A nurse accused of poisoning patients looked elated when he told colleagues one had gone into respiratory arrest, a court has heard.
Nurse Benjamin Geen denies two counts of murder
Benjamin Geen, 25, is on trial for murder and causing grievous bodily harm by administering substances to 18 patients to make them stop breathing.
The prosecution alleges he was the only medic present during the respiratory collapses of all the 18 patients.
Mr Geen, of Banbury, denies two charges of murder and 18 counts of GBH.
Anthony Bateman, 66, was admitted to the Horton General Hospital in Banbury in the early morning on the recommendation of his GP.
The jury heard he was suffering from serious lung disease, severe rheumatoid arthritis and was thought to have an underlying cancer.
But according to junior doctor Sally Hughes, he was sitting up in bed, "chatting", eating breakfast cereal and drinking tea when she visited him at about 1000 GMT.
She instructed Benjamin Geen to set up a saline drip containing potassium, ordered an x-ray and left.
Less than half an hour later she was told Mr Bateman's condition was rapidly worsening. Just over one hour later he was dead.
Co-ordinating nurse Anne Shea said that Mr Geen said, "Oh no, here we go again", as Mr Bateman turned blue and began to fight for breath.
Describing Mr Geen's attitude during the resuscitation attempts, she said: "He [Mr Geen] was alive, his eyes were bright and it's true we try to lighten the situation but it just felt that he was elated."
The court heard in previous evidence that experts said Mr Bateman's arrest was "highly unlikely" to have been caused by his underlying illnesses and his life would have been prolonged by resuscitation.
They concluded he had been given a muscle relaxant - the prosecution say by Mr Geen.
Dr Hughes said that she was left "puzzled" by the sudden deterioration.
She told prosecutor Michael Austin-Smith QC: "It stuck in my mind enough that after Mr Geen was arrested, I phoned up my old consultant - I had moved hospitals by then - and said that this was a case the police should probably look at."