A paranoid schizophrenic suffered a fatal cardiac arrest after being restrained by police officers for more than 20 minutes while in custody, a court heard.
Thomas Orchard, 32, died in hospital seven days after the incident in which he was held down, handcuffed and a large fabric webbing belt put across his face - to try and prevent him from biting or spitting.
He was then freed from the restraints - making little or no movement - and left lying face down on a mattress in his cell at Heavitree Road police station in Exeter, Devon.
It was a further 12 minutes before the officers re-entered Mr Orchard's locked cell and discovered he was not breathing.
The church caretaker, who was suffering a relapse in his mental health, had been arrested on the morning of October 3 2012 in the city centre on suspicion of a public order offence.
Custody Sergeant Jan Kingshott, 44, and civilian detention officers Simon Tansley, 38, and Michael Marsden, 55, are on trial at Bristol Crown Court accused of killing Mr Orchard. They each deny two charges of manslaughter.
Pathologist Dr Russell Delaney told the jury that he concluded Mr Orchard had died from severe hypoxic-ischemic brain damage.
He said this had been caused by a prolonged cardio-respiratory arrest "following a violent struggle and period of physical restraint", including a "prolonged period in a prone position and the application of an emergency response belt across the face resulting in asphyxia".
"The primary cause of death was severe hypoxic-ischemic brain damage, which is brain damage due to a lack of oxygen supply to the brain tissue," Dr Delaney said.
"This was caused by a prolonged cardio-respiratory arrest. It was the combined effects of the various factors that happened during the incident.
"It is not possible to isolate one factor as being more important than another. This is one situation where the factors happened together over time and they have contributed together to lead to him having a cardio-respiratory arrest.
"It is the fact of prolonged physical exertion which is involving a struggle and a period of struggle against restraint.
"To maintain a prolonged struggle you need increasing oxygen intake and you need to be able to maintain it.
"As part of that prolonged physical exertion your heart rate increases and your blood pressure increases and that prolonged physical exertion involves increasing levels within your blood stream of adrenalin and that makes your heart pump faster and stronger.
"The effect of a prolonged struggle is you will become physically tired and potentially leading to exhaustion at the end of that process.
"That tiredness and exhaustion effects your ability to breathe or it can reduce your ability to breathe effectively.
"During a long period of time spent in a prone position it can affect the mechanics of breathing especially in the context of prolonged physical exertion when you are getting tired."
Dr Delaney said the application of the emergency response belt across Mr Orchard's face would have also impeded his ability to breathe.
"I believe it would reduce the effectiveness of oxygen intake because it is covering the nose and the mouth," he told the court.
"I do not think the use of the leg cuffs and hand restraints have decreased his ability to breathe but his struggle against that has increased his need for oxygen.
"The significance in my opinion is there is no immediate support of the head apart from the application of the mask.
"The only thing that is preventing his face from collapsing into that mask is his own muscular strength.
"His nose and mouth would become covered by it and that would reduce the effectiveness of the air intake."
At the time of Mr Orchard's death he had been proscribed the anti-psychotic drug Clozapine to help treat his schizophrenia.
The court heard that one of the side-effects was hyper-salivation - an excessive production of saliva - or spitting and with the application of the emergency response belt it could have made Mr Orchard's breathing harder.
"I would expect the fabric to let less air through it if it was becoming more wet and less dry," Dr Delaney said.
Prosecutor Mark Heywood QC asked Dr Delaney about how being left face down on a mattress in his cell for 12 minutes would have affected him.
"I believe that position at that stage would have reduced the effectiveness of breathing at a time when his oxygen demands have increased because of the physical exertion that occurred previously," the pathologist replied.