In this image from video, defense attorney Eric Nelson, left, and defendant, former Minneapolis police Officer Derek Chauvin, right, listen as Hennepin County Judge Peter Cahill presides Thursday April 8, 2021, in the trial of Chauvin at the Hennepin County Courthouse in Minneapolis, Minn.
Fowler's testimony differs from that of medical experts called by the prosecution.
A medical examiner's report that Fowler signed said that Black's death was an accident caused by his heart issues.
Floyd, a 46-year-old Black man, died in May 2020 after Chauvin placed him under arrest, restraining him with his knee for more than nine minutes.
Still unknown is whether Chauvin will testify, which would leave him open to cross examination by the prosecution.
The only time Chauvin has been publicly heard defending himself was when the jury listened to body-camera footage from the scene last May.
Eric Nelson, Chauvin's lead lawyer, spent many minutes asking Fowler to describe the toxins found in vehicle exhaust and how it can kill people even in an outdoor area.
Dr. David Fowler, who was Maryland's chief medical examiner until his retirement in 2019, also told the jury he believed exhaust fumes from the police auto next to which Chauvin pinned Floyd to the road may also have contributed to Floyd's death in May 2020.
He said this was a result of his "atherosclerotic and hypertensive heart disease" and came during Floyd's restraint by police, using medical terms to describe the narrowing of blood vessels and heart problems caused by high blood pressure. Fowler ruled Black's death an accident but Black's family said Fowler and other Maryland officials "covered up and obscured police responsibility" for Black's death.
Prosecutor Jerry Blackwell launched an aggressive cross-examination, attacking Fowler's findings. He got Fowler to acknowledge that even someone who dies from being deprived of oxygen ultimately dies of an arrhythmia. Blackwell further accused Fowler of jumping to conclusions and suggesting to the jury that Floyd had a white pill in his mouth in the video of his arrest.
"Are you critical of the fact that he wasn't given immediate emergency care when he went into cardiac arrest?" asked Blackwell.
"I certainly don't have medical degrees, but I was always trained and feel it's a reasonable assumption that if somebody's, 'I'm choking, I'm choking, ' well, you're not choking because you can breathe", he said.
"In my opinion, the law enforcement subdual, restraint and the neck compression was just more than Mr. Floyd could take by virtue of those heart conditions", he said. He said Chauvin's kneeling on Floyd was not a significant factor in his death and that Chauvin's knee wasn't near Floyd's airway.
Chauvin, who is white, has pleaded not guilty to murder and manslaughter charges.
He said he doesn't believe Chauvin and the other officers used deadly force when they held Floyd down on his stomach, his hands cuffed behind his back and Chauvin's knee on his neck or neck area.