Interestingly, with drugs and intervention like this, cardiac events are very survivable. You know what's not? Drug overdoses, especially when there is no immediate diagnosis indicating naltrexone/Narcan.
My belief is that Floyd died of an opiate overdose that was misdiagnosed and as such he did not get appropriate acute treatment. Your mileage may vary.
I think this is going to be a cornerstone of the defense. When the cops tried to put him in the car, he looked like he was freaking out on an upper and not a downer. Not a junkie, don't play one on TV, but junkies don't usually try to fight four cops at once.
In either the Keung or Lane body cam, you can see flecks of foam around Floyd's mouth when he's walking to the squad car, and one of the cops, I think Lane, asks while they are pinning Floyd what kind of drugs cause nystagmus. One of the cops responded that PCP can cause that.
While the MMA guy was testifying, they played the part of the video where Thao is seen telling the crowd, "Don't do drugs," and everyone in the crowd is like "Aw hell naw" because there was only one bystander there when Floyd was fighting four cops at once, and they couldn't see from where they were standing that Floyd was still struggling when he was on the ground.
Then, when Floyd is in the ambulance, which you can see in Lane's bodycam video, there's no point where the EMTs even consider the possibility that he's OD'ing on opiates, and I'm sure any ambulance crew in this day and age passes out Narcan like it's going out of style. Of course resuscitation efforts are going to fail if the underlying opiod overdose isn't addressed.
Interestingly, the defense's opening statement also said that Floyd's nose was bleeding before they got him on the ground, and the attorney attributed it to Floyd banging his face against the plexiglass in the squad car, but if I were the defense, I might also float the theory that his blood pressure was through the got dam roof and he popped a blood vessel in his nose, which then smeared on the plexiglass. The autopsy doesn't seem to mention any injuries to the nose.
For the people talking about whether or not he could breathe, the prosecution's opening statement said that they would be calling witnesses to educate everyone about agonal breathing. (But instead they called a witness to say that Chauvin had Floyd in a blood choke and that killed him. Whoops!)
Not to keep harping on the point, but the prosecution needs one very clean narrative about what happened, whereas the defense is free to argue that he was both freaking out on meth, triggering a blood pressure spike and subsequent heart attack, while also arguing that he was OD'ing on opiods, triggering respiratory depression.