So I did some sleuthing the fentanyl will be an interesting argument that might be able to create enough reasonable doubt to end-up with Chauvin getting something like gross negligence for failing to provide medical aid.
George Floyd had 11 nanograms of fetanyl in his blood. Mixed with other drugs, concentrations as low as 7.6 have resulted in death. But the concentrations of other drugs in his body were not high, and didn't suggest recent use of more than a day or two.
For addicts with a tolerance, fatal doses can be @ 24 nanograms. 11-12 can be fatal to inexperienced users.
I'm guessing Floyd was probably not an inexperienced user, so I don't believe he had taken a fatal dose. The argument I'm sure will be made is in combination with Covid-19 and other drugs, it could have been fatal. And, obviously, the knee contributed. JMOG has said several times his instructor said that move is fatal. I'm sure the defense will have experts that claim it isn't.
Again, it's not about proving the COD. Only creating reasonable doubt that the restraint isn't fatal without the contributing high levels of fentanyl. This is really just about whether he'll be convicted of murder as multiple other charges pretty clearly should stick.