"I explained to him that we are trying to resucitate a child and cannot move the ambulance," the paramedic told Bild newspaper. "But he took up a threatening posture in front of me, as if he was about to hit me."
"I don't care who is being resucitated," the motorist said, according to the paramedic.
Police had to be called to restrain the motorist so paramedics could reach their equipment. The paramedics were able to save the child from immediate danger, but he is now in an induced coma in hospital while doctors battle to keep him alive.
The child, who has been named as Leonard Funke, was suffering ventricular fibrillation, a form of irregular heart beat that can be fatal if untreated.
"This man's behavior is unbelievable and inhuman," Thomas Funke, the child's father, said. "My son's life was in danger, and he only thought of himself."
The incident took place last week, but it was only over the weekend that the full details emerged.
The unnamed motorist is now facing charges of destruction of property for damaging the ambulance. If found guilty, he could face a fine or two-year prison sentence. The paramedics chose not to press charges over his threats of violence.
But prosecutors could yet decide to charge him with the more serious offence of obstructing rescue workers, which carries a possible three-year prison sentence.