Testimony on behalf of the defense is scheduled to resume Thursday in the murder trial of Derek Chauvin, whose fate could be in the hands of the jury within days.
The fentanyl and methamphetamine found in Mr Floyd's blood and carbon monoxide poisoning from the exhaust fumes of the adjacent police auto may have contributed to the death, Dr Fowler said.
A number of medical experts called by prosecutors have said Floyd died from a lack of oxygen because his breathing was constricted by the way he was held down.
Chauvin, a 45-year-old white man, is on trial on charges of murder and manslaughter in Floyd's death last May after his arrest of suspicion of passing a counterfeit $20 at a neighborhood market.
A lack of oxygen would impact the brain first, causing someone to become disoriented and speak incoherently, Fowler said.
Fowler appeared to dispute at least some of the findings of Dr. Andrew Baker, Hennepin County's chief medical examiner, who ruled Floyd's death a homicide caused by Chauvin and other officers restraining Floyd in a way that starved his body of oxygen.
Eric Nelson, Chauvin's lead lawyer, asked Fowler if Floyd died in a homicide or by some other manner.
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Under questioning by the defense, Brodd also testified that bystanders yelling at police to get off Floyd complicated the situation for Chauvin and the others by causing them to wonder whether the crowd was becoming a threat, too.
Dr Fowler listed a multitude of factors: Mr Floyd's narrowed arteries, his enlarged heart, his high blood pressure, his drug use, the stress of his restraint, the vehicle exhaust, and a tumour or growth in his lower abdomen that can sometimes play a role in high blood pressure by releasing "fight-or-flight" hormones.
In December, Fowler was sued in federal court by the family of Anton Black, who was 19 when he died after police pinned him facedown on the ground for several minutes as he cried out for his mother. The moment came about after Blackwell asked him twice about his testimony on "sudden death" while questioned by Nelson. 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.
"So, when we are in this space", asked Blackwell, "where there is a space between cardiac arrest and the actual death, are you suggesting that though Mr. Floyd may have been in cardiac arrest, there was a time when he may have been revived because he wasn't dead yet?"
"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.
"Are you critical of the fact that he wasn't given immediate medical care when he went into cardiac arrest?" A cardiology expert rejected the notion that Floyd died of heart problems, saying all indications were that he had "an exceptionally strong heart".
"The amount of force that was applied to Mr. Floyd was less than enough to bruise him", said Fowler, testifying that "all of Floyd's injuries were in areas that Chauvin's knee did not press on".
"Immediate medical attention ... may well reverse that process, yes", Fowler answered.