If you'd tried to enter the arrest process as a citizen, you would have been arrested too, with possible physical force being applied to you as well. As another cop, you could have tried to intercede, but there was a supervisor (Sergeant) on the scene, and she did not step in. Officers follow their superiors orders. She obviously did not believe that any departmental policies were being violated. In fact, the Breitbart article says that all uniformed staff present did not feel that Garner's life was being threatened throughout the process.
At that point, he as at the whims of the emergency personnel and his physical capacity dependend on their treatment. The lack of concern and lack of basic emt skills within the police is evident.
The NYC tax payers will be funding Mr. Garner family endeavors for a while.