I honestly don’t know. I’m not sure if that has been discussed, publicly...yet.
The picture that I saw did not show Chauvin constricting his air passage. In fact, the proponents of a “murder” charge insist that George was calling out that he could not breath. Talking is not something someone does when they are being choked or asphyxiated.
However, the knee on the neck did look like it had the potential of cutting off blood supply to the head. That could certainly result in lack of consciousness and even death, but it is not asphyxiation.