Lane, his partner Kueng, and Chauvin knew he didn't have a pulse. (Lane's partner Kueng checks at 18:03 and says "I can't find one"..and Lane and his partner then get off of GF at that time...and other times in video below they check) EMT knew he didn't have a pulse. That's the first thing they did when on EMS got on scene as shown in video below toward the end. (Can be seen more clearly on the bystander video which we've seen)
The reason they loaded and left so quickly, and told them they were going to a few blocks away was because at 20:15 it was said by LE "lets' get him out of here", probably due to the gathering crowd MOO. The video posted upthread toward the end shows Lane in the ambulance and what was done to finally give mechanical compressions at 59:13:00. It also shows that as Lane leaves, he comes back to the ambulance and the mechanical compressions that were going on had ended at 1:04:35.
MOO
WARNING - VERY UPSETTING AND GRAPHIC - ROLL AND SCROLL IF YOU DON'T WANT TO VIEW