The man accused of killing a Melbourne heart surgeon with a fatal punch may not have caused his death, his lawyer has argued in court.

Key points: Joseph Esmaili, 24 is charged with manslaughter over the death of Dr Pritzwald-Stegmann at Box Hill Hospital

Joseph Esmaili, 24 is charged with manslaughter over the death of Dr Pritzwald-Stegmann at Box Hill Hospital His lawyer argued the "direct cause of his death" was the decision to withdraw medical treatment, not the punch

His lawyer argued the "direct cause of his death" was the decision to withdraw medical treatment, not the punch He also said the issue of self-defence may be raised at trial

Defence lawyer John Desmond will argue at trial that Dr Patrick Pritzwald-Stegmann died because his life support was turned off, not because of the punch itself.

Joseph Esmaili, 24, is facing a Supreme Court trial charged with the manslaughter of Dr Pritzwald-Stegmann at the Box Hill Hospital, in Melbourne's east, in May last year.

Mr Esmaili allegedly punched the doctor, knocking him unconscious after a dispute about smoking in a designated non-smoking area of the hospital.

The 41-year-old surgeon died in the Alfred Hospital four weeks later.

Mr Desmond, for the defence, told the court the actual cause of death is an issue which would be raised during the trial.

"It is an issue of causation … the sanctity of human life," he said.

"It was a deliberate decision made by collective medicos in conjunction with his wife, and perhaps other family members, to withdraw medical treatment and to facilitate death."

He told the court the "direct cause of death was the decision to withdraw medical treatment and death ensued shortly thereafter".

'I shouldn't have hit him'

Mr Desmond indicated the issue of self-defence may also be raised during the trial.

"It's the doctor that continues to pursue," he said.

"He puts down his bag, his backpack, he is up for it. He is not walking away or retreating, he is up for it."

During the first day of the trial the 13 jurors were shown security vision of the moment the accused allegedly delivered the fatal blow.

The court heard Mr Esmaili fled from the scene, telling his friends outside they "needed to leave because he had just hit someone".

He was arrested the following day after his father saw the media reports about the incident and contacted police.

Chief prosecutor Brendan Kissane told the court that during a police interview, Mr Esmaili said he didn't mean to hit the victim hard.

"I wasn't trying to hit him hard. I shouldn't have hit him in the first place," he said.

The trial continues.