of Maryland Dr. David Fowler answers questions Wednesday, April 14, 2021, during the trial of former Minneapolis police officer Derek Chauvin in the death of George Floyd in Minneapolis. (Pool via REUTERS)
In this still image from video, former chief medical examiner of Maryland Dr. David Fowler answers questions Wednesday, April 14, 2021, during the trial of former Minneapolis police officer Derek Chauvin in the death of George Floyd in Minneapolis. (Pool via REUTERS)

Fowler, who together with a panel of forensic experts concluded Floyd died of sudden cardiac arrhythmia during his restraint by Minneapolis police last year, said that was because Floyd was pinned to the ground the night of May 25, 2020, near a running law enforcement vehicle.

Fowler did not label the manner of Floyd's death a homicide, as the medical examiner for Hennepin County did.

According to Fowler, Floyd's potential exposure to carbon monoxide, underlying heart conditions and drug ingestion made it "very difficult to say" just what the specific manner of his death was. Nelson has several times sought to paint Floyd's death as having to do mostly with his heart health and ingestion of methamphetamine and fentanyl.

Tobin on Thursday said he disagreed, however, with the level of carbon monoxide Fowler estimated Floyd was exposed to. Tests conducted on Floyd's blood found an oxygen saturation level of 98%, he told prosecutor Jerry Blackwell, meaning just 2% of it contained carbon monoxide at the most, all of which he said would be "normal."

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Blackwell also had Tobin reiterate that applying pressure to the neck can constrict the hypopharynx. Prosecutors have argued that by kneeling on Floyd's neck the night he died in south Minneapolis, Chauvin fatally deprived him of air.

Asked by Blackwell one day earlier if "pressure on the soft side of the neck also narrows the side of the upper airway, the hypopharynx," Fowler said he had "not seen any literature that indicates that that happens."

Cahill allowed Tobin to testify in a limited capacity, and did not allow new evidence into court that prosecutors sought to admit Thursday.

According to Blackwell, a separate blood-gas test conducted as a routine part of Floyd’s autopsy was found Wednesday not to have been submitted to the court. Their results may have rebutted Fowler's carbon monoxide hypothesis.

Dr. Andrew Baker, the chief medical examiner for Hennepin County who performed Floyd's autopsy, searched for and found the test records after hearing Fowler testify, Blackwell said. Nelson, Chauvin's attorney, told Cahill Thursday he had only been provided with copies of them that morning.

In this still image from video, Dr. Andrew Baker, medical examiner for Hennepin County, testifies in the trial of Derek Chauvin on Friday afternoon, April 9, 2021. (Forum News Service)
In this still image from video, Dr. Andrew Baker, medical examiner for Hennepin County, testifies in the trial of Derek Chauvin on Friday afternoon, April 9, 2021. (Forum News Service)

He said that they should not be presented in court and that he would motion for a mistrial if they were. Cahill agreed and told prosecutors the February submission of Fowler's assessments in writing "gave sufficient notice to the state."

The judge said if Tobin were to testify about the blood-gas test results, he himself would declare a mistrial. Cahill said he did not mean to make an accusation of bad faith but that "it seems odd that Hennepin County Medical Center, when they’re asked to turn over all their records, that they don’t include records that are maybe just buried a little deeper."

"I think their response to the subpoena was probably insufficient in and of itself, and that they should check their standard operating procedures so that, in the event that this happens again, they will supply all the information they have," he said.

Court will not reconvene this week. It is scheduled to resume Monday, April 19, for closing arguments, with jury deliberations to follow.