A paramedic who is suing the Health Service Executive for nervous shock has told the High Court his lifelong friend was killed when he fell out the side door of an ambulance and on to the road as they transported a patient.

PJ Cahill, who was driving the ambulance on the N3 Cavan to Dublin Road, said he heard a thud and looked in a mirror to see his 43-year-old friend and father-of-six Simon Sexton hit the ground.

Mr Cahill, who has begun a High Court action for nervous shock as a result of witnessing the accident, said he jammed on the brakes and ran back to find Mr Sexton lying face down near the grassy verge.

Three years ago the HSE was fined €500,000 for health and safety breaches as a result of the paramedic's death in June 2010.

In the High Court yesterday, Mr Justice Raymond Fullam was told by Mr Cahill's counsel Frank Callanan SC the case would centre on the side door.

Counsel said Mr Sexton had seen a crack of light at the top of the side door and went to secure it as the ambulance was moving with a patient and a nurse in the back as well.

Counsel said the side door of the ambulance did not open in the direction of forward travel and that was the immediate cause of the death of Mr Sexton.

He said in 2007 there had been an accident with an ambulance from the same batch in Kerry which had a similarly fitted door.

In evidence Mr Cahill said when he radioed into the Cavan Ambulance Service that a paramedic had fallen out of the ambulance there was "for a second" silence at the other end of the radio before he was told another ambulance was being dispatched to the scene.

"I grabbed the resuscitation bag. I started resuscitation and kept it going until the ambulance arrived."

He said he travelled in the second ambulance attempting to resuscitate Mr Sexton until they reached Cavan General Hospital where a consultant had been put on standby.

Mr Sexton was pronounced dead shortly aftertwards.

Mr Cahill, 50, from Kilnagarbet, Stradone, Co Cavan has sued his employers - the HSE and the German manufacturer of the ambulance Wietmarscher Ambulanz Und Sonderfahrzeug GMBH - for nervous shock.

He has claimed an ambulance was supplied which permitted the side door to open against the direction of travel and there was an alleged failure to ensure a motion lock was fitted to the door to ensure it could not be opened while the ambulance was in motion.

The claims are denied by both defendants.

Opening the case Mr Callanan said Mr Sexton and Mr Cahill were lifelong friends and from the same area. Mr Sexton had followed Mr Cahill into the ambulance service.

Counsel said the paramedics were transferring a patient to St James's Hospital in Dublin and were only 15 minutes outside Cavan when the accident happened.

Mr Callanan said there was no debriefing or counselling offered to Mr Cahill afterwards and he suffered flashbacks and nightmares.

Mr Cahill, he said, was also involved in the vain attempt to save Mr Sexton's life and stayed with him attempting resuscitation as he was brought in another ambulance back to hospital.

"It had a profound effect on Mr Cahill. He has an onerous sense of responsibility over what transpired and struggles to live with it," Counsel said.

The case before Mr Justice Fullum continues tomorrow.