Duffy then kicked the paramedic in the head.
The prosecution told Manchester Magistrates Court: "The defendant took a run up and then rugby kicked [Mr Shortman] with his right foot to the left side of his head and ear, causing him to fall on the floor and be stunned for around two to three seconds.
"When he came to, the male had been detained by security staff.'"
Paramedic Shortman was then taken to hospital to be checked over before being discharged with minor injuries.
Duffy was arrested at the scene.
He was later charged with assaulting an emergency worker and pleaded guilty this week.
In a victim impact statement, Shortman said: "I do this job to help people and I find it disturbing that someone would do this to me.
"He called for us to attend and see his father, I can't believe this person kicked me to the head, it could have caused an end to my career, massive consequences and even an end to my life.
"This is the most life threatening situation I have been involved in my 20 years in my career as a paramedic."
Duffy made a full confession, saying he had eight or nine pints. He said he remembered an argument with a paramedic and believes he lashed out because he was "overly protective".
He'll be sentenced on September 8.